Survey of the development of petro-forge forming machines

lnl. J. Mach. Tool Des. Res. Vol. 25, No. 2, pp. 105 197, 1985. Prmtcd ii't Great Britain

0021)-7357/85 $3.(X) + (I.(10 Pergamon Press Ltd.

SURVEY OF THE DEVELOPMENT OF PETRO-FORGE FORMING MACHINES S. A . TOBIAS*

(Received 1 February 1985) Abstract--Of the many HERF hammers developed in the 50's and 60's three types have survived: Dynapak, CEFF and Petro-Forge. Dynapak and CEFF machines are pneumatically actuated devices. They continue to be used in industry, having found a segment of the market in which they compete successfully with conventional forming machines. The operational characteristics of these devices are discussed and a critical appraisal of their design features is given. Petro-Forge is a combustion actuated hammer. A survey of the development of these machines is offered within the context of the practical experience relating to Dynapak and CEFF. After discussing their operational principle, the various types and models constructed are surveyed, in particular the Petro-Forge Mk.IF and Mk.IIF, the "'Slow-Speed Petro-Forge', demonstrating that combustion actuation can be used also for relatively low impact speeds, and the "Counterblow Hammer", the largest device built. Combustion actuation provides a 6 to 7 fold intensification of the primary power source. For the same blow energy output, a combustion actuated hammer requires a basic powerpack which is less than V6th of that of any other, conventional or HERF, forming device. The smaller powerpack leads to substantially lower capital and maintenance costs and somewhat lower running costs. The automation of Petro-Forge, for hot and cold forming in single and multi-stations, fed by robots or special purpose feeding devices, is described. The designs of the automatic, adaptively controlled, Petro-Crop cropping machines are discussed. Utilizing a diameter measuring device and weighing/sorting system in conjunction with a micro-computer, these are capable of producing high quality billets with close weight tolerances from black bar. The control system of the Series F Petro-Forge and Petro-Crop machines is contrasted with that of the latest, Series G, models which are micro-computer controlled. These are capable of adapting themselves to changing operational conditions and they have built-in fault diagnostic capabilities to facilitate maintenance. Individual hammers can be controlled by a central processor and this allows a further deskilling of forming operations. Micro-computer controlled Petro-Forge/Petro-Crop machines represent a major step towards the computer integrated forming shop. The efficiency of forming machines is a product of the blow efficiency and the efficiency of energy transfer. The former is generally high, provided the machine is attached to a large foundation mass, or it is of the counterblow type or, in the case of HERF hammers, it is supported by a very low frequency suspension. The efficiency of energy transfer is related to the hardness of the blow. It can drop to low values and it is dependent on load arising when dies are allowed to clash. The effect of slight excess energy on the forming load is discussed and a method for its prediction is offered. A comparison of conventional machines and Petro-Forge hammers is offered by using the concept of the forming capacity; it is shown that the forming capacity of a Petro-Forge Mk.II machine corresponds to that of a 4-5 MN (400-500 tonf) screw press or a 4.5 MN (450 tonf) crank press. The survey is concluded by a discussion of the machine advantages/limitations of HERF machines in general and Petro-Forge in particular. High speed machines are small and compact devices; for the same forming capacity, high speed hammers are 1/gth or less in bulk and weight in relation to conventional forming machines, irrespective of their method of actuation. In view of their small, compact size they have a significantly lower capital cost than conventional equipment. High speed machines usually do not require special expensive foundations and therefore their installation cost is also very much lower. Petro-Forge provides the additional benefits of a very high cycling rate, accurate control of the blow energy and a very short dwell time leading to improved die life in hot forming operation.

I. INTRODUCTION IN THE 1950'S s e v e r a l f o r m i n g p r o c e s s e s w e r e d e v e l o p e d w h i c h h a d t h e c h a r a c t e r i s t i c f e a t u r e o f i n v o l v i n g t h e c o n v e r s i o n at a v e r y h i g h r a t e o f s o m e f o r m o f e n e r g y i n t o forming work--hence t h e s e p r o c e s s e s a r e k n o w n as H i g h E n e r g y R a t e F o r m i n g ( H E R F ) . T h e m o s t t y p i c a l o f t h e s e is e x p l o s i v e - f o r m i n g t (Fig. 1 . 1 ( a ) ) in w h i c h t h e e n e r g y o f a n e x p l o s i v e c h a r g e , t r i g g e r e d by a d e t o n a t o r , is t r a n s m i t t e d t h r o u g h a m e d i u m ( w a t e r o r s a n d ) to i m p i n g e o n a s h e e t / p l a t e o r t u b e , to a c c e l e r a t e this i n t o a d i e . E l e c t r i c a l e n e r g y s t o r e d in a c a p a c i t a n c e c a n b e u s e d in a n a n a l o g o u s m a n n e r ; by d i s c h a r g i n g it t h r o u g h a t h i n w i r e , o r e l e c t r o d e s s u b m e r s e d in a fluid m e d i u m , a h i g h * Professor of Mechanical Engineering and Head of Department, University of Birmingham, Great Britain. + A detailed exposition of this forming process can be found in References [1.1. 1.2, 1.3]. 105

106

S . A . TOBIAS WATER TANK

CHARGING RESI

EXPLOSIVE

\

[ /[LI

DISCHARGE SWITCH

-VACUUM

/r:::: BLANK

(a)

CHARGING SWITCH

DISCHARGINGSWITCH

(b)

!

COMPRESSEDGAS.

I

WORKPIECE

/MACHINE FRAME

DIES c

~MAGNETIC COIL

I //////~//////////,

(e)

(d) FIG. 1.1. Principle forms of HERF processes. (a) Explosive forming, (b) Electro-hydraulic forming, (c) Electro-magnetic forming, (d) HERF bulk forming.

intensity shock-wave is generated which can be utilized similarly, the process being called electro-hydraulic forming* (Fig. 1.1(b)). In magnetic forming* (Fig. 1.1(c)), electrical energy is used by discharging it through a magnetic coil. The magnetic field generated induces a current in a nearby conductor (the workpiece) and this produces a second magnetic field. The two magnetic fields repel each other and the workpiece is accelerated into a die. The three processes mentioned so far are suitable for the forming (and joining) of sheets, plates and tubes. For bulk forming, that is the forming of billets with dies, high speed hammers (Fig. 1.1(d)) were developed which used the energy contained in compressed gas (usually nitrogen or air), acting on a piston which accelerated a die to impinge another, to form a hot or cold billet. The first HERF hammer was developed by the Convair Division of the General Dynamics Corporation, and it was marketed under the name of Dynapak. The machine was launched with a great deal of publicity--as a "'revolutionary concept of metal forming"--and as a result, in rapid succession other HERF machines appeared in the USA--the Clearing Hermes (U.S.I.) Machine, the Verson Hammer, the Ken-O-Matic, etc. Almost all industrialized countries followed suit and produced their own models--the CEFF Machine (West German and USA), the Krupp Hammer (West German), the Hi-Fomac (Japanese)--as well as copies of the original Dynapak, built in British, Hungarian and Russian research establishments; all actuated with compressed air or nitrogen, having a maximum impact velocity of 18-25 m/sec (60-82 ft/sec) and primarily intended for single blow hot forging operations. It is important to appreciate that these feverish design and development activities and the costly large scale trials, were backed by little research if any. Research was carried out very much later, not by the manufacturers of HERF equipment or their industrial users, but by research establishments. These were aiming to substantiate the original claims, the most interesting of which was that materials show a hydrodynamic behaviour when deformed under high enough pressures at high enough velocities. *A detailed exposition of this forming process can be found in References [1.1, 1.2, 1.3].

Development of Petro-Forge Forming Machines

107

By the early 70's the initial largely unreasoned enthusiasm for H E R F had been replaced by largely unreasoned revulsion and the real potential and advantages of H E R F had been forgotten. By the end of the decade most of the H E R F devices had disappeared from industry; some had been mothballed, some donated to educational establishments and a few were broken up and sold for scrap. Only Dynapak and CEFF continued to be used by a very small number of industrial companies. These succeeded to build up, what appears to be, a highly lucrative business by carving out for themselves a segment of the forging component market in which they could compete successfully, this being the forging of special materials, such as stainless steel and that of special components, such as compressor blades. Petro-Forge, the original combustion actuated forming machine, hybrid of an internal combustion engine and a hammer, was a relative latecomer among H E R F devices. In fact, in 1964 when its first laboratory model was about to become operational, there were a large number of other H E R F hammers already in the field; experience with these was accumulating and it had already become obvious that some of the early exaggerated claims were untenable. However, Petro-Forge had some marked advantages over its established competitors which gave its development exceptional promise: higher cycling rate, shorter dwell time and greater versatility. Moreover, it was the only H E R F hammer which could claim that its development was supported by basic and applied research on a wide range of aspects of the machines themselves, as well as processes that can be carried out with them. Already during the initial stages, machine R & D was progressing in parallel with process R & D. In addition to establishing a basic design theory of Petro-Forge drive units, problems of machine dynamics and control, of forming efficiency, of mechanical handling of components and machine automation, were investigated in parallel with the characteristics of forming processes, such as hot/warm/cold forming, blanking/piercing and cropping, powder compaction/forging, rubber forming, etc., when performed at high speed. After machines had already been fully developed, in the last decade or so, R & D had been concerned mainly with the automation of these processes, with bar and tube cropping, with mechanical handling and robotic feeding, with the computer control of single and multi-station machines, computer aided fault diagnostic and the exploration of some peripheral problems, noise generated by impact forming machines in general and its suppression, the CAD and CAM of forming dies, etc. This survey gives an outline of the most important results achieved in these interrelated investigations. The subject matter falls quite naturally into two parts. The present first part deals with the machine design/development aspects, including automation; aiming to elucidate the machine advantages~limitations of H E R F hammers in general and of Petro-Forge in particular, arising as a result of high impact speeds. A second part [1.4] will consider the process advantages~disadvantages that can be attributed to high forming speeds. The bulk of the material presented will relate to Petro-Forge machines and processes, respectively. Other H E R F devices, (and process experience gained with these), will be covered only for providing the technological background, and for discussing the difficulties encountered by industrial users and ways and means found, or proposed, for overcoming these.

2. GENERAL CHARACTERISTICSOF HERF HAMMERS

2.1. Classification of forming machines Machines used for the bulk forming of materials, such as hot forming, can be divided into three groups (Lange [2.1]), as summarized in Fig. 2.1: (1) Load-restricted machines (hydraulic presses) in which the maximum forming load is determined by the maximum pressure delivered by the hydraulic powerpack and the diameter of the drive piston.

108

S.A. TomAs l 1

l

LOAD-RESTRICTED

In

STROKE-RESTRICTED

WORK-RESTRICTED

FORMING MACHINE GROUPS

~'///, v

VELOCITY FORMING STROKE CHARACTERISTIC

Vo'~ S

DIE IMPACT SPEED m/sec

HYDRAULIC PRESS 0.03-0.5

$

CRANK PRESS~ TOGGLE PRESS/ 0.03-1.5

$ FRICTION SCREW PRESS 0.03-1.5 DROP HAMMER t,.- 6 POWER HAMMER 5- 8 COUNTERRLOW HAMMER 6-14 HERF HAMMER 5-25 EXPLOSIVE FORMING 20-12C

FIG. 2.1. Classificationof forming machines and their important characteristics. Column l.: Load-restricted, Column lI.: Stroke-restricted, Column lII.: Work-restricted machines,

(2) Stroke-restricted machines (crank presses, toggle presses, etc.,) in which the stroke-time characteristic of the platen is determined by the kinematics of the drive mechanism. (3) Work-restricted machines (screw presses and hammers in general) in which the forming force is determined by the kinetic energy of the moving tup or platen/ram assembly at the time instance when the forming stroke commences and the length of the stroke over which this energy is absorbed by the forming process. Fig. 2.1 summarizes the most important features of these three groups of machines, the velocity-stroke characteristics and the ranges of their die impact speeds. Load-restricted machines (hydraulic presses) work with a forming velocity which is generally very low and can be varied only within narrow limits. In theory, the forming velocity is constant, as set, but in practice it is dependent on load. These machines cannot be overloaded since the maximum force they can exert is a design parameter. Thus, the maximum amount of deformation they can impart in, say, a hot forming operation is given by the machine specification (and particular process conditions); this deformation cannot be increased by a restriking of the component, beyond that obtained by the first application of the maximum load. Stroke-restricted machines (crank presses, etc.,) can be overloaded since when the driving crank passes through its bottom dead centre the thrust exerted reaches infinity, in theory. In practice, there is invariably providcd a load limiting device to prevent machine damage. The maximum amount of deformation that can be achieved is set by the stroke length; as in the previous case, a restriking of the component does not increase the deformation obtained in the first stroke cycle. The primary distinction between load- and stroke-restricted machines on the one hand and work-restricted devices on the other lies in the increasing maximum load which the latter can exert in successive blows; multiple blows result in additional deformation though of a diminishing magnitude, because of the drop of the forming efficiency, as will be explained in Section 7.4. The ultimate limit of the progressively increasing forming load is set by the elasticity of critical machine components, that of the screw spindle in friction screw presses or of the anvil/die/tup system in hammers. Quite in general, in

Development of Petro-Forge Forming Machines

109

relation to their size, work-restricted forming machines are capable of exerting higher maximum forming loads than other types of forming devices. The secondary distinction between work-restricted and the other two groups of forming machines lies in their very much higher forming velocity. Even with stroke-restricted machines, in that range of the stroke in which the forming operation takes place, just before the bottom dead centre is reached, the forming velocity approaches zero (see Fig. 2.1). This has important effects with some forming processes, which will be considered in [1.4]. The classification of forming machines discussed was originally proposed for conventional machines. Nevertheless it covers also HERF devices, these falling into the group of work-restricted machines. As a matter of fact, they represent an extension to the characteristic advantages of this category by virtue of their even higher specific work capacity. This feature is a consequence of the fact that the kinetic energy of a moving mass is proportional to the square of its velocity. Figure 2.1 shows that the maximum impact velocity of HERF hammers is about 3 times that of conventional drop hammers. This means that for the same tup mass, the kinetic energy stored in the tup of a HERF machine is about 9 times that of a slow speed hammer. Alternatively, for the same impact energy, a high speed machine will have about '/gth the tup mass of that of a slow speed device. As a result, high speed machines are very much smaller than slow speed machines, as will be shown later by reference to concrete examples (Section 9.1). It must be pointed out that the deformation velocities attained with HERF hammers are relatively low in comparison with those of, say, explosive forming, as is clear from the figures given in column III., third row of Fig. 2.1. 2.2. Classification of work-restricted machines The major types of conventional work-restricted forming machines and some of their characteristic features, including their particular advantages, are summarized in Fig. 2.2 With the exception of counterblow hammers, these machines require substantial foundations, partly because of their size and weight and partly because of the need for absorbing unbalanced forces. Drop hammers and single-acting power-assisted hammers are generally bolted to massive concrete blocks which are supported by a substantial layer of resilient material incorporating high internal damping, or by a system of springs and dampers. According to the German Standard DIN 4025 the mass ratio of the foundation block mass M t. to that of the tup mass mp should be Mf/mp = 80. When the

WORK-RESTRICTED OROP HAMMER

FORMING MACHINES FRICTION SCREW PRESS

COUNTERBLOW HAMMER

POWER-ASSISTED HAMMER

mt

MACHINE CONFIGURATION

b

/,

"~////////////, DIE IMPACT SPEED

~-6 m/see

IMPACT ENERGY

E= mgh=½mv 2

ADVANTAGES

SIMPLE, LOW COST,

LONGLIFE,

S-Sin/sec h E= mgh+ A,[p(h)dh =

o =½mY2

6-11.m/scc 1 E=2 Atp(h)dh =

1

0.03-1.5 m / s ¢ c E=½],w2

o =½1m,*mblv2

COMPACT, /HIGH FORMINGENERGY, HIGH PRECISION, HIGH FORMINGSPEED. /HIGH FORMINGSPEED, HIGH FORMINGENERGY.

HIGH FORMINGSPEED

FIG. 2.2. Classification of work-restricted forming machines and their important characteristics.

S . A . TOmAS

110

foundation block is supported by springs then the stiffness of these is designed to ensure a natural frequency of the suspension between 3 and 5 Hz. The damping is chosen so that oscillations excited by a blow are damped out before the next occurs. The reason for these expensive precautions is that drop hammers and single-acting power-assisted hammers are exposed to severe impact forces, which unless isolated are transmitted through the foundation over long distances to cause disturbances affecting either or both humans and machinery. There is a second reason for bolting these hammers onto massive foundation blocks, as will be explained in detail in Section 7.2. It will be seen that when a mass mp (tup mass) moving with velocity v~, and hence containing kinetic energy Eg = V2mpv 2, impacts on a stationary mass M r (anvil/frame/foundation mass) then the amount of energy available for conversion into forming work can be expressed as ~bE~, where the efficiency of the blow is given by 1

"qh = i + ~m ~vtfl;--t,/"'¢

(2.1)

Thus, with the aim of utilizing as much as possible of the kinetic energy the mass ratio

Mf/mz, should be as large as possible. It will also be shown that with the counterblow type of hammer, if the momentum of the two moving masses at the time of impact is identical, then "rlb = 1. Thus, with these machines the blow efficiency is not affected by the mass of the foundation block; the combined kinetic energy of the two masses is available for conversion into plastic work during the forming operation and no shocks are transmitted through the foundation. H E R F hammers are related to the conventional power-assisted or counterblow hammers, although they are designed so that the acceleration imparted by the driving (pneumatic, hydraulic, etc.) medium is very much greater than the gravitational acceleration. The three basic machine configurations that have been evolved, with a fourth which is essentially a variant, are summarized in Fig. 2.3. Figure 2.3(a) is a schematic diagram of a single-acting configuration in which the frame is firmly bolted to the foundation. This arrangement has been used only rarely, generally

; ; : r - ' l - ~ Z L k-'~

. , ~ . .v.,F i

t

~N

q×~


x",~

SINGLE

,,]

SINGLE-ACTING

DOUBLE-ACTING

COUNTERBLOW

COUNTERBLOW

~

x~" "~\~\\\\\\\\\\,,~,\\\\\\\x~ PARALLEL

SINGLE-ACTING

ACTIHG

(a)

(b)

(c)

COUNTERBLOW

(d)

FIG. 2.3. Machine configurations of HERF .:ammers. (a) Single-acting. (b) Single-acting countcrblow. (c) Counterblow (double-acting) with or without synchronization. (d) Single-acting counterblow with drive units coupled in parallel.

Development of Petro-Forge Forming Machines

111

with experimental machines or when the blow energy is low and hence only small shock loads are imparted to the foundation. The arrangement most frequently used is that of Fig. 2.3(b) in which the machine frame is supported on soft springs. This configuration is called single-acting counterblow; during the working stroke a force equal to that driving the ram downwards is acting on the machine frame and is lifting this up. If the natural frequency of the frame suspension is sufficiently low (see Section 7.2, Organ [7.1]) then at the time of impact the frame is moving in opposition to the ram, with nearly equal momentum though at a much lower velocity, in counterblow fashion. A variant of this configuration is shown in Fig. 2.3(d) in which two drive units are coupled in parallel. Finally, the counterblow type configuration (Fig. 2.3(c)) in which the frame is bolted to the foundation and two.drive units are acting in opposition. The upper and lower moving masses may be synchronized, though this is by no means necessary. The moving masses may be unequal provided they have appropriate velocities ensuring that their momentum is equal.

J-

1

R~

POWER GENERATOR

PLAT

-RELEASE MECHANISM TO, WORKPIE

RECOCKINO MECHANISM

•M A C H I N E

L.-

SUSPENSION

..J

FI6.3.1. Principal elements of HERF hammers.

3. DESIGN FEATURES OF HERF HAMMERS

It has already been mentioned that the characteristic feature of H E R F hammers is that energy stored in some convenient form is released/discharged at a high rate and converted into forming work. After completion of the working stroke the power unit is recocked, i.e., recharged, for another cycle to commence. On the basis of this general principle of operation, it is convenient to discuss the design of H E R F hammers by reference of the four principal elements/features required at the successive stages of the working cycle, as shown diagrammatically in Fig. 3.1: (a) The power (drive) unit in which the energy used in the forming process is either stored or

t12

S.A. TOBIAS

generated just before the commencement of the forming stroke. (b) The release mechanism which when activated permits the ram/platen/tool assembly to commence the power stroke, in the course of which the potential energy stored in the power unit is converted into kinetic energy. (c) The recocking mechanisms for returning the ram into its original position after its kinetic energy has been converted into forming work. (d) The machine suspension/support determining the foundation requirements, which are dependent on the overall configuration of the device, as explained in the previous Section. These are, of course, not the only essential elements of H E R F machines; an anvil and lower die, a workpiece ejector(s), a platen/tup guidance system, etc., are also required. In practical designs they may play an important part in the performance of the hammer, having at times a crucial effect on reliability. However, the four principal elements/ features listed above and summarized in Fig. 3.1 are of particular importance since they differentiate one design from another and they are characteristic of H E R F machines in comparison with conventional hammers. It is convenient to divide H E R F machines into families on the basis of the energy source used for actuation and the recocking mechanism. For instance, most machines are actuated by compressed air and recocked with hydraulic rams and this family of devices can therefore be described as "'pneumatic-hydraulic" H E R F machines. A variant of one of these designs uses a crank mechanism for recocking and will hence be called a "pneumatic-mechanical" device. There is a family of machines which can be classified as "hydraulic-hydraulic" in which the hammer is actuated hydraulically and is also re-cocked by such means. Petro-Forge, the machine which is the main topic of discussion of this paper, falls into the category of "'combustion actuated-pneumatic", as will be seen later. 4. PNEUMATIC-HYDRAULIC HERF MACHINES These machines are actuated pneumatically and recocked hydraulically; into this category fall the first machines offered commercially and they form by far the largest family of H E R F hammers. 4.1. The Dynapak machine* This was the first H E R F hammer brought onto the market; it has gone through extensive design development since its first appearance. The version discussed here is the one of which more than 80 models have been built for the USA and most industrialized countries and which inspired similar machines all over the world.t Dynapak was (a) actuated by compressed nitrogen, substituted later by compressed air for reasons of running costs. The energy contained in the compressed gas is (b) released by a pneumatic mechanism which is an integral part of the power unit. This (c) power unit is recocked by a pair of hydraulic jacks. The overall configuration of the machine falls into the category of (d) single-acting counterblow devices (Fig. 2.3(b)). The principle of operation of the power unit with the release mechanism is explained with the aid of Fig. 4.1. At the beginning of the working cycle, the ram with the mushroom-like enlargement at its top, is pushed upwards to make contact with the rubber seal contained in the end-plate of the drive cylinder. The drive cylinder is filled with air (or nitrogen) at pressures up to 138 bar (2000 psi). The small amount of gas contained within the circular rubber seal is allowed to vent to atmosphere. In the position shown in Fig. 4.1(a) the ram is pressed against the seal with force Po = p(A2 - A l ) , where p is the pressure in the drive cylinder, and A 2 and At are the areas contained

*Originally built and marketed by General Dynamics, Electro Dynamics Division, Avenal, N.J. All rights now belong to Precision Forge Company, Santa Monica, Cal., a subsidiary of Macrodyne Industries, Inc. tThe original Dynapak machine has been described by Carpenter [4.11 and a development of this by Feddersen [4.2]. The stage of machine development considered here is taken from Wang [4.3, 4.4, 4.5], Gallagher and Yoblin [4.6] and private communications.

Development of Petro-Forge Forming Machines

113

A?.. SEAL-~

VENT SPACE

VENT

q3,

+ /AIV///+

Jii~ HIGH PRESSURE CHAMBER ~ /

Po--

L:.:

!.'.4,;C...',;

:~::

~

pA I 2=iAI

PI = PAl

(a)

(b)

UNIT

R/~ GUIDINGS'IRIPS

.-

EJECTOR

HYDRAULICJACK~ Ill)AMCE

FORGED FRAME

AIR SPRINGS

FABRICATEDFRAME

(c) FIG. 4.1. Principle of operation of Dynapak power unit. (a) Recocking of ram. (b) Firing of ram. (c) Principle elements of Dynapak Model 1220D.

within the seal and the cross-section of the ram, respectively. T h e r a m is n o w " c o c k e d " , ready for firing. Firing is effected by injecting into the vent space a small quantity of high pressure gas which causes the seal to crack. With this the high pressure gas in the drive unit acts on the top of the r a m surface, generating a d o w n w a r d s force of m a g n i t u d e Pt --- P A1 (Fig.

114

S . A . TOBIAS

4. l(b)). This accelerates the ram/platen/tool assembly downwards until it impacts with the workpiece resting on the lower die. Upon completing the forming operation the ram/platen/die is lifted into its top position by two hydraulic rams (shown in Fig. 4. l(c)). As soon as the seal is operating, the gas contained in the vent space is exhausted into the atmosphere and with this the system is "recocked", ready for the next firing cycle. A diagrammatic drawing of the Dynapak Model 1220D is presented in Fig. 4. l(c). The drive unit, consisting of the integrated power generator and release mechanism, is attached to the top of a forged alloy-steel frame. This contains the ram/tup guiding strips and the two hydraulic jacks for recocking the ram. Hydraulic ejection cylinders are provided for ejecting the workpiece from the top or bottom dies. The forged frame is supported by two air,cushion springs (latterly ordinary coil springs), permitting the machine to operate in the manner of a single-acting counterblow device. The forged frame is guided in an outer fabricated frame. The complete working cycle of a Dynapak machine is presented in Fig. 4.2; it is controlled sequentially, there being no overlap between successive stages of the firing cycle. In principle, the firing cycle can be started with the recocking jacks already withdrawn to their bottom position; for safety reasons they are generally lowered as the first step. This increases the cycle time and it also has other disadvantages in hot forging, as will be explained in Section 4.4.

SAFE POSITION

READY POSITION

TRIGGERED

IMPACT POSITION

RECOCKING

(a)

(b)

(c)

(d)

(e)

FiG. 4.2. Operational cycle of Dynapak. (a) Safe position, hydraulic jacks up. (b) Ready for firing, hydraulic jacks down. (c) Triggered by injecting gas into vent space. (d) Forming operation completed. (e) Machine recocked with hydraulic jacks.

The essential specifications of the four most important Dynapak machines* are presented in Table 4.1. The smallest machine, Model CP-16, which had a maximum energy rating of 8.13 kJ (6,000 ft lbf), was intended specifically for powder compaction. It is by no means typical of the Dynapak range, being actuated by 7.0 bar (100 psi) air pressure. This machine and the Model 400, with a maximum rating o 17 kJ (12,500 ft lbf) were relative latecomers of the Dynapak range. The next two sizes, Model 620D and 1220D, with maximum energy rating of 54 kJ (40,000 ft lbf) and 305 kJ (225,000 ft lbf), respectively, were primarily intended for hot forging. An improved version of the last mentioned machine with a maximum rating of 407 kJ (300,000 ft lbf) is currently under construction. The Dynapak Model 620D and 1220D machines have been built, sold and used in far larger numbers than any of the other H E R F hammers, not only because they were the first in the market and were backed by far the greatest sales effort, but also because of their simple design and consequent high degree of reliability. The limitations of the machines, and in particular, difficulties encountered in their application under production conditions, will be discussed in Section 4.4. *In the literature, Dynapak and other similar designs discussed in this section are called "pneumaticmechanical" devices. The name has been changed to "pneumatic-hydraulic" because this is more descriptive.

Development of PetroiForge Forming Machines

115

E

,c

~-' ~'~

-:::

r-- r-i

~- ~:~--~

-- xe ~l ' T M

z z

~C ,,,k r,

r,

.,J

~.C

r."',

"r'4-

<

F<

r~- X

~

3C

""3"

u% X

""4"

t "l

~4 Z ~

~J

<

~

_

~x

Z ©

~1 X 0

z

=',~-

.<

~,,-;

~E

r*b

o~ X

n-

~J"

N

~q

-

6 N Z m o Z ©

X

~m

T-

E

c.

J~

"=

MTDR 25:2-B

~4

~,

"~

.~ .~

E

116

S.A. TOBIAS

4.2. The CEFF machine* The Controlled Energy Flow Forming Machine [4.8, 4.9, 4.10] is characterised (see Fig. 3.1) by (a) being actuated by compressed nitrogen, (b) the energy of which is released by mechanical latches, (c) recocking, i.e., the recompression of the expanded gas, is effected by hydraulic rams, and (d) the machine is mounted directly on a solid foundation, being of the counterblow (double acting) type (Fig. 2.3(c)) though with unequal moving masses. A cross-sectional diagram of the machine, showing also its principle of operation, is presented in Fig. 4.3. Referring to Fig. 4.3(a), the upper and lower drive units consist of the high pressure gas cylinders, containing compressed nitrogen, acting on the upper ram and lower bolster. At the beginning of the firing cycle, while the machine is awaiting activation (Fig. 4.3(a)), the ram and the bolster are held apart by a knuckle jaw latch system, contained in a frame (cradle) supported by hydraulic rams. There are four latches engaging the upper ram and when the machine is activated, the latches are unlocked by pressurized hydraulic fluid being injected into each of the latch cylinders. The latch system is self-locking for safety reasons.

IOP DRIVECYLINDER

EJECTOR CYLINDER I

RAN

LATCH

FR,abIE

\ BOTTCMDRIVECYLINDER/

\HYDRAULIC CYLINDER

(a)

(b)

(c)

F=G. 4.3. Operational cycle of CEFF. (a) Latches engaged, safe and ready position. (b) Forming operation completed. (c) Latches engaged and hydraulic rams energized, machine being recocked.

Upon completion of the forming blow (Fig. 4.3(b)) the hydraulic rams supporting the trigger carrying frame are lowered for the latches to engage. Ram and bolster are separated, the nitrogen in the drive cylinders is being recompressed and the machine is made ready for another cycle of operation by energizing the hydraulic rams. The recocking of the machine does not entail any gas losses. The completed workpiece is ejected by hydraulic ejectors contained in the upper ram and lower bolster. All moving masses are guided within a four-piece welded frame which is pre-tensioned by tie rods. The machine has two unusual features: Firstly, since the moving masses are unequal (a typical mass ratio being 1:4) the momentum of the upper ram and the lower bolster is balanced by a higher gas pressure in the bottom drive cylinder, the pressures in the two being independently adjustable. By such a differential pressure setting, it is ensured that

*Originally built by MaschinenfabrikWeingartenA.G., Weingarten,Wurtt. West-Germany:marketed in the USA by the CEFFEquipmentCorporation,El Cajon, Cal. Rightsnow belongto PrecisionMetalProducts, Inc.. E, Cajon, Cal., a subsidiaryof MacrodyneIndustries, Inc.

Development of Petro-Forge FormingMachines

117

no shocks are transmitted to the foundation. Secondly, as an option, on the bigger machines the stroke can be reduced by a limited amount to suit the tool height. For this purpose the bottom drive cylinder is supported by a hydraulic piston, as shown in Fig. 4.3, which can be raised by up to 100 mm (4 in.). Of the range of six CEFF machines offered only Type HE10, with maximum energy rating of 98 kJ (72,000 ft lbf), nominal cycle time 5 sec and Type HE55, with maximum energy rating of 542 kJ (400,000 ft lbf), nominal cycle time 8 sec, respectively, were built. The specification of these is also contained in Table 4.1. The total number of CEFF machines built was small, probably not more than 10 or so and most of these were of the Type HE55. Nevertheless, they have survived as production machines by finding a special niche of the component market in which they can compete successfully, this being the forging of large compressor blades and similar components. 4.3. Other H E R F machines Of the commercially available machines two further pneumatic-hydraulic/mechanical devices must be mentioned briefly. The first because it was sold in numbers second to Dynapak. The second because it was designed to overcome the main limitations of the Dynapak design. The "Clearing Hermes" or USI H.E.R. Machine* [4.11, 4.12, 4.13], in accordance with the classification explained in Fig. 3.1, was (a) compressed gas actuated with a (b) pneumatic release mechanism, in which (c) recocking was effected by hydraulic means. Thus, basically this design showed some similarities to Dynapak but there were important differences. The machine structure was fixed to the foundation and (d) was of the single-acting counterblow type, containing two drive cylinders coupled in parallel in the manner shown in Fig. 2.3(d). This had both advantages and disadvantages as will be seen later. It was built in 4 sizes, spanning a maximum blow energy range of 22 kJ (16,000 ft lbf) to 407 kJ (300,000 ft lbf). The nominal cycle time was between 6 and 12 sec. The essential specification of this series of machines is also given in Table 4.1. The Hi-Fomac Machinest [4.14, 4.15, 4.16] were a development of the Dynapak concept. They deserve special attention by virtue of their improved operating principle related to three aspects: (1) Reduction of gas lost from the vent space in the final stage of recocking (see Fig. 4.1(a)). (2) Reduction of "'dead time", i.e. the delay between initiating the firing sequence and actual firing, due to the time necessary for lowering of the recocking/safety rams (see Fig. 4.2(b)). (3) Reduction of "dwell and ejection times" at the end of forging cycle. Improvement (1) is important from the point of view of running costs, particularly when nitrogen is used as energy storing gas. The significance of improvements (2) and (3) will be appreciated in the light of the discussion of the limitations of pneumatic-hydraulic HERF machines, presented in the following section. They were implemented in two separate designs utilizing either an "oil hydraulic jacking (recocking) system" or a "mechanical jacking (recocking) system". The latter was a flywheel and crank mechanism, the machine being essentially a hybrid of a HERF device and a crank press. Hence, this machine falls into the category of pneumatic-mechanical devices. A range of 5 machines was offered but only 2 of these were built in small numbers. Their specification is presented in Table 4.1. Note that their cycle times were much shorter than those of other pneumatic-hydraulic HERF machines. All models had a maximum die closing speed of 16 m/sec (52 ft/sec). As far as their mechanical action is concerned, Hi-Fomac machines are clearly an improvement on the original Dynapak. They are more complex and may therefore be more liable to breakdowns. However, no information is available as to their performance under workshop or even laboratory conditions. *Marketed by U.S. Industries, Inc., Chicago, Ill. tMarketed by Kobe Steel Ltd., Fukiai-ku, Kobe, Japan.

118

S . A . TOBIAS

4.4. Limitations of pneumatic-hydraulic HERF machines' According to a well informed guestimate, the total investment in pneumatic-hydraulic H E R F machines in the U.S.A. alone was of the order of $150 million (in terms of the 1970-72 value of the dollar), involving the installation in industry of 120 machines or so. Little of this investment has paid off so far and it is therefore reasonable to ask two questions: Firstly, what went wrong with pneumatic-hydraulic H E R F machines'? Secondly, what is their future potential? The main reason for pneumatic-hydraulic H E R F hammers not coming up to the original commercial expectations was that the technology was oversold. With hindsight, even without overselling, they had only a limited chance to succeed: (a) They worked on a single "blow basis" with a long cycle time. This made high demands on the dies and it also resulted in a low rate of output. (b) Most machines were designed for large blow energies; die failure was consequently very costly and this discouraged experimentation and process development. (c) The major process development effort was devoted to hot forging of complex components; the new machines had to compete with the well established presses and hammers on jobs which were very difficult if not impossible to produce with reasonable die life. (d) They were often sold with inadequate technological back-up as to die design, process optimization and general usage, to companies with a traditional background, lacking time, manpower and capital to develop the necessary "knowhow". (e) Because of their principle of operation and design characteristics they suffered from excessive dwell time and die bounce, two very serious limitations. These general comments are not absolutely correct in all individual cases. For instance, both Dynapak and USI did offer models for relatively low blow energies. However, they came too late and were therefore not able to establish H E R F outside the hot forging area. Some work was done on cold forming and powder compaction, but cropping and powder forging did not receive any attention. The following discussion will elaborate on the general points made above, with particular reference to machine design and process experience. It will cover not only published data but also information collected in visits to users and through private communications. Die life will be dealt with only very briefly, a detailed discussion being held over for [1.4]. (a) Dynapak machines. From the design point of view, their main limitation was the shape of the single-piece forged steel frame, carrying the drive unit at the top (Fig. 4.1(c)). This was able to take up the maximum blow energy only at forming strokes longer than about 12 mm (1/2 inch). Hard blows, or at the extreme, a clashing of dies had to be avoided because of the danger of frame fracture and a number of frames did indeed do so. In the improved model currently under construction this frame consists of a number of individual pieces held together by pre-loaded tie bars. The single-piece forged frame was unsatisfactory also from the point of view of tup guidance. Gib strips guiding the tup were bolted to the sides of this frame and it was found very difficult to keep the bolts tight. Opinions differ as to whether these bolts were shaking loose because of inertia forces acting on their heads and the gib strips or because of the impact induced vibrations of the frame. Basically, the frame has an O shape and when impact loaded performs a ring-type oscillation. This reduced the clearance between the gib strips and the tup, tending to bend the former. The loosening of the gib strips can be most troublesome, so much so that machines are used without these, the tup guidance being effected over the final stage of the forming stroke by guide pins protruding from the lower die pot. This is not a satisfactory arrangement because such guide pins cannot correct any misalignment; they are likely to wear excessively. The design of the attachment of the gib strips has also been changed in the latest version. Keeping bolts tight has been a trouble encountered with all H E R F machines.

Development of Petro-Forge FormingMachines

119

Originally, tapped bolt holes in the tup and the anvil contained locking wire-inserts, partly to lock the bolts holding down the dies and prevent them from shaking loose and partly to safeguard the threads from damage. Experience showed that under production conditions such locking wire-inserts lost their effectiveness very quickly. Die holding bolts were often not tightened hard enough and once loose they caused a bouncing of the die pot on the anvil/tup, damaging mating surfaces. With the latest improved version die holding bolts have been dispensed with and the die pot is clamped by traditional wedges. Dynapak machines performed well under production conditions. Die bounce was troublesome with shallow workpieces, as it was with all pneumatic-hydraulic HERF machines, causing components to be ejected from the die and damaged by restriking. Initially, difficulties were experienced with the fracture of dies, due to inadequate die design and process procedures, the latter leading to excessive die loads. As more experience was accumulating these became rare. However, all along die life was a problem as far as the raajority of users were concerned. A few reported favourable experiences; one user found a very large improvement in die life and the man who has made Dynapak pay in the long run claimed: "once the techniques are developed for a particular part, dies life is better than with conventional forging equipment" (see comments summarized in [1.3] p. 152). Perhaps insufficient effort went into process development but it cannot be denied that by the end of the 60's users generally held that die life was poor in comparison with that achieved on conventional machines. This more than anything else caused a disillusionment with pneumatic-hydraulic HERF hammers. By the early 70's it became clear that poor die life in Dynapak (and all other pneumatic-hydraulic HERF machines) was due to excessive die heating. This caused a softening of the material which in due course produced excessive die erosion. Excessive die heating is one of the consequences of the operational mechanism of pneumatic-hydraulic HERF machines, in particular of the manner in which their drive unit is being recocked. Consider the Dynapak firing cycle, as summarized in Fig. 4.2. Heating of the bottom die starts as soon as the hot billet is inserted. Before the firing, button is pressed, the operator has to step back to avoid being in close proximity to dies impacting at high speed. "Operator movement" is unavoidable with HERF devices, but it is unnecessary with conventional machines, such as slow speed hammers or presses. Pressing the firing button, first causes the recocking hydraulic jacks to descend. This "dead time" takes up to about 1 sec and during this period the billet loses some further heat to the die. Similarly, at the end of the forming stroke, the tup and the anvil are separated by the action of the jacks and until they become operative the dies remain closed for a further 1-2 sec. During the "dwell time" the contact between workpiece and dies is intimate and hence heat loss and die heating are considerable. In fact, contact between the workpiece and the lower die is further extended by the "ejection time"; the dies separate only slowly in the recocking process, delaying ejection. Attempts have been made to reduce excessive die heating by speeding up the recocking process. Increasing the power of the hydraulic powerpack actuating recocking is not very cost effective. A preferable solution is to store its output during idling periods in an accumulator, to augment the flow when the jacks are operational. Alternatively, the recocking can be done mechanically and this led to the design of the Hi-Fomac machines already discussed. It is not known how far these attempts were successful. (b) CEFF machines. The striking feature of the CEFF design is in the use of mechanical latches for the release of the blow energy. At the energy levels at which such mechanical latches are supposed to work, trouble might be expected from the points of view of wear, alignment and synchronism of operation. The latch surfaces engaging the ram are exposed to very high surface loading, which is likely to lead to excessive wear. Moreover, it is bound to be difficult to ensure that the engaging surfaces of all four latches are aligned sufficiently accurately so that they carry comparable loads. This and other mechanical effects (friction in the latch releasing cylinders) may cause the latches to disengage at slightly different times, producing a

120

S.A. TOBIAS

misalignment of the ram at the beginning of the stroke; impossible to correct once the ram is accelerated and likely to cause wear in the ram guide bush (which does not have a favourable length/diameter ratio, as Fig. 4.3 indicates) and on the gib strips. CEFF machines have been used for the bonding of sheeets and good results were obtained even with highly dissimilar materials. In such operations blows of particular severity arise because of the very short stroke over which the energy has to be absorbed. Die loads of the order of those arising in die clashing are generated: the machines are designed to withstand these. With the CEFF machine, the blow energy for a given stroke can be varied in the ratio of only about 1 : 5. This is a rather limited range, The facility of varying the stroke length, a feature which no other pneumatic-hydraulic H E R F machine can offer, proved to be useful for some jobs. As far as excessive die heating is concerned and its effect on die life, the same considerations apply as have already been summarized in connection with Dynapak; CEFF machines cannot be expected to perform better than other H E R F designs in which hydraulic rams are used for recocking. Die bounce is likely to be troublesome with hard blows, as arising in coining~ but it does not seem to prevent the machines being used for the forging of large compressor blades. (c) USI machines. The main complaint against this design was lack of reliability. The principle of operation is clever but complex, too complex for reliable working during severe impact conditions: the parallel coupling of drive units doubles the number of components which can fail. It was claimed that servicing and/or repairs were necessary only too frequently. In addition, the machine suffered from severe platen bounce. However, there was no limitation as to the stroke length over which the energy could be used up. This is the only pneumatic-hydraulic H E R F hammer with which controlled comparative die life tests were carried out by producing three typical components on a USI machine, a drop hammer and a crank press. A large number of these were forged but nevertheless the conclusions drawn were only "'tentative". The results deduced from these [4.17, 4.18] will be discussed in detail in [1.4]; at this stage only some of the major points will be summarized. The dies of the three components behaved substantially the same way on the USI hammer and the conventional machines. On the whole, no significant difference in the rate of change of salient component dimensions was noted when changing from the slow speed to the high speed process. Exception to this arose in some cases, as for instance on a corner adjacent to the flash gap on one of the dies, which on the H E R F machine gave 2-3 times as much wear as when used on the conventional machine. However, generally die wear was not appreciably greater on the H E R F hammer. Die life was limited by surface cracks which appeared after fewer components on the USI machine: it was shorter with this device but the difference was by no means as large as was expected on the basis of the reputation of the process. The picture that emerged was complex and somewhat confused, particularly so since in the associated laboratory tests "wear on the 'Petro-Forge" machine dies was less than on the (crank) press dies, whatever the combination of forging stock and die". This was attributed to the much shorter dwell time of Petro-Forge. Clearly, this investigation was of crucial significance, (as a matter of fact, it was part of the initial phase of the Petro-Forge Development Programme, though carried out by the Drop Forging Research Association (D.F.R.A.), Sheffield); it did not arouse the interest it deserved and was not taken far enough. It proved that when using standard dies, i.e., dies designed for conventional equipment, die life with pneumatic-hydraulic H E R F hammers was shorter. It did not answer the claim that "once techniques are developed for a particular part, die life is better than with conventional forging equipment" [1.3, p. 152]. It confirmed the importance of dwell time and the need for closely repeatable blow energy. The picture that appears is twofold: Firstly, nowhere nearly enough R & D had been

Development of Petro-Forge FormingMachines

121

done in relation to die life in HERF. Secondly, excessively short die life seems to have been at least partly due to users trying to produce components of such complexity which were very difficult, if not impossible for conventional equipment. Not altogether surprisingly, such components proved too much even for HERF machines--they could be formed but only at a large expense of die life. (d) Hi-Fomac machines. There is nothing known about the performance of these machines under real or simulated production conditions. (e) Conclusions. By far the major part of the production experience gained with pneumatic-hydraulic HERF machines relates to hot forming and hence the following comments relate to that process. It is accepted that such devices can produce components which cannot be manufactured with conventional equipment, but usually only at the expense of die life. Components particularly suitable for pneumatic-hydraulic HERF fall into one of two categories: (A) Components, usually of a complex shape, made from materials needing a high rate of deformation in the course of forging for achieving the required material properties. (B) Components which have a very low heat capacity, because of thin, deep sections, which would cool rapidly when deformed slowly, preferably produced from materials with a low forging temperature. High forging speed is required with components of category (A) for achieving material structure and for category (B) for avoiding forging loads becoming too large. With the former die costs are of little or of secondary importance. With the latter, the avoidance of having to use very large conventional machines, of which there are few available, is a major advantage. Indeed, the two remaining major users of pneumatic-hydraulic HERF equipment have succeeded by specializing on these two types of components. Two riders ought to be added: Firstly, it has been shown that the high rate of die wear, caused by a long dwell time, can be reduced by the use of heat resistant die steel [4.19]; current users of HERF machines do not seem to make use of this possibility. Secondly, not all components with a low heat capacity are suitable for pneumatic-hydraulic HERF hammers. Flat shapes, even if twisted, i.e., compressor blades, are eminently suitable; ring shaped components, with zero draft angles, are not. The reason for this is that the latter when cooling tend to shrink onto the punch and therefore require very rapid tool retraction which these machines cannot provide. In addition, there is a wide range of components for which pneumatic-hydraulic HERF machines offer some advantages, which are however not as decisive as for those of categories (A) and (B): (C) Components, usually of cylindrical symmetry, which can be forged in one blow from either a billet or preform, requiring improved dimensional accuracy, high surface detail and/or surface finish, with reduced draft or machining allowances. These constitute a range in which pneumatic-hydraulic HERF machines and conventional equipment compete and on which the former have not been able to take a hold because of their inherent limitations. Thus, whereas in principle the range of applications of pneumatic-hydraulic HERF machines encompasses wide areas of the market of hot forged components, in practice they are in a strong competitive position only in relation to a narrow sector of this. This may well be the reason for the current lack of enthusiasm among forgers for such machines. However, this may not be the only reason and not even the main one. In the late 50's and the 60's these types of machines were marketed by machinery manufacturers, who because they were in competition with each other, and makers of conventional plant, tended to employ high-pressure salesmanship and thus oversell the process. The manufacturing rights of the remaining two pneumatic-hydraulic HERF machines (Dynapak and CEFF) now belong to the two main users of such equipment, both subsidiaries of the same holding company. Both companies seem to operate very successfully in rather special areas of the forging component market but they do not seem

122

S . A . TOBIAS

to exert themselves to sell their machines. The reasons behind the commercial policy pursued by these companies, and in particular the relative importance attached by them to their dual activities as machine makers and users, can only be guessed. However, it does not need much imagination to appreciate that at some stage they must have realized that they have two policy options which are mutually exclusive: They can exploit the technology and the know-how they have developed or they can sell the HERF equipment. They cannot do both because in the long run selling machines means inviting competition. An assessment of the size of the potential component market may well have suggested to them that not selling the equipment is the safer and/or more profitable commercial policy. Such considerations may well explain the absence of any current drive behind the sale of pneumatic-hydraulic H E R F machines. 5. COMBUSTION ACTUATED-PNEUMATIC HERF HAMMERS

The literature contains references to three combustion actuated high speed hammers: (1) Petro-Forge, (2) Repco High Energy Head, and (3) Russian hammers. The Petro-Forge development was initiated in late 1963. Successive stages of the work were published in a large number of papers and theses, both in relation to machine design/development and process R & D. The principle of combustion actuation was demonstrated in several design configurations and counting these and the successive re-designs, over 25 machines were built. Of these 8 were sold, including one machine (Mk.IIE) to the USSR in 1976. In 1971 Repco Research Pty Ltd., Australia, designed and built a combustion actuated hammer, allegedly on the basis of papers discussing Petro-Forge hammers. This machine was used for cropping 25-29 mm steel blanks used for cold extrusion purposes. Over 10~ components were produced and on the basis of the experience gained a "low cost patented Energy Head" [5.1, 5.2] was developed. This was more a test rig or laboratory model than a machine tool, intended primarily for the educational market. Hardly anything is known about Russian developments. [5.3] by the "'Chairman of the High Speed Forming Council", briefly summarizes the high velocity forging programme in progress in the USSR in 1981, (which was very extensive indeed), and refers to "'high speed forging hammers with a wide range of impact energies, based on the internalcombustion engine . . . . gun powder charge, pneumatic mechanical, etc., principles under design" [5.3]. 5.1. Petro-Forge machines* Petro-Forge [5.4, 5.5, 5.6, 5.7, 5.8, 5.9, 5.10] is essentially a hybrid of a hammer and an internal combustion engine which has the following principal features (Fig. 3.1): (a) Its power unit is actuated by high pressure gas generated by the combustion of a mixture of hydrocarbon fuel, generally propane or natural gas, and air. (b) The piston/ram assembly is released by a pneumatic device, automatically, just before the combustion pressure reaches its peak value. (c) Recocking, that is, the return of the piston/ram assembly, is usually effected automatically as soon as the working stroke is completed, by an air cushion acting on the underside of the piston (back-pressure): generally, the release and the recocking mechanisms are integrated with the power cylinder. (d) Both single-acting counterblow (Fig. 2.3(b)) and double-acting counterblow (Fig. 2.3(c)) models have been built but only the former have been fully developed and have been used for process R & D [1.4]. In its normal mode of operation, Petro-Forge is appropriately described as a "combustion actuated-pneumatic" device. However, it can also be actuated by high *The Petro-Forge System has been developed under a Ministry of Technology Contract in the Department of Mechanical Engineering, University of Birmingham. The development has received support also from SERC (process R & D), the Wolfson Foundation (Section 6.2) and industry [5.13]. Petro-Forge is marketed by Engineering Design Research Development Ltd., Birmingham. England.

Development of Petro-Forge Forming Machines COMBUSTIONCHAMBER

Pc

123

RESERVOIR

SEAL I

SACK PRESSURE

F~ J

I PLATEN" ~

I

I

Ca)

(b)

(c)

I..

~

I

Cd)

Fl~. 5.1. Petro-Forge release mechanism and back-pressure control. (a) Downwards stroking, (b) Upwards stroking, (c) Supercharged drive head, (d) Release of back-pressure.

pressure air only, and indeed provision is generally made for doing this with the aim of extending the range of blow energy to very low levels. In this second mode of operation, it can be categorized as a "pneumatic-pneumatic" H E R F hammer. The principle of operation of the power unit with the integrated release and recocking mechanisms is explained with the aid of Fig. 5. l(a). The unit consists of (1) a combustion chamber, (2) an expansion chamber/drive cylinder, (3) connected by a constriction known as seal. This seal is closed by a cylindrical plug projecting from the top of the piston. At the beginning of the cycle, the piston/ram assembly is lifted into its top position by the back-pressure acting on the annular underside of the piston. If the back-pressure is Ph then it generates an upwards force, acting on the piston, of magnitude Ph = pm4h where Ab is the area of the annular underface of the piston. The combustion chamber can now be filled up with a combustible fuel/air mixture to a charge pressure p,. which causes a force Pc = p,Ac to act on the seal/piston in the downwards direction, where A,. is the seal area. As long as Ph > Pc. the piston will not move. Somc of the fuel mixture or air, respectively, may leak from the combustion chamber through the seal, or from the back-pressure chamber across the piston, into the space on top of the piston. To prevent a pressure build-up there, which would result in premature firing, the so called vent space, is connected to the atmosphere (not shown in Fig. 5.1. see Fig. 5.3). Upon ignition of the fuel mixture the pressure in the combustion chamber increases n fold, where n depends on the fuel and the efficiency of the combustion process; for propane 7 < n < 8. Somewhat before the combustion pressure reaches its maximum level, the downwards force acting on the piston P,. overcomes the upwards force due to the back-pressure Ph; the piston starts to move and the seal is opened. With this the combustion products stream into the expansion chamber to act over the whole area of the piston, causing a force surge which accelerates the piston/ram assembly downwards to impinge on the workpiece and the lower die. During the downwards stroke of the piston the back-pressure Ph is intensified and as soon as the forming operation is completed this intensified back-pressure lifts the piston/ram assembly off the workpiece and returns it to its original position, the return stroke being facilitated by the exhaust of the combustion products. The return stroke of the piston/ram assembly is not necessarily effected by a back-pressure. By inverting the power unit and making it upwards stroking, as shown in Fig. 5.1(b), that function can be achieved by gravity, if necessary augmented by a back-pressure. This solution has been adopted in the "'Slow Speed Petro-Forge'" [5.10] to be discussed in Sub-Section 5.3 (Fig. 5.10). Furthermore, a substantial part of the energy lost by intensifying the back-pressure can be regained by bleeding some of it off through

124

S . A . TOBIAS

a non-return valve and storing it in a reservoir for use as charge pressure in the following cycle, as shown in Fig. 5.1(c), and incorporated in the "Supercharged Petro-Forge" [5.11]. The back-pressure can be exhausted to the atmosphere by reducing the cross-section of the ram over part of its length (see Fig. 5.1(d)). This arrangement requires separate recocking jacks; it is wasteful on air and increases the dwell and cycle times. However, when the back-pressure chamber is small, or when the energy loss incurred by the intensification of the back-pressure is undesirable, and the hydraulic power for recocking is already available, as was the case with the "Counterblow Petro-Forge" (see Sub-Section 5.4 and Fig. 5.13(c)), then it may offer advantages. 5.2. Petro-Forge Mk.l and Mk.ll machines Of these two models a total of 5 and 18 machines, respectively, were built. They reached their present stage of development after 6 cycles* of re-design, summarized in Fig. 5.2. The figure relates to the Mk.ll machine; the Mk.I series used the same control

REDESIGN CYCLE"B" TWO MACHINESBUILT AND USED SINCE 1957. IHPROVEMEMTSOVERMk.llk NOBEL:ENlARGED BACK - PRESSURE CHAMBER FOR INCREASED ENERGYOUTPUT,INCREASEDMASSANDR I G I D * ITY. IMPROVEDVIBRATION ISOLATORS.

_._|

v/Y///////////7//////~ ~////////.

i

I

i

i

"/2P///////////>2;/ "//X////V//,~//////~ ~.

REDESIGN CYCLF "A"

REDESIGN CYCLE "C"

THREEMACHINESBUILTAND USEDFROM1966 TO 1977. UPDATEDBY INTRODUCTIONOFNEW IMPROVEDFUEL SYSTEMAND VIBRATIONISOLATORS.

HAMHERCOMPLETELYREDESIGNED:INTEGRAL RAM-PlATEN ASSEMBLY, WATERCOOLING OF COMBUSTIONCHAMBER, IMPROVED LIQUID FUEL INJECTION SYSTEMAND CAMTIMER CONTROL.

"I

i

REDESIGNCYCLE "E" HAMMERREDESIGNED: NEWBASEAND VIBRA TION ISOLATIONDESIGN,ENlARGEDDIE SPACE ENCLOSEDWITHSOUNDABSORBING SHIELDS, SOLID STATECONTROL~VSTEM, IMPROVEDGAS C~'

REDESIGNCYCLE "D"

REDESIGN CYCLE "F"

CONTROLCONSOLREDESIGNED, GAS ACTUATION INTRODUCEO, IMPROVEDSPARKSYSTEM. TOTAL OF 8 MACHINESSOLD.

IMPROVEDCONTROLCONSOLGAS CONTROLFOR MORE ACCURATE BLOWENERGYCONTROL. IMPROVEDNOISE ISOLATING PANELS,SIMPLIFIEDCOMBUSTIONCMN4BER, SIMPLIFIED ASSEMBLY OF SUPPLY LINES GENERALFACELIFT,

FIo. 5.2. Development of Petro-Forgc Mk.ll Series A to U Machines.

*The cycles of re-design are denoted by the letters A to F.

Development of Petro-Forge Forming Machines INJECTION '

CHARGING/___(~_. _

®

,/,

WORKING

'°"'I'"ION

RETURN

125 STROKE

STROK E

OIL SUMP

(a)

(b)

(c)

(a) INJECTION At the beginning of the firing cycle the ram/piston assembly (A) is he/d at the top of i t • •txo~e by i o * premsuze a i r in the b a c ~ - l ~ m ~ chm~m-r (B). The aeal (C) clcmem the o o ~ t i n m chamber porting. ~ne e ~ t valve

(D) as ~

M che ".,ent '.~J.'~e (V) are o~n and the pm~=uce in the o~tion d ~ (E} i= at]Km~t~ric. iocemsln~ the firin~ button the fuel injection @aas~ starts; the ~ust vale- (D) is cloee~ and the gaseous fuel is ac~itt~ o~tion ch~ (E) via the gas v a l ~ (F).

(d) ( c ) WORKING STROKE

into the

( b ) CHARGING ~fter clcaing the gas valve {F) the combustion d ~ r is charged by admitting osmpressed air through the ip/et %~ive (G). As ~ as charging is c ~ l e t e d , the inlet val%~ (G) is closed and the ~ r / ~ mixture is ignited by the spark plug (H}. This results in a s e ~ to eight fold r i ~ of the ~essuze in the c ~ b ~ s t i ~ ~ r . The v~mt val~e (v) is closed igdition takes p l y .

AS l~on ~ the fozoe d~e to the "~l-~tl~n ~ act/n~ on ~ mmmll area (I) at the top of the ~ (C) i~ sufflcismtly laz~e to o w m ~ e th~ ~ % ~ e d ~o~oa d~e to t/~ Ira* b a c k - ~ t ~ m = e in che ~ (S), a ~ u ~ on ~ annular l o ~ r face ¢~ the piston (&), the l a t ~ star~ ~ n~. ~s • r~l~It the porting b e ~ the ~ - t l o n ~ (E) end che ~ l ~ o n c~llnder is oiwmed and ~ t~t/~ ~ are pel~t~ell to ~ ~i~ to act ~ the Whole piston area. ~ qemez~te~ ~ larva fo~oe ~ttrge which accelerates the pis~m/ram assembly dmcma~Is to impinge on the workoiece.

(d) RETURN STROKE Duzin9 the working s t ~ e the baak-~emaure in mace (B} is intensified and conseq~ntly acts ~ a return sp~ing am s ~ n M the forming ol~rati~ is completed, thus rapidly s e d a t i n g the dies. ~im ~ u r n of the ram/pist~ ~sembly to its initial position is completed by the o ~ n i u g of the ea(ha~t valve (D) which [~rmi~l the ~ t l o n products to leave through the duct {J). Finally, the ve~t valve {V) is ~pened and as socm as the seal (C) is closed the machine is r ~ d y for ~otber cycle.

FIG. 5.3. Operational cycle of Petro-Forge drive unit using gaseous fuel. (a) Injection of gas charge. (b) Charging up with air. (c) Workingstroke, combustionproducts expanding. (d) Return stroke with the aid of back-pressure and exhaust of combustion product.

console but a smaller, scaled down, hammer. Note that series A to C were liquid fuel actuated and that gas actuation was introduced with series D. Current development of these models is concerned with their micro-computer control and diagnostic and these aspects will be discussed in Section 6.5. That section will also describe the pneumatic control circuit of the Mk.IIE and Mk.IIF machines, contained in their control console. The Mk.I and Mk.II are downwards stroking machines in which the drive unit is situated on the top of the hammer. Figure 5.3 describes the cycle of operation of the drive unit with gaseous fuel; the figure is self-explanatory. Figure 5.4 is a photograph of the Mk.IIE model. Note that its die area is enclosed by sound isolating/absorbing panels and that access to it is gained through a front (and/or a rear) trap door(s), interlocked with the firing cycle, as will be explained in greater detail later. As far as outside appearance is concerned, the latest Mk.IIF model does not differ greatly from the Mk.IIE model, though it does contain some significant design improvements. With the series F machines the hammer design has been frozen and hence the following discussion will be concerned with that model. (a) Designfeatures. Figure 5.5(a) shows the general appearance of the machine. The control console and the hammer are connected by a flexible duct containing pipes carrying service requirements; the gas and air charge, the pneumatic signals for actuating inlet/exhaust valves and the ignition voltage. The drive unit is supported on four pillars, which also serve for platen guidance. These pillars extend right through the anvil, the cast iron stands and the stand base, pre-stressing the former. The whole hammer is supported on four vibration isolators and two air bellows to ensure a suspension of very low natural frequency (see Section 7.2). There are also two guide pins for providing vertical alignment. Figure 5.5(b) gives a quarter section through the hammer. Detailed design features of the drive unit, that is, the combustion head, the drive cylinder and the return buffers, are presented in Figs. 5.6 to 5.8, respectively.

126

S.A. TomAs

FIG. 5.4. Photograph of Petro-Forge Mk.IIE.

It was seen in Fig. 5.3 that the combustion chamber (E) contains poppet valves for charging it up with gas (gas valve F), with air (inlet valve G) and for exhausting the burnt mixture (exhaust valve D). In early models there were separate valves for these functions and also a vent valve to prevent pressure build-up above the piston, All these valves were air operated, receiving their actuating signal from solenoid valves contained in the control console. In the current design (F series) there are only two air operated poppet valves, as can be seen from Fig. 5.6, the inlet and the exhaust valves. The operation of both valves is augmented by springs to ensure that the inlet valve is closed and the exhaust valve is open should their air supply fail: the system is fail-safe. The inlet valve is used for both gas and air charging, as will be seen from Fig. 6.23(a) which shows the pneumatic/gas circuit producing the fuel/air mixture and actuating the poppet valves, The combustion head (Fig. 5.6) contains also a small shuttle valve for venting (vent valve) which is kept open by a spring during the charging process, when it bleeds off into the exhaust duct any fuel/air mixture leakage: it is closed by air pressure during the combustion and expansion parts of the cycle. A cross-section through the drive unit is presented in Fig. 5.7. The cast iron drive cylinder unit consists of two concentric cylinders attached to a flange. The inner cylinder contains a cast iron liner surrounded by a water .jacket. Cooling water enters through an inlet port in the flange of the drive cylinder casting and, after rising, flows into channels

127

Development of Petro-Forge Forming Machines

GUIOE

BAS

(a)

(b)

(c)

FIG. 5.5. Construction of Petro-Forge Mk.IIF machine. (a) Control console and hammer but without sound isolating/absorbing panels enclosing die area. (b) Segmental section of hammer, showing construction of drive unit. latches, standard die bolster and anvil/stand with ejection cyclinder. (c) Fully enclosed die area and front/rear flap door for workpiece feeding/removing.

SPARK . . . . .

~LATE

COOLING CI

N CHAMBER

INLET V~

VALVE

PIST(

VALVE

CYLINDER

~L

WATER J~

YLINDER

FIG. 5.6. Cross-section of combustion head. encircling the combustion chamber (see Fig. 5.6), to prevent overheating of the inlet and exhaust valves, leaving through an outlet port in the cover plate. The drive cylinder casting is pre-compressed by 4 (Mk. I model) or 6 (Mk. II model) tie rods (Fig. 5.7), which pass through the space between the outer and inner cylinder walls. That space is in fact part of the back-pressure chamber, being connected with passages to the space below the piston, as can be seen in the schematic diagram Fig. 5.3. The connecting passages serve as an oil sump, the oil contained therein lubricating and

128

S.A. TOBIAS

CYL

ACKET

RAI

ROD

BA( HEAD

INLI

8UMP

PL~

&RING

HYDHfiuLI~

Durrcn

rt.~l~N

uulul:: BUSH

FI~;. 5.7. Cross-sectionof drive unit.

splashing/cleaning the inner cylinder wall as the air is pushed outwards during the working stroke and reverses while the piston/ram assembly is being recocked. In spite of the large volume of the back-pressure chamber, during the working stroke the back-pressure is considerably intensified. This has the effect of returning the platen/ram/piston assembly at a considerable speed. To avoid a severe impacting of the piston on its seal, in the Mk.II model, the final stage of the platen return stroke is cushioned by two hydraulic buffers, contained in the cross head which supports the drive unit. The position of the buffers is shown in Fig. 5.5(b) and their operation is explained with the aid of Fig. 5.8. Each buffer consists of a poppet valve, closing a port to the oil sump in the back-pressure chamber, and a hydraulic piston. These two elements are pushed apart by a spring which is fully compressed when the platen is in its top position, as shown in Fig. 5.8(a). During the forming stroke (Fig. 5.8(b)) the poppet valve and the piston are separated by the action of the spring and at the same time both are pushed downward by the action of the back-pressure; the space between the two elements being filled out with oil from the back-pressure chamber. During the last 15-20 mm of the platen return stroke (Fig. 5.8(c)) the poppet valve closes the direct connection to the oil sump and as a result the piston squeezes the oil through a number of holes, past an adjustable needle throttle, via a bypass, back into the sump. The cushioning desirable can be set with the throttle. Figure 5.5(b) also shows that the drive unit and the anvil are connected by two bridges containing pneumatically operated latches. These serve a safety function only, preventing the piston/ram/platen assembly from sinking down should there be a leak in the back-pressure chamber. They are withdrawn as the first stage of the firing cycle, as will

Development of Petro-Forge Forming Machines

......

....

io

129

WATERINiET PORT

WATER JXgKE~

/

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

/

/

¢ ..........................

[jT,

2

PLATEN/RAM/PISTON ^SSEUBLY (a)

(b)

(c)

Fi6. 5.8. Operation of return stroke buffer. (a) Platen in starting position. (b) Platen in downwards stroke, buffer being recharged. (c) Platen in return stroke, buffer operational.

be explained in Section 6.5. Finally, workpiece ejection is accomplished by a hydraulic jack, located between the stands, bolted to the stand base. Figure 5.5(c) shows the front (and/or rear) door of the die space, constructed of sound absorbing/isolating material and containing the pneumatically actuated flap door. This is automatically closed at the beginning of the firing cycle, simultaneously with the withdrawal of the latches. It re-opens at the end of the firing cycle, as soon as the platen has returned to its top position, to facilitate the removal of the completed component and the feeding of a new billet. The front (and/or rear) door can be rapidly opened by a pair of toggle clamps, to facilitate fast die changing. A compressor providing the pneumatic power supply, a propane gas bottle and a hydraulic powerpack actuating the workpiece ejector, are external to the machine and are not shown. (b) Performance characteristics. The essential specification of the Petro-Forge Mk.I and Mk.II machines, series F (or earlier D and E) is given in Table 4.1. Note that their energy ratings are specified by two sets of figures: (i) Nominal maximum performance given as 6.8 kJ (5,000 ft lbf) and 13.6 kJ (10,000 ft lbf), respectively, achieved in conjunction with a 7.0 bar (100 psi) compressor, at an impact speed of 15 m/sec (50 ft/sec). (ii) Ultimate blow energy specified as 13.6 kJ (10,000 ft lbf) and 27.2 kJ (20,000 ft lbf), respectively, is achieved in conjunction with a 12.5 bar (180 psi) compressor, at an impact speed of 22 m/sec (72 ft/sec). Particularly hard blows, such as arising in coining or cropping, should not be exerted in continuous running at energies above the nominal maximum output, as explained in greater detail on Section 8.0. The blow energy can be adjusted down to l/sth of the nominal maximum (l/16th of the ultimate performance) when operating the machine with combustion actuation and to even lower levels by switching to compressed air actuation. The performance characteristics of a Petro-Forge machine can be presented in the form of a calibration chart, of the type shown in Fig. 5.9 for Mk.IIE and Mk.IIF hammers. This gives the blow energy Eg kJ as a function of the platen stroke s mm, for a series of values of the charge pressure Pc bar. Figure 5.9(a) corresponds to the case when the nominal back-pressure, set on the appropriate gauge on the control panel (see Section 6.5, Figure 6.22(a)), is equal to the minimum back-pressure Ph,,,m. This

130

S. A, TOBIAS

CHARGE PRESSURE Pc bar gauge I

S

/ 2~

CHARGE PRESSURE Pc bar gauge 2a

I

_11 2¢

"~"

x

L~J" 20

Z

-12

./ .,0

i

-7 ~.j

-6

12

o ~ cs~

.

~

-~

16

~

/

~

-8

12

3o

~

-s

8 .......---

50

100 STROKE

--2

150

200 -

s

250

300

4

0

mm

50

fO0 STROKE

Ca)

150

200 -

s

250

300

mm

(b)

FI(~. 5.9. Theoretical calibration charts of Petro-Forge Mk.IIF. (a) Normal back-pressure, set automatically, (b) Normal back-pressure plus 2.0 bar, excess being set manually. For a bounce-free operation working point must lie to the right of the optimum energy-stroke line (chain dotted).

back-pressure is just sufficient to balance the weight of the piston/ram/platen/tool assembly and lift it into its top position. The actual back-pressure acting on the piston is in fact larger by an amount dependent on the charge pressure p, set on the control panel, in accordance with the blow energy required. The actual back-pressure is automatically set by the control system to be p , , = pt,,,,i,, +

(p/5)

(5.1)

Note that in Fig. 5.9(a), for each value of the charge pressure p,, the blow energy increases with increasing stroke and after reaching a maximum value drops again. The maxima of all the curves are connected by a chain dotted line which represents the optimum combination of blow energy and stroke length. This curve corresponds to the condition when at the end of the stroke the forces acting on the two sides of the driving piston, due to expanded gases and the intensified back-pressure, are in equilibrium. Bounce-free operation (required for coining or when forging shallow workpieces) and a particularly short dwell time, of the order of milliseconds, are achieved when the effective stroke length is greater than the optimum stroke length, i.e. when the working point is to the right of the chain dotted line in Fig. 5.9(a). As already stated, Fig. 5.9(a) corresponds to the nominal back-pressure setting which is Ph,,i, = 0.75 bar (10.9 psi). Figure 5.9(b) shows the calibration chart of the machine when the back-pressure (set on the back-pressure gauge) is increased to Ph,,m = 2.0 bar (29 psi). Note that the curve of the optimum blow energy versus stroke length has shifted to the left and the area of working conditions ensuring bounce-free operation has been substantially increased; as long as the stroke is larger than s = 275 mm the machine will work bounce-free at all energy levels. However, this has been achieved at the expense of lowering somewhat the maximum blow energy for each charge pressure. The nominal back-pressure can be raised beyond Prom,, = 2.0 bar (29 psi) to ensure bounce-free working at very short stroke length at small energy levels, which is a useful feature of the machine for particular applications.

Development of Petro-Forge Forming Machines

131

The calibration charts can be found experimentally or theoretically [5.6], with good correspondence between the two. There are now simple and sophisticated [6.26] programs available for the CAD of drive units. A detailed description of the control panel and of the operation of the control console of the Mk.I and Mk.II machines is given in Section 6.5. This deals with the electronic timer control system used in D to F series of designs and it also gives an outline of the principle of operation of the micro-computer control which is being installed in existing machines. The Petro-Forge Mk.I and Mk.II, series E and F, machines differ from other H E R F hammers in a number'of respects, as ought to be clear by now. One of these, not as yet mentioned, is their versatility; they are general purpose hammers which can be used for a wide range of forming processes, hot/warm/cold forming, powder compacting/forging, cropping, blanking and piercing, etc., to be discussed in detail in [1.4]. The Mk.IF machine, or its drive unit, form a constituent part of the General and Special Purpose Petro-Crop machine which is described in Section 6.4. 5.3. Slow-Speed Petro-Forge The Petro-Forge principle, that is, the use of combustion energy for the actuation of a hammer, can be applied also for machines working at relatively low speeds. The die closing speed is reduced simply by increasing the moving mass. The Slow-Speed Petro-Forge was built to demonstrate the versatility of combustion actuation and provide a facility which spans the gap between low speed conventional equipment and the high-speed Petro-Forge machines, required for research on the effect of forming speed on process characteristics. It was originally liquid fuel actuated as all Petro-Forge hammers were at that stage of development. Its operating cycle did not differ significantly from that of current gas actuated devices described in Fig. 5.2, the difference being solely a reversal of the order of fuel injection and air charging, as explained in [5.10]. As far as machine development was concerned, it was used to investigate the performance of a range of alternative fuels, from pure petrol (gasolene), to mixtures of petrol with gas oil (diesel oil), to pure paraffin. It performed successfully with all of these. The Slow-Speed Petro-Forge [5.10] is an up-wards stroking machine, operating in the manner shown in Fig. 5.1(b). It was designed in 1967, roughly at the same time as the Petro-Forge Mk.IIC and it is therefore convenient to compare its performance with that of that machine. (a) Design features. The construction of the Slow-Speed hammer is shown in Fig. 5.10. The piston is attached to the hollow ram which contains the lower ejection cylinder. The anvil, which houses the upper ejection cylinder, is supported on four pillars which guide the massive platen. For facilitating die alignment, the platen can be lifted by two hydraulic jacks contained in the fabricated box structure, pre-stressed by a series of tie rods. This is bolted to the base, supported by vibration isolators. These provide a natural frequency of the machine in the vertical direction of the order of 3.0 Hz, which is sufficiently low to prevent shock loads generated during forging to be transmitted into the foundation. (b) Performance characteristics. The calibration chart of the machine, showing the blow energy output Ei, as a function of the stroke length s, for a series of values of the charge pressure and the appropriate back-pressure, is presented in Fig. 5.1 l(a). As with the chart relating to the Mk.IIF machine (Fig. 5,9(a)), the optimum working conditions are along the chain dotted line connecting all the maxima of individaal curves for constant charge pressures. Note that up to a charge pressure of pc = 7.3 bar (106 psi) the back-pressure was set to Ph = 0 bar (atmospheric). One of the advantages of the design configuration used is that it obviates the need for a back-pressure; that function can be performed by gravity. However, when back-pressure is applied it can be used for regulating the blow energy within wide limits, as can be seen from Fig. 5.11(b). This shows the blow energy for a

MTDR 25:2-C

132

S . A . TOBIAS

BOT1

COMBUS1

VII IS(

(a)

(b)

FIG. 5.10. C r o s s - s e c t i o n and side view of S l o w - S p e e d P e t r o - F o r g e . (a) Cross-section. (b) Side view.

CHARGE PRESSURE PC bar gauge

BACK-PRESSURE Pb bar" gauge

4o

l/

0/

1

\~

o L t ° - - Y ' ° ~ I"-1°

f

CHARGE PRESSURE Pb bar gauge

25

1.4

e71 .~ °~" w....o..-.--

1.0

I-~

/ ,,o

.o/t/

30

o

/ / o~o

, .o/I/

0 _J El

o

='--8 7

. - ~- --I-~O .

i

o

I

•

i~-sJ)

//9"1"'" o~ ._o-~,_ o...,._., 3

IJJ >-I IZ LU Z UJ

o

/,

Z0

if

Pl

I

"

0.7

h

0.5

.3C t~

o

-/i_o.~ -o-/~--od //,o-oZL. !'-~.~ /o Io~o I I °~ L .o/I o o/.I I-'" .o-M / t°\! o 0

......o....--o-- e"--o .....6 I o ' t "''o~

Io

I"--52 I "

,,.oI -..~o/~ "--.o"~'o ~ o ~ l o1~--&.5 .,,.o -o---~"~o~

~

I ~"o~

I ~-3.8

I >-

20

15

W Z U.I

,.,--O--.d

,__o.~. \o\-O., ' --0.7

~Z

10

~ ~

q 50

-

o o

0

75

i 125

175

i 225

~v,, Z5 2"/5

--I.0 _l.t.

m o

0 (OPEN}

1.7

--2.0 0

o

75

S T R O K E - s mm

125

17S

STROKE-

(a)

225

2"/5

s mm

(b)

Fro. 5 . t L Experimental calibration charts of Slow-Speed Petro-Forge. (a) Calibration chart for optimum back-pressure values. (b) Effect of back-pressure on blow energy at constant charge pressure p , = 5.85 bar gauge.

Development of Petro-Forge Forming Machines

133

TABLF 5.1. COMPARISONOF SLow-SPEED PETRO-FORGE AND PETRo-FORGE Mk.IIC Slow-Speed Petro-Forge Weights and dimensions Weight of moving mass [kg (lb)] Total machine weight [kg (Ib)]

127l

(2.80/))

127

(280)

11,690

(25,750)

3,314

(7,300)

Moving mass/stationary mass Die area [mm x mm (in x in)] Maximum dimensions [mm x m m x mm (in x i n xin)] Performance--Nominal maximum: Max. impact speed [m-see (ft/sec)] Blow energy [kJ (ft Ibf)] Perff~rmance--Ultimate: Max. impact speed [m/see (ft/sec)] Blow energy [kJ (ft Ibf)] Cycle time [scc]

Petro-Forge Mk.IIC

I/~.2

711 x4(16

1/25

(28x 16)

3048 x 1828 x 1219 (120 x 72 x48) with a 7.() bar 5.6 20

7.9

(12x 12)

254[) x 11893 x 965 (100x47 x 381 (100 psi) compressor

(18.4)

14.6

(48)

(14,7110)

13.6

(10,000)

with a 12.5 bar

411.[)

305 x 305

(180 psi) compressor,

(26.0)

23.1

1711)

(29,600)

34.11

125,0011)

1.0

1.0

charge pressure of pc = 5.85 bar (100 psi) (and constant fuel/air ratio) as a function of the stroke, for a series of values of the back-pressure. The top curve corresponds to the case when the back-pressure chamber was connected to the atmosphere and hence no appreciable intensification takes place. By increasing the back-pressure the blow energy can be reduced to quite low levels, particularly at long strokes. For instance for s = 280 mm, the adjustability of the blow energy was greater than 1 : 16. The essential characteristics of the machine are presented, and compared to those of the Petro-Forge Mk.IIC, in Table 5.1. The moving weight was ten times that of the Mk.IIC model, which resulted in a maximum impact speed of about l/srd of that of the high speed device, at a comparable energy level. Its platen area was more than twice of that of the high speed machine. The bulk of the machine and total weight were kept relatively low by reducing the ratio of the moving mass/stationary mass from 1/2sto I/8.2. The result of this is that during the working stroke the Slow-Speed Petro-Forge lifts about 3 times as much as the Petro-Forge Mk.IIC, which is probably unacceptable in a production version. Table 5.1 also contains the performance specification of the two machines in terms of the nominal maximum and ultimate blow energy and corresponding impact velocities. The cycle time for both is 1.0 sec. As far as the machine design aspect is concerned, the Slow-Speed Petro-Forge represents an obsolete stage of the technology. Nevertheless, it does demonstrate dramatically the essential difference between slow and high speed machines. The reduction of the maximum impact velocity was achieved by a 3-fold increase of the machine mass, approximately at about 3-fold the capital cost. In an improved design the machine base mass would almost certainly have to be increased quite substantially and this would further increase the capital and installation costs. However, the machine does have one attractive feature and that is that it has a substantially enlarged die area which greatly extends its range of application. The machine has recently been converted to gaseous fuel actuation under microcomputer control (see Section 6.5). A perspective drawing of it in its latest form is presented in Fig. 5.12. Note that to facilitate the feeding of billets, and avoid the sinking of the hammer into a pit, the control console (and the operator) has (have) been placed

134

S . A . TOBIAS TOP DIE

GUIDE PILLAR

.J

BOTTOM DIE

PLATEN MICROCOMPUTER

,FURNACE CONTROL CONSOLI

CAJARD

OPERATOR PLATFORM

BASE

FIG. 5.12. Perspective drawing of Slow-Speed Petro-Forge after conversion to gas actuation with computer control.

on an elevated platform. Between this and the machine, there is a pneumatically operated guard panel, safeguarding the operator. The introduction of the guard panel increased the total cycle time to 2.5 sec. The blow energy output and consequently the calibration chart is not significantly different from that shown in Fig. 5.11. The principle of operation of the micro-computer controlled system is discussed in Sub-Section 6.5. 5.4. Counterblow Petro-Forge [5.12] This machine was designed and constructed with the aim of demonstrating that the Petro-Forge principle is also applicable for very large blow energies. It was originally (a) compressed air actuated with two identical drive cylinders synchronously coupled with a multi-mode hydraulic "closed chain" mechanism. The design of the drive cylinders was similar to that of the Dynapak drive units and hence (b) the energy was released by the pneumatic mechanism shown in Fig. 4.1. (c) Recocking was affected by the platen synchronization mechanism operating in its second mode, the third and fourth being the inching and locking of the platens. (d) The machine was mounted directly on the foundation, as per Fig. 2.3, being a perfectly balanced counterblow device with equal moving masses. (a) Designfeatures. The front view of the hammer, with a section, is presented in Fig. 5.13(a). The machine consisted of two drive units linked together by two round bars which also served as guide pillars of the platens. The guide pillars were attached to the forged outer casing of the drive units with the aid of taper-lock bushes. This whole assembly was supported by fabricated stands each of which contained on the outside the multi-function hydraulic platen synchronization mechanism already mentioned. The position of this mechanism is shown in Fig. 5.13(b). It is essentially a hydraulic

Development of Petro-Forge Forming Machines

135

fSTA.O J

q

////////,4, , ,///////// (a)

(b)

F16.5.13. Construction of Counterblow Petro-Forge. (a) Front view and cross-sectionof machine--air driven version. (b) Side view of machine, showing position of hydraulic synchronizingmechanism. (c) Front view and cross-section of machine after conversion to liquid fuel actuation. "'chain" or closed loop, consisting of oil columns and rams bearing onto both platens and thus ensuring that the movement of one is transmitted to the others. A detailed explanation of its operation is given in [5.12]. Hydraulic ejectors contained in the top and bottom platens were supplied by telescopic pipes connecting the drive units and the platens. At a later stage the machine was converted to liquid-fuel combustion actuation by the introduction of combustion chambers and appropriate pistons with seals, as can be seen in Fig. 5.13(c). These modifications left the drive units with a very small back-pressure chamber which caused large losses. To avoid these, the back-pressure was allowed to exhaust during each stroke through recesses milled into the ram, in the manner explained with the aid of Fig. 5.1(d). The subsequent recocking of the platen was affected hydraulically. As already mentioned, this is not a satisfactory arrangement because of the increased air consumption and the extended dwell and recocking times. However, without a major redesign no other solution was possible. The major dimensions and general appearance of the machine in its liquid-fuel actuated form can be seen in Fig. 5.14. In its final version, the Counterblow Blow Petro-Forge can be described as a "combustion actuated-hydraulic" H E R F device. (b) Performancecharacteristics. The maximum blow energy of the machine was about 100kJ (75,000 ft lbf), achieved at a relative platen velocity of 24 m/sec (80 ft/sec). The cycle time was 5.0 sec, including unlocking of the hydraulic mechanism and relocking it at the end of the cycle. Without relocking, for instance, in multiple blow operation, the cycle time dropped to 2.0 sec. The development of this machine was not taken very far because from the late 60's onwards effort was concentrated on the Mk.I and Mk.II models. 5.5. Advantages of combustion actuation [5.14, 5.15] The H E R F hammers discussed, work on the principle that a certain volume of gas

136

S.A. TOBIAS

FABRICATED STAND~

I% UPPER DRIVE UNIT

UPPER PLATEN

J

J

DIES \

LOWER PLATEN

LOWER DRIVE UNIT

J EJECTOR OIL SUPPLY"

! "i

////// //////2////

-FUEL INJECTORS

CONTROL CONSOLE o Lt~ o ro

mmmmm~mm~ o

T

o m

il

V / / J / ;~ / / /

///////4//////////~

, / , / / / / / ~ 9,

3480

FIG. 5.14. Outlines and major dimensions of Counterblow Petro-Forge in its liquid fuel actuated form.

contained in an expansion chamber (combustion chamber for Petro-Forge), at a pressure Pi, is made to expand by driving a piston. The work done by the expanding gas is converted into kinetic energy of the moving mass (piston/ram/platen assembly) and this is then utilized for forming purposes. In pneumatic-hydraulic HERF hammers, at the end of the forming stroke the expanded gas is recompressed with the aid of hydraulic rams (recocking cycle) to prepare the machine for the next forming stroke. Thus, the useful forming work and the inevitable losses are covered by a hydraulic powerpack acting as the primary energy source. With combustion actuated machines, the principle of operation of which is shown in Fig. 5.1, at the end of the forming stroke the expanded gas is discarded (open loop) by opening an exhaust valve and the piston/ram/platen assembly is returned to its original (firing) position by the back-pressure. Thus, with such machines the energy is derived from two sources: (1) A low pressure air compressor supplying the air charge of the combustion chamber, and (2) the combustion of hydro-carbon fuel (propane). The hydro-carbon fuel acts essentially as a means for intensifying the primary power source. (a) The specific power out-put of combustion-actuated machines. Figure 5.15 shows the thermodynamic cycle of Petro-Forge machines and indicates that this consists of three parts. Part I involves the compression of V~, volume of atmospheric air (pressure Po) into volume V,. (combustion chamber volume) at pressure p,. (charge pressure), point A in Fig. 5.15, represented by a pV = constant line in the pressure-volume diagram (interrupted line in the figure). Part II is the combustion stage of the cycle in which fuel-air mixture at pressure p,. and volume V,. is ignited. With this the pressure rises six to eight fold to Pi (point B in Fig. 5.15). Part III involves the expansion of the gas mixture and the conversion of its cnergy into kinetic energy. This takes place again along a pV Y= constant line (BC in Fig. 5.15). Calculating the useful forming work during Part IlI and dividing it by the work input during Part I leads [5.14, 5.15] to the specific power output (SPO)pF. This gives the useful

Development of Petro-Forge Forming Machines

137

[3

p.I I

12.1 tr O0 U..I tr n

xA

/-'pV = Constont

'

~

~

~

vc

"-I

"--'~--

~

?

ve VOLUME

_

, ,0

-

V

FI6. 5.15. Schematicthermodynamiccycle of combustion actuated drive unit.

power output available for forming per each unit of electrical power input going into the primary power source (compressor) as

(SPO)pF

•

1

TICm 'IIPF "qau - "Y

Pi

1

_ ( V,, 1 (1-'~) \vi/

(5.2)

Pc"

--1

where "qCm = compressor efficiency, ~PF Petro-Forge efficiency, covering seal opening losses, arising during the initiation of the expansion, and back-pressure losses (partly recouped during return stroke of piston), -,/ = polytropic index of compressor cycle, ~/ = 1.4, via, = air utilization efficiency, covering scavenging and other air losses, Ve/V,. = expansion ratio, expanded volume/combustion chamber volume. =

Consider a Petro-Forge Mk.IIF (or Mk.IIE) machine operating under the following conditions: (a)Blow energy Ei = 13.6 kJ (10,000 ft lbf), (b)cycling rate n = 60 per min. This means that according to Fig. 5.9(a) the following machine setting had been made: (c) Charge pressure Pc = 7.0 bar abs. On the basis of previous research [5.6, 5.7], it can be assumed: (d)Combustion pressure Pi = 7p,. bar abs. (e) Petro-Forge efficiency, made up of 13% seal and 17% back-pressure losses, "qur = 0.7. (f) Air utilization efficiency ~a,, = 0.77. Furthermore: (g) Compressor efficiency, generally between 0.6 and 0.9, assumed "qCm = 0.7. (h)Expansion Ratio Ve/Vi = 4.82. Substitution of these values into equation (5.2) yields for the specific power output achieved, at the blow energy level of the "nominal maximum performance", (SPO)pF = 1.20. It is easy to show with the aid of equation (5.2) that at lower energy outputs the (SPO)eF will be larger and that at higher levels the opposite is the case, although the

138

S . A . TOBZAS

drop will not be big, for reasons to be explained later. (SPO)pF is not as large as might be expected, bearing in mind the additional power introduced by the combustion of hydro-carbon fuel. The reason for this is that (SPO)p F measures the power output with respect to the electrical power input into the compressor and therefore contains all losses arising in the system. It will be seen that the (SPO) for a combustion actuated machine is very much higher than for a pneumatic-hydraulic system, the ratio between the two being measured by the

relative merit factor RM. (b) The specific power output of pneumatic-hydraulic machines. With a pneumatichydraulic machine the power is derived by the expansion of the compressed gas from pg to p,, (line BC in Fig. 5.15). The specific power output (SPO)D, i.e., the useful power obtained for unit electrical input power is simply

(SPO)D -

WfDWfD + Wlo = "rid

(5.3)

where Wrt~ = power available for forming, WtD = overall losses, "qr) = overall efficiency.

WjD can be calculated but there is no need for this since it can be found from the blow energy and blow rate, both of which are contained in the machine specification. The power losses W/D can be estimated from the machine specification. Table 5.2 lists the information required for these purposes, giving for three types of pneumatic-hydraulic machines the maximum blow energy, the blow rate and the kW rating of the primary power source. The power available for forming WfD for each machine is obtained by multiplying blow energy and blow rate and this, in terms of kW, is given in the fifth column. Dividing WfD by the input power contained in the fourth column, yields WeD + Wu) and with this equation (5.3) yields the (SPO)D, presented in the sixth column. Table 5.2 shows that the specific power output (SPO)o of pneumatic-hydraulic H E R F hammers tends to increase with machine size, being approximately (SPO)t~ = 0.18 at small energy ratings and about (SPO)D = 0.34 at the top of the range. The former value is relevant since only machines of comparable energy ratings can be compared. For a Petro-Forge Mk.IIF the (SPO)pF = 1.20 and dividing this by (SPO)D = 0.18 gives a relative merit factor RM = 6.7, valid at an energy rating of about 13.6 kJ (10,000 ft lbf). This means that for the same blow energy output, a pneumatic-hydraulic H E R F machine requires a primary power source which is about 6.7 times that of the combustionactuated Petro-Forge Mk.IIE hammer, i.e., for the particular design configuration and

"I'ABI F 5 . -3. SPI!CIFICATIONAND SPECIFIC POWER OUTPUT OF PNEUMATIc-HYDRAULIC H E R F MACHINES

Trade

Model

I1~t[1]¢

Dynapak CEFF

U S1

Blow energy (k J)

Blow rate (rain i)

Input power

Useful work

(Wtt~ + WII~)

Wlt~

(kW)

(kW)

(SPO)D='q~,

400 620D 1220D

17 55 306

20 12 8

30.28 37.85 113.55

5.83 11.17 41.94

0.192 0.295 0.368

HEI0 HE55 210C 500C 2000C 3500C

98 544 22 68 204 408

12 7.5 7.5 10 7.5 5

121.12 233.16 15.14 45.42 90.84 90.84

20.14 69.95 2.79 11.66 24.19 34.97

0.167 11.30 0.184 0.256 0.288 0.385

Development of Petro-Forge Forming Machines

139

charge pressure, combustion actuation achieves a 6.7 fold intensification of the primary power source. Three riders should be added: (1) The comparison is based on the "nominal maximum performance" of the Petro-Forge. Using the "ultimate performance" (see Section 5.2) would lead to roughly the same RM. Although the (SPO)pF drops for increasing charge pressure, this is partially compensated by a drop of some of the air losses, those originating in scavenging and the actuation of valves, which do not increase with the output. (2) For the same reason, and also because according to equation (5.2) (SPO)pFis independent of the blow energy and dependent only on the PiPe, Ve/Vgand Pc/Po ratios, Petro-Forge machines with a much larger blow energy can be expected to have a somewhat higher relative merit factor RM. (3) The (SPO)pF depends on certain design features; it was lower for earlier models (for the Mk.IID (SPO)pF = 0.87 and hence RM = 4.5 [5.13, 14]) and it can be increased further, though not much, by increasing the expansior~ ratio Ve/V i. 5.6. Operating costs of Petro-Forge The point to stress is that if the expansion ratio V,,/Viis large then the intensification of the basic power source is achieved at a low cost, as the following example will show. The primary and secondary power costs of a Petro-Forge Mk.IIF and an equivalent pneumatic-hydraulic machine, both exerting 1 blow per second at an energy level of 13.6 kJ (10,000 ft ibf), are summarized in Table 5.3. The figures provided are based on current (May 1984) electricity and propane costs in Great Britain: (i) Electrical power: 0.0517 £/kWh, average figure for industrial user, (ii) Propane: 0.35 £/kg, bulk purchase price. The fuel consumption per blow of 13.6 kJ (10,000 ft lbf) can be calculated by considering the following: The combustion chamber of the machine, Vc = 4,916 cm 3 (300 in 3) contains an air/gas mixture at p,, = 7.0 bar abs. Assuming that the chamber is filled with air only, the mass of its content is Ga = 42.327 g (specific mass of air at atmospheric pressure 0.00123 g/cm3 (0.07651 lb/ft3)). The ratio of air to propane is about 24:1 but the mass of propane is 50% higher than that of air. Thus, the propane consumption per blow is (G,/24)x 1.5 = 2.65 g. The propane cost per hour (3600 blows) is therefore £3.34.

TABLE 5.3. HOURLY ENERGY COSTS OF PETRO-FORGE M k . I I D AND PNEUMATIC-HYDRAULIC H E R F OR CONVENHONAL FORMING MACHINES. BLOW ENERGY: 13.6 kJ (10,000 ft Ibf). CYCLING RATE: 60 PER MIN

Electrical energy

Pctro-Forgc Mk.IIF Pneumatic-hydraulic HERF or conventional forming machinc

Combustion energy (Propane)

Total

(kWh)

(£)

(kg)

(£)

(£)

11.3

0.584

9.54

3.34

3.92

76

3.93

3.93

The primary power source (compressor) of Petro-Forge requires an electric drive motor rated at 13.6/1.20 = 11.33 kW. A pneumatic-hydraulic HERF machine needs a (primary) power source which is 6.7×11.33 = 76 kW. The table shows that the basic energy costs of the Petro-Forge Mk.IIF are practically the same as those of a pneumatic-hydraulic machine. For obtaining the running costs, amortization and maintenance costs of the primary power source, with the associated pipe work, valves, etc. must be added and these are significant. For instance, current (January 1984) average capital

140

S . A . TOBIAS

cost figures of suitable compressor/hydraulic powerpacks are: Compressor, Hydraulic powerpack,

7.0 bar, 12.5 kW 135 bar, 75 kW

£2,400 £4,100

It is reasonable to assume that the cost of associated equipment (valves, etc.) and that of maintenance is proportional to the capital cost of the primary power source. Finally, the energy required for performing a certain forming operation is roughly independent of the forming speed. It follows that the total running costs of a combustion-actuated machine are significantly less than those of a pneumatic-hydraulic hammer or any conventional slow-speed machine. 6. A U T O M A T I O N OF P E T R O - F O R G E M A C H I N E S A N D P R O C E S S E S

6.1. Automation of HERF machines Very little work has been done in the USA and elsewhere on the automatic feeding, or automation in general, of H E R F machines. The Dynapak Model CP-16 powder compaction machine was equipped with automatic powder handling facilities but nothing is known about its performance. Robotic feeding was tried but did not prove successful because of the low output rate achieved. For instance, the Denver plant of the Sundstrand Aviation Corp. had an installation in which a Unimate was used for linking a furnace to a Model 1220D Dynapak. The subsequent manipulation, i.e., feeding the workpiece into a trimming press followed by a second Dynapak and trimming operation, were carried out manually, the total production time being 45 sec per component. Work on the automation of Petro-Forge originated by a number of inter-related reasons: (1) It became clear at an early stage that a cycling rate of 60 blows/rain could not be utilized by manual feeding; in hot forging a manual cycle time of 10 to 15 sec appeared to be the minimum maintainable over long periods. (2) Potential users frequently enquired about machine life; life tests were run on a Mk.IC machine by multipleblow upsetting of cold billets. Over 1.8 x 105 blows were clocked up in continuous running but the life of hammers is measured in millions of blows and that did not seem feasible with manual feeding. (3) Questions were frequently asked about tool life in hot and cold forming and cropping and this could be established only by the forming of many tens of thousands of billets. (4) Potential customers deprecated the absence of automatic feeding accessories. (5) Finally, in the early 70's, the forming industry became very conscious of noise pollution. All hammers cause severe impact noise and Petro-Forge was no exception. A large impact noise project was initiated [5.16, 5.17] which aimed at the design of hammers with reduced noise emission. As this work progressed, it became increasingly apparent that it might be easier to solve the noise problem by automating the forming process to a stage when operators were not required in the immediate vicinity of the machines. This led finally to the computer control of single or multi-station forging machines, fed by special purpose feeding devices or general purpose robots. 6.2. Automation of hot ]brming It was a matter of the highest priority in the course of development of Petro-Forge to establish reliable and realistic data on die wear under high speed forging conditions. It has already been mentioned (Section 4.4(c)) that work by D F R A [4.17, 4.18] had ascertained that die wear in hot upsetting on a small Petro-Forge hammer was substantially less than on a crank press. This and work done on the relative die life performance of presses and hammers in general (see for instance [6.1]) suggested that dwell time was one of the key factors. An investigation was therefore initiated to explore the effect of dwell time on die wear. (a) Automated die life tests. Preliminary tests showed that reliable and repeatable results are obtained only if all process variables, i.e., cycle time, forging energy, dwell

Development of Petro-Forge Forming Machines

141

time, billet temperature, quantity of lubricant, etc. are kept within narrow tolerances. This requirement led to the development of an automated forging system. The results achieved with the installation and their relevance to realistic practical conditions will be discussed in [1.4]. At this stage only the mechanical design and its subsequent evolution will be considered. The problem was essentially the linking of a 75 kW induction furnace to a Petro-Forge Mk.IIA machine by an automatic transfer mechanism. The billets were upset under appropriate conditions of blow energy and dwell time between flat die plates that had been cleaned by airjets and lubricated by an automatic lubricator applying a fixed quantity of fluid. After forging, the upset billet was cleared by a rake and the process repeated at a constant cycle rate. The major system elements, that is the furnace, the Mk.IIA hammer with its control console and the automatic die lubricator, are shown in Fig. 6.1. The billet loading station and the transfer mechanism are presented in Fig. 6.2. The billet emerged from the heating coil (Fig. 6.2(a)) at intervals determined by a timer, and dropped down a chute onto the loading platform. Up to that instant, the feeding mechanism, consisting of the billet gripper and the rake, both pneumatically operated and linked by a cross-bar, were in the position shown in Fig. 6.2(a). The arrival of the billet on the loading platform was registered by a photo-cell and this initiated the feeding cycle: The loading platform was slightly lowered by the platform cylinder. Simultaneously, the cross-beam with the gripper and rake were advanced by the feed cylinder, for the gripper to pick up the hot billet and the rake to clear the die surface. The whole transfer mechanism, the gripper/cross-beam/rake assembly was now slightly raised, withdrawn, indexed into the

FIG. 6. l. Major elements of single station automatic forging machine used in die wear tests.

142

S. A, "I'()BIAS CHUTE

~_~.f.

HEATING COIL

I

!

)

U

I G R P P IE R ~

CROSS-BEAM

FORM CYLINDER -- - " ' ~

(a)

(b) /

/ .? 5. LOWERBILLET

RELEASE BI

GRIP BILLET

,,

NOE ASSE X ]. STARTCYCLE I

(d) FIG. 6.2. Automatic billet transfer mechanism. (a) Manipulator awaiting arrival of billet on loading platform. (b) Gripper picks up billet from loading platform. (c) Gripper places billet onto centre of die. (d) Path of gripper during complete cycle.

position shown in Fig. 6.2(c) and advanced again. The billet, having arrived over the centre of the die, was placed onto the forging die plate by lowering the assembly and opening the gripper. The gripper/cross-bar/rake assembly was then rapidly withdrawn by the feed cylinder and indexed back into the position shown in Fig. 6.2(a). The spatial m o v e m e n t of the gripper is presented in Fig. 6.2(d). The small vertical lifting and lowering of the billet served the purpose of preventing a scraping of the loading platform and the die plate surface which was coated with an appropriate lubricant. The cycle was completed with the firing of the Petro-Forge and the upsetting of the billet. The whole system was fully automatic, being interlocked to prevent forging while the gripper was still in the die area, or when the die area had not been cleared by the rake, or when a new hot billet had not been placed into the centre of the die, or the billet temperature was outside preset limits. There was a starting up procedure applied as the furnace was running up to full load: billets were fed and ejected but the firing of the machine was delayed until the forging temperature was reached. All essential major elements, furnace, Petro-Forge, transfer mechanism and automatic lubricator were controlled by individual cam-timers and linked sequentially. The normal cycle time of the system was 5 sec between blows. (b) Automation of multi-station machines. The system of billet feeding discussed was primarily intended for die wear tests, as has already been mentioned. However, it is obvious that with the application of two identical transfer mechanisms three Petro-Forge hammers can be coupled in series, leading to an automatic transfer forging machine performing preforming (de-scaling), forging and coining/trimming operations. An out-

Development of Petro-Forgc Forming Machines

143

PLATEN

PETROF -O

GRP IPER

(TROLCONSOLE

/

TRANSFER

BASI

//1/"/

/

•/I/,

//

/,

(a)

PETRO-FORGE Mk.IID

TRANSFER MECHANISM

____.__.-BASE BLOCK

~.-~ BASEPLATE/

:

r 'l

i 3048

i'

I

I~'-1.~ "

' ~ VIBRATION ISOLATOR

CONTROL CONSOLE

(b)

FIG. 6.3. Concepts of three-station forging machines with automatic transfer. (a) In-line arrangement, front and side views. (b) Arc arrangement, front and phm views.

line drawing of such a machine is shown in Fig. 6.3(a). In fact, this machine configuration ([6.2, 6.3]) was never constructed since its limitations became obvious in the course of the die life work on the single station arrangement. The lessons learned, and later incorporated in a more sophisticated machine, were as follows: (1) The transfer mechanism should be independent of the machine, attached to the base plate only; any mechanism bolted onto the hammer has a tendency to shake loose because of the severe shocks to which the machine structure is exposed. (2) Special provision must be made for ensuring the permanent alignment of the transfer mechanism

144

S.A.

TOBIAS

with the furnace on the one hand and the Petro-Forge h a m m e r on the other, or between successive hammers. The hammers should be supported on vibration and shock isolators, to minimize ground reaction. Hence, during the forging blow each h a m m e r lifted about 10 to 12 m m , later reduced to 4 to 6 mm by increasing the h a m m e r mass. After the vertical oscillations have damped out, the die centres must return to their original position. (3) For manufacturing and maintenance reasons, it is desirable that identical transfer mechanisms be used. (4) The transfer mechanism should ideally be placed between successive forging hammers, and the h a m m e r s themselves are best arranged along the arc of a circle, both to facilitate the changing of dies and also to reduce the transfer time. (c) Wolfson Transfer Forging Machine. A schematic picture of the machine which emerged on the basis of these considerations is presented in Fig. 6.3(b).* It consists of three standard M k . I I D hammers, but without the side bridges containing the latch mechanism, arranged along the arc of a circle segment and supported on the customary vibration isolators. There are four identical transfer mechanisms, transferring the billet from the loading station (furnace) into the pre-form (de-scaling) stage, and from there to the central main forging h a m m e r , and finally through the coining/trimming stage into a storage container. The workpiece is moved between the platen guide rods, leaving the front face of the machine accessible for die change or manual single/multiple blow operation. The control console is situated behind the three hammers, as shown in the plan view contained in Fig. 6.3(b). A perspective drawing of the front of the machine in its fully developed form is presented in Fig. 6.4(a). Experience has shown that special provision must be made to ensure an accurate alignment of the centre of each of the three dies. For this purpose, each h a m m e r is guided at both the top and the bottom. At the top there is a central guide pin attached to the cover plate of the combustion chamber and this slides in a bush contained in an independent fabricated structure firmly bolted to the foundation. At the bottom keys are attached to each base block, both at the front and the rear and these are guided in brackets bolted to the foundation.

GUIDESTRUCTURE

GUIDEPIN

PETRO FORGEMk.IIDHAMMERS

PYROMETER

HEATINGCOIL

TRANSFERMECHANISM

TRANSFERGuIMECHANISM D

/ VIBRATIONISOLATOR (a)

BABEBLOCK

CONTROLCONSOLE

INDUCTIOr

(b)

Fro. 6.4. Perspective drawing of Wolfson Computer Controlled Transfer Forging Machine. (a) Front view showing hammers and transfer mechanisms. (b) Rear view showing control console and furnace.

*This machine has been built with the aid of a grant from the Wolfson Foundation.

Development of Petro-Forge FormingMachines

145

Figure 6.4(b) gives a rear view of the installation, showing the control console and the furnace. Provision is made for the independent setting of blow energy of each forming stage and on the central hammer (main forging stage) two successive blows at differing energy levels can be exerted. The temperature of the billet/workpiece is measured by pyrometers at a number of points along its path through the machine. The signals generated by these pyrometers as well as those produced by a series of micro-switches, indicating the position of the component grippers and that of the hydraulic workpiece ejectors, are used as inputs into the interlock system which insures safe operation of the machine complex. In its original form, the three Mk.IID hammers and the four transfer mechanisms were each controlled by a cam-timer. The machine is now being converted to micro-computer control by linking two interacting control systems. System I., which is already operational, supervises the automation, i.e., billet handling and the interlocking of this with the firing of the machines. System II. will set and maintain within close limits the blow energy (see Section 6.5). The purpose of System I. is to check with the aid of the pyrometers whether the billet/component temperature lies between the set tolerances and whether the loading station and the three dies have been cleared of the billet/component produced in the previous cycle. Successive hammers fire only when these conditions are met, the component ejectors have been withdrawn into their bottom position and the component grippers have been removed from the die area. After the billet/component has been forged in a particular stage, it is immediately ejected and transferred into the next forging stage or the final storage bin. This means that if there is a breakdown or delay with the furnace, preventing the hot billet to be fed into the first, pre-form, stage, all dies are cleared to avoid excessive die heating. If the billet/component temperature lies outside the set tolerance, the particular hammer will not fire and the billet/component is simply transferred into the next die, right through the three stations, to keep the dies warm. Similarly, machine firing is suppressed when one of the component ejectors gets stuck or any other fault arises in the system. It takes 15 sec for the billet/component to pass through the three forging stages and therefore, in principle, one component can be produced every 5 sec; special purpose transfer mechanisms are relatively cheap and they operate fast, giving a short cycle time. However, they are inflexible. The Wolfson Transfer Forging Machine can be justified only for a limited range of components, components which are complex and which even at high forming speeds require at least two forging stages for completion; it requires a substantial capital investment since it cannot be evolved by piecemeal automation. Control aspects of the automation of the hot forging process with Petro-Forge have been discussed in [6.5, 6.6, 6.7, 6.8, 6.9, 6.10, 6.11, 6.12] though these investigations have so far not had any significant influence on machine design and development. (d) Robotic feeding. A two-station forging system, which was developed piecemeal from standard single station Petro-Forge machines, is shown in Fig. 6.5. It consists of two Petro-Forge Mk.IID hammers with their individual control console, linked to a furnace with the aid of a Unimation robot. The principle of control of this system is similar to that of the Wolfson Forging Transfer Machine: the temperature of the billet emerging from the furnace is measured by a pyrometer which decides whether it lies between the set limits. It is transferred into the first stage by the robot and forged; transferred into the second, forged and ejected from the die. As the gripper is about to insert the preform into the finish-forging stage, it sweeps the die surface clear of the finished component and this drops onto a chute. After the gripper has inserted the preform into the second stage, it returns to the furnace to pick up a new billet. The die temperature of the forging stages and their clearing of the semi-finished/ finished components is observed by pyrometers which are interlocked with proximity switches indicating the position of the ejectors. The whole system is supervised by the micro-computer controlling the operation of the robot.

146

S . A . TOBIAS CONTROL CONSOLE

PYROMETER HEATING COIL

#> HYDRAULIC EJECTOR

INDUCTION FURNAC,E

HYDRAULIC POWERPACK

UNIMATIONPaDBOT

FiG. 6.5. Robotic kinking of furnace with two Petro-Forge Mk.IID machines.

The cycle time of the machine is 15 seconds. This relatively slow performance is partly due to the low furnace output and partly to the speed with which the robot can operate. The versatility of the robot is off-set by its high capital cost and slow speed of operation. 6.3. Automation of cold forming (a) Robotic feeding. Figure 6.6 shows an early attempt to automate a one-blow cold forming operation, with the aid of a Versatran [6.13, 6.14]. The need for this arose by a life test in which 50,000 cropped billets, of 41.28 mm (15/8 in) dia by 44.45 mm (13/4 in) height, were sized for subsequent cold forming. The installation was developed to a stage when it could work for long periods without supervision. "Fail-safe" operation was achieved by 6 interlocks operating on the machine ignition system. To keep the cycle time as short as possible, the charging cycle of the Petro-Forge and the billet feeding cycle were allowed to overlap. However, the machine was fired only if all 6 interlocks were completed. Unless this condition was met, the spark was suppressed and by starting a new feeding cycle the unforged billet was ejected. The positions of the interlocking sensors are numbered in Fig. 6.6. Billets were stored in a magazine and their arrival at the loading station was sensed by a proximity pick-up marked 1 in Fig. 6.6. Pick-up 2, contained in the robot gripper, stopped the process if the billet was lost during its transfer into the die. The completion of the insertion of the billet into the die was checked by an optical beam 3 scanning the die surface. This registered also the ejection of the billet, its removal from the die surface being effected by the gripper inserting a new billet into the die. Micro-switch 4 sensed that the ejector cylinder had withdrawn to its bottom position. Light beam 5 just grazed the end of the punch and was intended to stop the process in case of punch failure. Finally, proximity pick-up 6 recorded that the punch holding platen had returned to its top position and hence the hammer was ready to start another forging cycle. The cycle

Development of Petro-ForgeFormingMachines

147

CONTROL CONSOLE

MAGAZINE

DIE POT CHUTE

STORAGE BIN

EJECTION CYLINDER

VERSATRAN ROBOT

Fl6. 6.6 Robot fed Petro-Forge Mk.IID for cold forming.

time achieved was 6 sec, mainly because of the slow speed of the Versatran. One of the important requirements of this operation was to keep the forging energy between close limits; the operation was essentially a closed die one and excess blow energy might have caused a bursting of the die. These aspects of the work will be discussed in [1.4]. (b) Special Purpose Feeding Device. For forward/backward extrusion of components up to about 30 mm dia. the installation shown in Fig. 6.7 is employed. This was developed primarily for investigating as to whether the sizing of billets can be avoided, and with this the production cost significantly reduced, by using high quality billets, cropped with a Petro-Crop machine to be discussed in the next Section. In the installation shown in Fig. 6.7, billets are fed from a bowl-feeder via a flexible tube to a special purpose feeding device. This incorporates a gripper which moves the billet to the centre of the die and allows it to drop into the die container. In the course of the feed motion of the gripper the component, formed in the previous cycle and ejected from the die, is swept onto a chute to fall into a storage bin. Component ejection is effected by a hydraulic cylinder, actuated by the powerpack. There are a total of 10 interlocks applied, as numbered in Fig. 6.7. Interlock 1 registers the position of the hydraulic ejector. 2 signals the arrival of the billet at the loading point, after its descent through the feed tube. It also initiates the gripping and the forward motion of the feeder. 3 signals that the ejector is to be lowered and 4 opens the gripper jaws to allow the billet to fall into the container. During the return stroke of the gripper, 5 initiates the firing cycle of the machine but unless 6 is closed, indicating that the feeding mechanism had cleared the die area, the spark is suppressed. 7, 8 and 9 perform the same function as 3, 5 and 6, respectively, with the installation shown in Fig. 6.6, already discussed. Finally, 10 indicates that the billet has fallen down the chute and that therefore the top of the die has been cleared for another feeding cycle to commence.

MTDR 25:2-D

148

S . A . TOBIAS

HYDRAULIC EJECTOR

FEED MECHANISM

HYDRAULIC POWERPACK

FIG. 6.7. Automatic cold forming installation.

The rate of output achieved with this installation was 15 to 20 components per minute. Typical component examples, forward and backward extrusions, and combined forward/ backward extrusions, will be shown in [1.4]. 6.4. Automation of cropping--Petro-Crop High-speed cropping is one of the areas of application of Petro-Forge which has a very high industrial potential. For a wide range of metals, an increase of shearing (cropping) speed from the conventional to 5--6 m/sec (15-20 ft/sec) results in a substantial improvement of the geometric quality of the billet, that is, reduced axial and circumferential distortion, a flatter sheared face and closer weight tolerances. Such improvements are progressive up to a cropping speed of about 5-6 m/sec; beyond this range they are only marginal [6.15, 6.16, 6.17, 6.18]. Although die design for HERF applications lies outside the scope of this paper and will be dealt with in [1.4], as far as cropping is concerned, some aspects have to be introduced here to permit an appreciation of the machine design aspects of the automated cropping systems developed. A tool set which proved itself for the cropping of bars, in conjunction with Petro-Forge Mk.IC hammers and later models, but which also went through several cycles of re-design, is presented in Fig. 6.8; Fig. 6.8(a) shows a longitudinal cross-section, Fig. 6.8(b) is a three-dimensional cut-away drawing and Fig. 6.8(c) explains its operation in conjunction with the hammer and the bar-stop. The two main elements of the tool set are a stationary blade and a moving blade insert; the former is contained in a tool block and the latter in the moving blade carrier. Cropping is effected by the relative shear motion of the stationary and moving blades. The bar to be cropped is clamped pneumatically, by a device contained in the cylindrical projection at the rear of the tool set (Fig. 6.8(a) and Fig. 6.8(b)). It consists of a collet, compressed by a tapered ring, actuated by a pneumatic piston with spring return. The cycle of operation is explained with the aid of Fig. 6.8(c). At the start of the cycle, the bar to be cropped is pushed through the stationary and moving blades by a feed mechanism until it is stopped by the bar stop so that it projects beyond the shear plane by

Development of Petro-Forge Forming Machines

149 BLADE

MOVING BLA

)LLETT

COVER

(a) BAR:

HNG

~ISTON

DECELERAT(

PROXIMIT~

lING

MOVING

BLOCK

MOVING BLA

(b) COVER

SPRING

DECELERAT

Q

®

®

eKUXlMI FY PICK-UP

(c) CROPPING

TOOL RETURN, BAR FEED, BILLET EJECTION

ADVANCEOF END STOP

FIG. 6.8. Cropping tool set for bars up to 38 mm dia. (a) Cross-section, (b) Three-dimensional cut-away drawing. (c) Sequence of operations and off-cut length control.

a distance corresponding to the off-cut length. The pneumatic clamp is now applied and the bar stop is withdrawn, as shown in Fig. 6.8(c) as stage 1. The moving blade is struck by the Petro-Forge hammer platen and shearing takes place. The moving blade carrier has a short free stroke after which it impinges on the decelerator piston (Fig. 6.8(a)). The purpose of this is to absorb the surplus energy remaining in the moving blade carrier after billet separation has been effected; high

150

S . A . TOBIAS

speed cropping is always performed with some surplus energy, mainly to prevent a stalling of the shear blade in the bar stock because of a small fluctuation of the impact energy. The decelerator is guided in the top and bottom of the base plate of the tool set and is contained in a cylindrical cavity partially filled with oil and pressurized pneumatically, Energy absorption is achieved by forcing the oil to pass through the narrow gap between the piston and the housing [6.19], caused by its downwards movement. As soon as the surplus energy is absorbed the piston returns to its top position because of a net upwards force acting on it; its bottom cylindrical extension has a smaller diameter than the top one and because of the differential area there is a net upwards force. Similarly, the moving blade is also returned to its top position by one or two springs acting on its underside (see Fig. 6.8(b)). The proximity pick-up facing the bottom extension of the decelerator piston gives a warning when the piston stroke exceeds a certain maximum. This can arise when the surplus energy to be absorbed is excessive or when the oil in the decelerator needs replenishing. (a) The dynamics of high speed cropping [6.20]. Two fully automatic cropping systems have been developed: (1) The Petro-Crop System GP (General Purpose) and (2) the Petro-Crop System SP (Special Purpose). The former incorporates a Petro-Forge Mk.IE hammer, with a die set of the type shown in Fig. 6.8; it is intended for cropping small diameter bars. The latter can crop large diameter bars as well as tubes (Sub-Section 6.4(e)) and it incorporates a special high speed hammer, not a Petro-Forge Mk.ll, as might be expected. Tests showed that for fundamental reasons the Petro-Forge Mk.lI is not suitable for cropping on a production scale, though in the laboratory steel bars up to 70 mm dia. were in fact cropped with it. The reasons which led to a special purpose machine, out of line with the general design policy, will be understood by considering the dynamics of high speed cropping. It will be clear, on the basis of the previous discussion and in particular Fig. 6.8, that a high speed cropping machine consists essentially of three masses impacting on each other in succession. These are: (i) the piston/ram/platen mass Mt, (ii) the moving blade mass M 2, and (iii) the decelerator piston mass M3. For a Petro-Forge Mk.IE hammer and a die set of earlier design than that shown in Fig. 6.8, a schematic drawing of these three masses, with their actual magnitude, is presented in Fig. 6.9(a). An overall picture of the transfer of energy between these three masses and of their terminal velocities (after energy transfer has been accomplished) is easily found by the use of the principle of conservation of energy and momentum, by considering one pair of masses at the time [6.20].

0

/

M,

20

Ln=

./'CZ--~LOCITY' R*;'O '

(73.5 kg)

Z w

R12 " M I I M 2

=

( l i . 9 ko) R23 - M2/M 3 M 3 (S.S ko)

d

-

M2 - -

i[ri fJll

f

1.2

J

"

I/

: i

i"~,., L

I

!

~ ENERGY RATIO

I

i I I

[~

J

TI >

0

1

3

k

5

?

MASS RATIO - R

(a)

(b)

FI6.6.9. Effect of mass ratio on velocity ratio and energy ratio. (a) Moving masses of Petro-Forge M K . I D . (b) Velocity and energy ratios as functions of mass ratio.

Development of Petro-Forge FormingMachines

151

Consider the piston/ram/platen mass (M1) impacting on the stationary moving blade mass (M2) and assume that the process is loss-free (coefficient of restitution equal to 1.0) and that no external forces are acting on either mass (friction and cropping forces are neglected). The ratio of terminal velocities vz/vl and that of the appropriate kinetic energies E2/EI, as a function of the mass ratio R is obtained from Fig. 6.9(b). For the piston/ram/platen impacting on the moving blade the mass ratio is R~2 = M1/M2 = 6.17 and according to the figure that gives vz/v~ = 1.72 and Ez/Et = 0.48. This means that the terminal velocity of the moving blade is 72% above that of the platen. This was achieved at the expense of a low efficiency of energy transfer of 48%; after impact there is still 52% of the kinetic energy left in the platen which therefore continues to move with reduced velocity. The second impact takes place between the moving blade (M2) and the decelerator piston (M3) and for these the mass ratio is R23 = M2/M3 = 2.2 which according to Fig. 6.9(b) gives v3/v2 = 1.37 and E3/E2 = 0.85. Thus, of the kinetic energy left in the moving blade (after the cropping operation has been completed) 85% is transferred to the decelerator piston. The remainder (15%) stays in the moving blade and this will also continue to move. Since both the moving blade and the decelerator have a limited stroke, the platen is bound to catch up with the moving blade, and the latter with the decelerator, both impacting at least once more. The amplification of the terminal velocity of the moving blade with respect to the impacting velocity of the platen is beneficial since it ensures a high cropping velocity. Although above a threshold value of about 5-6 m/sec (15-20 ft/sec) any further increase of cropping speed produces only marginal improvement of billet quality, this amplification is advantageous since it ensures adequate speed even if the impact energy is low, as is the case with small bar diameters. The low efficiency of energy transfer limits the maximum utilization of the machine. This is not as serious a matter as might be thought since the maximum bar diameter that can be cropped on the Petro-Forge is limited not by the energy actually available for cropping but by the size of the die that can be accommodated. However, because of the low efficiency multiple impacts occur between platen and moving tool and experiments have shown that there can be as many as 12 to 14 of these. Multiple impacts put unnecessary loads on components and they generate a great deal of noise. According to Fig. 6.9(b), if the three moving masses are chosen to be equal then this results in a 100% energy transfer at both impact stages. At the end of the first impact process the platen assembly becomes stationary and the moving blade attains a velocity equal to the initial impact velocity of the platen. Similarly, the moving blade terminal velocity is exchanged with the initial velocity (zero) of the decelerator. As soon as the platen, and in turn the moving blade, become stationary they are returned to their starting position by the weak spring acting on them, the back-pressure in the case of the former and the return spring (see Fig. 6.8) with the latter; multiple impacts are automatically eliminated. The equalization of the mass ratios led to a new, special purpose, machine. Figures 6.10(a) and 6.10(b) show the geometric shape and weight of the three moving masses for the Petro-Crop Mk.ID and the redesigned model, respectively. The former was further developed into the Petro-Crop System G P (General Purpose), described in the next Sub-Section and the latter led to the Petro-Crop System SP (Special Purpose), considered after that. In the course of this redesign the combustion chamber and even the drive cylinder casting were left unaltered, i.e., the energy rating remained unchanged, but the energy utilization was improved. The moving blade was increased in size not only for increasing its weight but also to raise its strength so that the higher loads due to larger bar diameters cropped are easily taken up. The decelerator mass was also increased substantially and it was moved from the base plate of the die set (Fig. 6.8) into the machine base (Fig. 6.15(b) or Fig. 6.16(b)), as will be seen later. The analysis summarized is elementary and approximate. The coefficient of restitution is clearly not equal to 1.0. However, it is easy to show that its inclusion affects only the

152

S . A . TOBIAS

RAM

M I = 73.5 kg

PLATEN MOVINGBLADE~ DECELERATOR

MZ=11.8kg ~

M 3 = 9.5 kg

(a)

~

~

~_~

M 1 = 43.5 kg

M z = 43.5 kg

M,3 = 45.3 kg

(b)

Fx~. 6.10. Geometric shape and weight of moving masses. (a) Original design, Petro-Crop System GP, Series D. (b) Redesigned version for Petro-Crop System SP, Series E.

efficiency of energy transfer and hence the terminal velocity of the moving blade and that of the decelerator; it does not alter the essential conclusion that from the dynamic point of view optimum conditions are obtained when all three masses are equal. Moreover, strictly speaking, the mass equality condition should be enforced not only as far as weight is concerned but also in relation to the geometry and this was not possible. Figure 6.9(b) describes the relationship between the initial (before impact) and terminal (after impact) velocities and energy ratios. It gives no information as to the impact process itself and in particular as to the motion of the moving blade as it is accelerated from a stationary position to attain its terminal velocity. This takes a finite, though short, time interval during which the blade travels an appreciable distance, penetrating into the bar, first to deform it elastically and after that to initiate the separation process. A rigorous analysis requires the examination of the stress waves generated in the impacting bodies. This would be complex because of the complex shape of the bodies involved and in addition it would not lead to a closed analytical solution. An approximate method was therefore evolved which does lead to an understanding of the basic mechanism involved and which can be considered as a second order approximation. The method represents an impacting body by an equivalent single degree of freedom system. This consists of an equivalent mass, concentrated in the centre of gravity of the impacting body, which has equivalent springs on both sides [6.20]. The equivalent constants are easily found not only for simple cylindrical bodies, (for which there is a rigorous solution) but also complex ones, such as the platen/ram/piston assembly or the moving blade. Once the equivalent constants have been found from the design drawings of the impacting bodies, the equations of motion of these are set up and solved by considering the initial conditions. The latter are obtained by taking into account that the cropping process consists of three stages: Stage 1 starts at time t = 0 when the platen comes into contact with the moving blade and energy transfer commences. It ends at time t = t(,~ when the moving blade has traversed the radial clearance Cl between the bar and the bore of the blade insert. A schematic picture of the starting time instant is given by Fig. 6.1 l(a) and of the end by Fig. 6.11(b).

Stage 2 starts at time t = t~,/and ends when energy transfer has been completed, and the two components separate, at time t = t,. During this time interval the moving blade insert penetrates the bar, first elastically and then plastically. The elastic

Development of Petro-Forge Forming Machines t=O

153

t =tc.t

x l Qt t=tct

l

"4

|

A

0-/H/~-/,,s-,*//r//,-////,-~ H i /

IJH~F/I/HHH//H/IH/r

a..

~

t~ti

=to

~1 ~ t=ti

//'HH.//H//.I// e.

.//~rz

HH/¢I//Z/H

al t = t ¢

~///Ht//HI

d.

Fic. 6.11. Three stages of cropping process. (a) Starting point of Stage 1 during which clearance CI is traversed. (b) Starting point of Stage 2 during which energy transfer is completed. (c) Starting point of Stage 3 during which the cropping process is completed. (d) End of Stage 3, completion of cropping process.

penetration is short and can be neglected. The plastic deformation process will be represented by a constant force Pc acting on the blade. Figure 6. l l(b) represents the starting and Fig. 6.11(c) the terminal conditions. At the end of Stage 2 the cropping process is still in progress, the penetration of the bar being small, as will be seen later.

Stage 3 starts when the platen and the moving blade have separated, the latter continues to advance in free flight against the cropping force Pc, continuously losing momentum which is used for completing the cropping process. This stage is completed when billet separation has taken place at time t = t,., as shown schematically in Fig. 6.11(d). Experiments indicate that billet separation occurs when the tool has penetrated the bar by about 12% of the bar diameter. The process as arising on a Petro-Forge Mk.IE hammer (moving masses specified in Fig. 6.10(a)) when cropping a steel bar of 25.4 mm (1.0 in) dia. is illustrated in Fig. 6.12. The cropping force was assumed to be Pc = 300 kN (66.14 ton). The figure shows the platen velocity ,tl and moving blade velocity X2 as a function of the distance travelled by these, xl and x2, respectively. It contains two sets of curves corresponding to an impact velocity of vl = 6.8 m/sec (full curve) and vt = 5.83 m/sec (chain-dotted line). Considering the curve for vl - 6.8 m/sec (full curve) first, at the end of Stage 1, when the moving blade has traversed the clearance Cl = 0.052 mm, it has reached a velocity of ,t2 = 3.10 m/sec and impacts on the bar to be cropped. During Stage 2 the moving blade velocity Joe rapidly rises to 11.21 m/sec. When the maximum value has been reached the transfer of energy between the platen and moving blade is completed and the two separate. Thereafter, Stage 3 follows during which the moving blade velocity drops rapidly until billet separation takes place, i.e. when the blade has penetrated into the bar about 12% of its diameter, that is about 3 mm. Following that the moving blade

154

S . A . TOBIAS STAGE 1~

1 STAGE 2

~

STAGE 3

~n 12 ~: ~2 " 1 1 . 2 1 - -

m

•~i

,o

~

8

>. I--

0 C) v1 - 6.80

0 ..J uJ >.

LU 1:3

~

~

5.83-4

~ ~ - 8.,0I-.

_z •

O - 0.056

0.5

1.0

1.5

STAGE 2 STAGE

2.0

STAGE 3

2.5

x2t = 2.57

3.0

Jl

I PLATEN DISPLACEMENT

F[6.6. t2. Movingblade

- x I rnm,

M O V I N G B L A D E D I S P L A C E M E N T - x~ mm

and platen velocities as function of their displacement. Platen impact velocity v~ = 6.8

m/sec (full curve) v~ = 5.83 m/sec (chain dotted line).

continues with constant velocity (disregarding friction effects) until it impacts on the decelerating piston (see Fig. 6.8(a)). This final velocity is 4.4 m/sec and corresponds to a kinetic energy which is 15.4% of that transferred to the blade during Stages 1 and 2. The figure contains also the variation of the platen velocity x~, which drops from the initial v~ = 6.8 m/sec to 4.7 m/sec at the end of Stage 2 and remains constant thereafter. Because of the unfavourable mass ratio R~2 = 6.17, the residual kinetic energy left in the platen is 47.8%, of the same order as predicted by Fig. 6.9(b). The curve for Vl = 5.83 m/sec (chain dotted) shows a case when during Stage 3 the moving blade velocity ~tt dropped to zero after a penetration into the bar of only 2.57 mm. This is insufficient for billet separation to occur; a second blow is required for completing the process. The process depicted in Fig. 6.12 has two undesirable features. Firstly, the efficiency of energy transfer is low, which was to be expected from the elementary theory (see Fig. 6.9(b)). Secondly, the low efficiency produces a high terminal platen velocity, which causes multiple impacts, with all the undesirable consequences. Figure 6.13 is analogous to Fig. 6.12 but it refers to the case when the mass ratio R~2 = 1.0, that is, when the platen (ram) and moving blade masses have the geometry and values given in Fig. 6.10(b). Since the ram mass was reduced, to keep the impact energy the same its impact velocity was increased to Vl = 8.66 m/sec. All other conditions remained unchanged. Comparing the two figures leads to the following conclusions: When the mass ratio R~2 = 1.0, at the end of Stage 1, i.e. by the time the radial clearance (Cl = 0.05 mm) between tool and bar has been traversed, the moving blade impacts on the bar with a reduced velocity, (2.05 m/sec in comparison with 3.1 m/sec). This is beneficial from the tool life point of view. The maximum moving blade velocity at the end of Stage 2 is also lower (7.9 m/sec instead of 11.2 m/sec) but still substantially higher than that required for satisfactory cropping ( 5 - 6 m/sec). During Stage 3 the moving blade velocity decreases more slowly because of its larger mass and at the end of that stage, after a penetration of 3.0 mm, when billet separation occurs, it is substantially higher (6.0 m/sec) than for the unequal mass ratio case (4.4 m/sec). This suggests that the initial ram kinetic energy was higher than required for completing the cropping process and could be reduced; not unexpected in view of the higher efficiency of energy transfer. The most important feature in Fig. 6.13 is that it shows that the ram velocity, though initially higher (8.3 m/sec) than for Fig. 6.12 (6.8 m/sec), rapidly drops (because of

Development of Petro-Forge Forming Machines

155

STAGE 1

;!\

STAGE 2

o =

E

•~' +1 B.Be "

t

I-

>O 5 .a W

/

I

I

l

i l i!ll

I

II i

I

I

--

I

I- I! I

0

/j 0

-2

i

i

I I

I

I

I

I

]

I

I

i

1

"g

11

K:,

,.~

~

,

I

I

1.5 I

I

x2 = o . o s

°

I

I

")Z._l i ,i, ,+

oIE

t

MOVING BLADE J~

IIl'il

PLATEN ~'I ~

\

: I

I II

3

ilil i

/-"

W

F< n

STAGE

=i_

I

i 2O

L.+I 25

3.0

I

I i.II .~-

PLATEN

DISPLACEMENT

- x 1 ram,

MOVING

BLADE

DISPLACEMENT

- ~

mrn

FzB. 6.13. Moving blade and ram velocities as function of their displacement for R~2 = R23 = 1.0. Ram impact velocity v~ = 8.66 m/sec.

efficient energy transfer) and actually becomes negative. This means that after the impact has been completed the ram returns to its starting position and hence there is no possibility for multiple impacts to occur. According to Fig. 6.10(b) the ram velocity should be zero and its negative value is due to the inclusion in the analysis of the cropping load P,.. With a mass ratio of RI2 = 1.0 the efficiency of the process has improved to such an extent that a "stalling" of the moving blade in the bar, i.e., a penetration of less than 12% of the bar diameter (3 mm), occurs when the initial ram energy is less than 53% of the value corresponding to the case of Fig. 6.12. Even in that case, the maximum blade velocity would be about 5.5 m/sec, which is ample for ensuring a good quality crop. (b) Petro-Crop System GP (Series F). This is an automatic cropping system for producing billets from steel bars up to about 38 mm and light alloy bars up to 50 mm, at a rate of about 30-45 billets per min, depending on the off-cut length. The machine, a photograph of which is shown in Fig. 6.14, consists of a standard Petro-Forge Mk.IF-5K hammer with control console, the cropping die set discussed previously (Fig. 6.8), a bar rack and feed unit and a bar stop for determining the billet length, which functions in the manner explained in Fig. 6.8(c). The operation of the installation is fully automatic; after the energy required for the particular bar diameter and material and the bar stop have been set, in accordance with the billet length, the machine works without supervision. Bars are dropped from the bar-rack onto the feed mechanism, are fed into the die set and cropped, the first and last billet of each bar being rejected. If for some reason the bar gets stuck in the die, or if the blow energy set is too low or too high, or if the oil contained in the decelerator of the die set needs replenishing, the hammer automatically switches itself off. This is a general purpose (GP) forming system which by changing the tool set can be used for a wide range of operations, hot/warm/cold forming, powder compaction/ forging, blanking, piercing, coining, rubber forming, etc. The latest version of this machine (Series G) is controlled by a microcomputer which performs three functions: (1) It sets the blow energy and ensures that this remains constant, within + 5%, without any interference from the operator; it can adapt itself by correcting the slow drift of impact energy caused by heating up or other reasons. (2) When malfunctioning occurs, for instance when the system cannot adapt itself to a change of parameters, a diagnostic routine is called up which diagnoses the cause and instructs the operator how to correct the fault. (3) The energy of the machine can be set locally or from a central computer, thus deskilling the operation. A more detailed

156

S.A. TOBIAS

Fro. 6.14. Petro-Crop System GP (General Purpose) series F.

discussion of the computer control of Petro-Forge will be given in Section 6.5 [6.26, 6.27]. (c) Petro-Crop System SP (Series' F). This is a specialpurpose (SP) machine designed for cropping but suitable also for other forming processes, such as for coining and with some limitations for cold forming and compaction. Its maximum cropping capacity is 50-55 mm dia. steel and 65-70 mm dia. light alloy bars, at a rate of 20 to 35 billets per minute, depending on off-cut length. A three-dimensional drawing of the hammer and the control console of this machine is shown in Fig. 6.15(a). The main difference between this hammer and the standard Petro-Forge Mk.IF-5K hammer is that the former has a substantially enlarged die space to accommodate the larger tool set required for larger billet diameters. The die area is totally enclosed by sound absorbing/isolating panels which can be individually removed, except for the front panel which is hinged, to facilitate the tool set changing. Similarly, and for the same purpose, the bar stop mechanism is hinged to the front of the anvil and the base; it is hydraulically locked. Figure 6.15(b) gives a cutaway drawing and shows the internal construction of the hammer. The sound isolating/absorbing panels are shown in an "'exploded" manner. Note that the piston/ram/striker assembly is cylindrical and does not have the mushroomlike extension (platen) found on other hammers, this change being introduced so that the optimum mass ratio can be ensured. It is guided in a substantial bush and can be locked in its top position by two pneumatically actuated latches. Figure 6.16(a) gives a close-up view of the die area. The guide bush of the piston/ram/striker assembly extends into the die space and its lower end guides a clamping plate which is hydraulically actuated. This plate is used for clamping the tool set to the top surface of the anvil. The tool set is resting on an array of spring supported balls

Development of Petro-Forge Forming Machines

(a)

157

(b)

FIc. 6.15. Hammer, bar stop and control console of Petro-Crop System SP. (a) Three-dimensional drawing of hammer, bar stop and control console. (b) Cut-away drawing of hammer showing tool set space, piston/ram/ striker assembly, decelerator incorporated in base.

(a)

(b)

Flc. 6.16. Internal construction of hammer of Petro-Crop System SP. (a) Cropping tool set, clamping plate and energy absorber. (b) Cut-away drawing of hammer, tool set and energy absorber.

(ball transfers); by unlocking the bar stop and swinging it and the front sound isolating/absorbing panel out of the way, the die space is exposed and the tool set is easily slid out and removed; tool changing is effected within a matter of minutes. Figure 6.16(b) is a cut-away drawing of the die area. The hydraulic energy absorber [6.18] is integrated into the anvil and stand; contrary to the solution shown in Fig. 6.8(b)

158

S.A. TOBIAS

it is not part of the tool set. It consists of a large slightly tapered mass, guided at its two ends, which is in a tapered cylindrical container filled with pressurised oil. The radial clearance between the absorber mass and its container is reduced during the energy absorbing stroke, though otherwise the principle of energy absorption involved is the same as with the die set shown in Fig. 6.8(b). In view of its larger size, this absorber is water cooled. (d) Automatic billet weight control. Both Petro-Crop Systems, when cropping bars with a close diameter tolerance, for instance cold drawn bars, achieve a billet weight tolerance of + 0.3% [6.15]. With "black" hot rolled bar stock the weight tolerance is inevitably larger but it can be reduced by measuring the radial dimensions of the bar at intervals and correcting the off-cut length for minimal volume (weight) variation. A system of this kind [6.22] will be described with the aid of Fig. 6.17 to Fig. 6.19. Figure 6.17 shows an "exploded" view of a Petro-Crop System SP with automatic billet weight control. The bar to be cropped is fed by an airmotor driven chain into a bar feed unit which advances it in increments through the bar diameter gauge into the cropping tool set. The off-cut length to be cropped is set by a ball screw, driven by an electro-hydraulic stepping motor contained in the bar stop unit. Billets cropped are ejected into a chute which at the bottom end has a flap to separate the first and last billets, which are generally "damaged" and fall outside the volume/weight range set. The bar diameter gauge, located between the bar feed unit and the hammer, consists of 6 pneumatic cylinders, arranged so that their centre lines are equi-spaced and point through the bar centre. The ball-end of the piston rod of each cylinder contacts the bar surface when taking a bar radius measurement, the radial displacement of the piston rod

BAR

BAR FEED UNIT

BALL SCREW

AIR NOTOR

BAR DIAMETER GAUGE

CROPPING TOOL SET

ELECTRO-HyDRAULIC BAR STOP

~

~'SEPARATOR FLAP

FI~. 6.17. Exploded view of Petro-Crop System SP with electro-hydraulicbar stop. Sound isolating/absorbing panels enclosingdie space have been removed.

Development of Petro-Forge Forming Machines

159

,ECTRO-HYORAULIC BAR STOP

SORTER

WEIGHING STATION

FIG. 6.18. Petro-Crop System SP with billet weighing and sorting stations.

being measured by a built in LVDT. Measurements are made at intervals of one off-cut length, when the bar is stationary and clamped, just before the cropping blow takes place. After taking a reading the ball-ends withdraw, to ensure that the LVDT's are not damaged by shockloads. From the 6 radial measurements, the micro-computer controlling the system, (not shown in Fig. 6.17), computes a mean bar diameter and cross-section, as well as the appropriate off-cut length for constant volume/weight. This off-cut length is adjusted by the electro-hydraulic stepper motor actuating the bar stop. Clearly, the control system described so far is an open loop one. Its performance can be improved by closing the control loop with the introduction of a weighing device. The weight of the billet cropped is fed back to the micro-computer to improve the accuracy with which the mean cross-section is calculated. For certain forming operations, like closed die forging, it is desirable that the billets be classified into even closer weight tolerance ranges and these be used in succession of increasing weight groups, so as to compensate for the wear of the forming dies. For this purpose the weighing station is followed by a billet sorting device, shown in Fig. 6.18. Billets cropped and weighed drop onto an elevator which takes them to the sorting station where with the aid of flaps they are directed to fall into one of 4 containers, one of

160

S . A . TOBIAS

which receives the bar ends. The principle of operation of the closed loop system with billet sorting is explained by the block-diagram in Fig. 6.19.

SIGNAL

// MC IROC -OMPUTE ~R <~

INTERFACE

JN I TERRUP]----~ T PETRO_CRO[P

[ I

I

WEIGHING & SORTING

I cRcu,T ]

l

CONTROLCONSOL

f

I ISTEPPE R MOTOR

ICONTROLLERF _(~f

I CROPPING TOOLSET

,

~

AIR MOTOR& ENCODER F--CHAIN DRIVE . . . .

-I'-.AR DIAMETERGAOGE

,~\

\, BAR

WEIGHING STATION

Fl(;. 6.19. Block diagram of on-line billet volume control system.

The rate of output that can be achieved with this system when cropping black bar with 50 mm dia. is between 30 to 40 billets/min, depending on the offcut length. For a billet length of 125 mm the following weight tolerances can be maintained [6.22]: A. Open loop control only, i.e., without weighing and sorting: 1000 + 8.7 g, that is + 0.87%. B. Closed loop control, i.e., with weighing but without sorting: 1000 _+ 6.3 g, that is _+ 0.67%. C. Closed loop control with weighing and sorting: Batch tolerances can be set to, say, 1000-1001.99, 1002-1003.99 g, 1004-1006 g. Tolerance within each batch _+ 0.1%. (e) Tube cropping attachment. The Petro-Crop System SP can be used for cropping thick-walled tubes into rings, as required, for instance, for the manufacture of antifriction roller bearings [6.23, 6.24, 6.25], by replacing the bar stop mechanism with a special attachment. With tube cropping, means must be provided to prevent the off-cut and remainder from collapsing or deforming excessively and this is done by inserting a mandrel into the tube. Indeed, different methods for cropping tubes differ in the design of this mandrel [6.25]. With the design adopted for Petro-Crop the supporting function is performed by a three-part mandrel held together with the aid of a tie bolt. The essential features of the process will be explained with the aid of Fig. 6.20(a) which shows the design of an early tube cropping die set of the type used in conjunction with a Petro-Forge Mk.IID machine. The major parts of the die set, the stationary and moving blades, the bar clamp actuated pneumatically, are identical with those seen in the bar cropping die set shown in Fig. 6.8. The decelerator piston is supported by a polyurethane cushion used in earlier designs for energy absorption. The three-part cropping mandrel unit consists of the stationary plug, at the free end, the moving (shear) mandrel guided in the moving blade, and the guide block located in a bush contained in the end plate of the die set. The stationary plug prevents the tube

Development of Petro-Forge Forming Machines MOVING MANDREL

MOVING BLADE

STATIONARY BLADE

161 STATIONARY PLUG

HYDRAULIC RAM

TUBE CLAMP

TUBE

ii, J

STRIPPING UNIT , \ \

DECELERATORPISTON

POLYURETHANEPADf

GUIDE FINGER

(8)

STRIPPING CLAW

,y./////~NARY

PLUG

TIE BOLT

GUIDE BLOCK

CROPPEDRING

MOVING MANDREL

(b)

FIG. 6.20. Early tube cropping die set. (a) Sectionalview of die set. (b) Details of mandrel and stripping unit.

remainder from collapsing. The middle part, which supports the off-cut, can be displaced a limited distance, to effect the shearing motion in conjunction with the moving blade insert. The guide block is attached to the piston rod of a hydraulic cylinder which pushes the whole assembly into the tool set and withdraws it, thus stripping the ring-billet from the central moveable element. The stripping is effected by claws attached to a double gimble, as shown in Fig. 6.20(b), this assembly being bolted to the cover plate of the die set. The current design of the tube cropping system is shown in Fig. 6.21 which represents a section through the die and the decelerator of the Petro-Crop System SP. The die set is

S. A, TOBIAS

162

S,M STRIKER

DIE CLAMPING

CLAMP

THREE-PART MANDI

ING DIE

G BLADE

RETRACTING CYLINI

TOR PISTON

Flo. 6,21. Sectional view of tube cropping attachment for Petro-Crop System SP with die set and energy absorber,

identical with that used for bar cropping and the only difference is that for tube cropping the bar stop mechanism (Fig. 6.15(a)) is replaced by an attachment for the hydraulic actuation of the three-part mandrel. By far the major part of the development of tube cropping has been done with a material used for the manufacture of anti-friction roller bearings (En31), for two off-cut sizes: Outside dia. mm

Inside dia. mm

Width mm

63 46

50 33

18 14

The die set can be scaled up for larger diameter rings, of greater width, without any difficulty. The optimum process conditions, in particular, the optimum die/tube clearances required for achieving high quality rings, are discussed in [6.24]. 6.5. Computer control of Petro-Forge machines The Petro-Forge and Petro-Crop Series E and F models are controlled by a solid state timing device serving the purpose of the cam shaft in internal combustion engines. This method of control is perfectly satisfactory, being reliable and ensuring that the blow energy remains between fairly narrow limits, provided the cycle time is long. It was noted that when the cycling rate is high then for the first few hundred blows when starting from cold, or after the machine has been inoperative for a considerable time, there is a slow downwards drift of the blow energy. This appears to be due to the heating up of mechanical components and the consequent reduction of clearances between mating parts. This energy drift can, of course, be corrected by increasing the charge pressure but this places additional responsibility on the operator which is undesirable. The ideal control system should be able to adapt itself, within limits, to varying internal and external influences and ensure that operation is maintained at the set level. This and a great deal more can be achieved by a new microcomputer controlled system. In the following discussion the control system of Series F machines and that of a micro-computer based system (Series G) will be described in parallel.

Development of Petro-Forge Forming Machines C

F

E

B

I

M

163 T

B

U

H -----..

G~

w\

R--.. QJ

~r~o.

/

,go, "#

~'~

C-iN.

Ju

QI N

D

A

S

J

K

L

Dj N

A

(a)

S

L

(b)

A Key-Switch

I Gas Pressure Gauge

O AuxilUary Functions

S Power Indicator

J Blow-Selector Switch

R Hydraulic Ejector

C Air Indicator

K Fuel Switch

S Emergency Button

O inching Button

L Firing Button

T VDU Screen

E Back-Pressure Gauge

M Firing Indicator

U Numeric Pad

F Back-Pressure Regulator G' Charge Pressure Regulator

N Latch Release Switch O Blow Rate Selector

V Keyboard Socket W Gas Indicator

H Charge Pressure Gauge

P Blow Counter

FIG. 6.22. Control panel of Petro-Forge machines. (a) Solid state timer controlled machines (Series F). (b) Micro-computer controlled machines (Series G).

It is convenient to take as the starting point the control panels of the two designs, as the only feature of the control systems to which the operator is directly exposed. (a) Controlpanel of timer controlled machines. Figure 6.22(a) shows the control panel of the Series F machines. The machine is activated by turning the central key switch (A) to the right and thus placing the machine into the F O R G E mode of operation. The blow energy is set on the charge pressure gauge (H) with the aid of the charge pressure regulator (G). As already mentioned, the relation between the charge pressure and blow energy, for a particular platen stroke, is given by a calibration chart of the type shown in Fig. 5.6(a). The back pressure, read from the back-pressure gauge (E), is usually set to its "normal" level by the T R I M M E R (F), unless for process reasons (very short dwell time) it is increased. The machine is fired by turning the latch switch (N), to withdraw the latches (Fig. 5.5(b)), and pressing the firing button (L). Multiple blows, up to 3 at intervals of 1.0 sec, can be set by switching the blow selector switch (J) to MULTI. The firing cycle can be initiated by an external signal, for instance an interlock signal generated by a feeding device etc., by switching blow selector switch (J) from the SINGLE to the REMOTE mode. However, the charge and back pressures cannot be set from a distance but only by adjusting the appropriate regulators (G and F, respectively) on the control panel. Furthermore, if the blow energy drifts because of temperature effects, it must be adjusted/reset manually. Turning the key-switch (A) towards the left calls up the INCHING mode, used for setting up dies/punches; the latches are withdrawn and the platen can be lowered slowly by pulsing the inching button (D). (b) Control panel of micro-computer controlled machines. Figure 6.22(b) presents the control panel of the micro-computer controlled machine (Series G). As in the previous case, activation of the machine is achieved by turning the central key-switch (A) to the right, F O R G I N G mode, or to the left, INCHING mode. The latter is identical with that arising with the solid state timer controlled system. In the F O R G I N G mode, when the blow selector switch is turned to MANual, all the

MTDR 25:2-E

164

S . A . TOBIAS

control parameters, charge pressure, back pressure, number of successive blows, are entered by the operator with the numeric pad, in response to questions displayed on the VDU (T). The firing cycle, single or successive multiple blows, is initiated in the same way as before, by turning the latch release switch (N) and pressing the firing button (L). Turning the blow-selector switch to AUtomatic places the machine under the control of a central computer, which sets the blow energy (charge pressure and back-pressure), the number of successive blows and the total number of components to be produced, etc. of a group of machines. This information is displayed on the VDU (T) for information only and cannot be altered by the operator. However, the machine has adaptive capability and may alter the blow energy set initially, either to compensate an energy drift or to ensure that the component is completed. Further insight into the working of the two control systems will be gained with the aid of Fig. 6.23(a) and (b) which shows their respective pneumatic control circuit. Both can be divided into three sub-systems concerned with the control of the air charge (AIR SYSTEM), that of the gas charge (GAS SYSTEM) and that of the back-pressure (BACK-PRESSURE SYSTEM). These are enclosed in the figure by interrupted, chain dotted and dotted lines, respectively.

Si~l= u AIR V*n GASSYSTEM AIRSYSTEM ~ Vg2

, .'-c. V t

) .

' .

.

.

.

.

Vex

Pb

Val

'

<

COMBUSTO IN l ~

!

i L . . . . .

......................

_ _

~

Nb ....

AIR GA S

(a)

(b)

FIG. 6.23. Pneumatic system of Petro-Forge machines. (a) Solid state timer controlled machines (Series F). (b) Micro-computer controlled machines (Series G).

(c) Air~gas charging of timer controlled system. With both the timer controlled system (Fig. 6.23(a)) and the micro-computer controlled (Fig. 6.23(b)) system the air charging process and that of the gas works on the same principle. Hence, it will be sufficient if only one of these, the air charging system, is described. With the timer controlled system the charge pressure in the combustion chamber is regulated by reducing the air line pressure with the regulator R(, (G in Fig. 6.23(a)) working with the charge pressure gauge (H). The charging air flows through the solenoid valve V(, and the inlet valve Vi,, for a predetermined period T, which is the same for all charge pressure levels. Figure 6.24(a) shows the charging curves for a series of values of the nominal charge pressure Pc. For filling up the combustion chamber to the level of the

Development of Petro-Forge Forming Machines

165

nominal charge pressure Pc, the charging time would have to increase with the charge pressure, from Trnin tO Tma x. With a fixed charging time T, chosen so that the cycle time is kept reasonably short, at high charge pressure, the combustion chamber will not be charged completely; the actual charge pressure being significantly lower than the nominal charge pressure set. This means loss of blow energy at high charge pressure levels. (d) Air~gas charging of micro-computer controlled system. With the micro-computer controlled system (Fig. 6.23(b)) the head pressure of the airflow remains unaltered (being set by the regulator R,t) and the charging time is varied by switching a solenoid valve off when the required pressure is attained in the combustion chamber. This is illustrated with Fig. 6.24(b) which shows that the charging time is varied between To < T < T~o. The combustion chamber is completely filled for all charge pressures, resulting in a gain of blow energy; the cycle time, which is largely determined by the air charging time, increases with blow energy.

CHARGE PRESSURE

CHARGE .PRESSURE Pc bar NOMINAL

J~ Oo 10

.----r

A.

LM

8 I,LI nO.

< I 0

......

ACTUAL

10

,

Pc bar I~

:

ACTUAL

~e~

--lO

1(3

'"

,-8

e 4

'

/ / J j - i

......

' 0.2

.....

I i 2 o.e

0:4 Ti

CHARGING TIME T TIn

TIME-t sec

TI

max

-,--2 m

--6

,,,

--4

=~ < n0

--2 !1,

ii

ToT2

I. . . . . . .

I°'~ [

T4

Te

I °~

Te

~'~

TIO Tmax

TIME-t sec

FIG. 6.24. Charging characteristics of combustion chamber. (a) Solid state timer controlled machines (Series

F). (b) Micro-computer controlled machines (Series G).

The time instant at which the air charge valve V,I is closed is determined by a pressure transducer S, which, strictly speaking, should be located in the combustion chamber. To safeguard it from excessive heat and shock exposure, it is located upstream in the charging line, at the highest point of the control console. Because of the location of the transducer S, and the scatter of the finite switching time of the controlling solenoid valve, this system ensures accurate and repeatable charge pressures only if the air flow is highly throttled, i.e., for a very slow rate of charging. For this reason the charging process is effected by two solenoid valves in parallel; V,, which has a large orifice and is therefore slow acting and Va2 which is small and responds fast. During charging both valves are switched on together, the large valve is shut off when most of the air has passed into the combustion chamber and the small valve controls the final topping up. As is clear from Fig. 6.23(b), gas charging is effected in an analogous manner. The regulator Rg sets a constant pressure head. (e) Back-pressure control. The normal back-pressure Pb is related to the charge pressure Pc by equation (5.1). With the timer controlled system Pb is generated by a pneumatic circuit contained in the dotted enclosure in Fig. 6.23(a). Means are provided for increasing the back-pressure by increasing Pmin with the aid of regulator Rh2, this being the T R I M M E R (F) in Fig. 6.22(a). With the micro-computer control system (Fig. 6.23(b)) the back-pressure is set by the regulator Rbs actuated by a stepper motor, directly set by the micro-computer. (f) Measurement of blow energy. The primary purpose of the computer controlled system was to ensure that the blow energy, once set, is automatically kept within a narrow scatter band. For this purpose the blow energy is measured during every cycle at

166

S.A. TOBIAS

a point of the platen stroke which is slightly above that at which the forming operation commences. The velocity sensor used is a proximity pick up which is triggered by two small projections attached to the platen and located a known distance (say 10 mm) apart. These generate square pulses as they pass the pick-up and by measuring the time between the leading edges of these and dividing it into the space distance the velocity is obtained. (g) Design of computer system. The micro-computer used in the control system is a CBM 3032 or 8032. It performs the following functions in conjunction with the appropriate interface circuitry, A/D converters, etc: (i) Stores the values of input parameters required for achieving a specific blow energy level. Fixed input parameters: Drive unit geometry (combustion and back-pressure chamber volumes, piston/ram diameters, stroke, etc.), valve characteristics (opening and closing times). Variable input parameters: Air and gas charge pressures. (ii) Computes optimum back-pressure and sets it. (iii) Computes time parameters for firing cycle control, i.e. opening and closing times of gas and air and other solenoid valves, of inlet and exhaust valves of ignition and scavenging. (iv) Controls firing cycle by actuating valves and ignition circuit at appropriate times. (v) Samples the variation of the gas and air charge pressures and compare these with required values. (vi) Computes blow energy from impact velocity, establishes the moving average energy of the last N blows and by the use of this resets variable input parameters to ensure constant energy output for subsequent blows. The software for performing these functions is written in BASIC, except for one of its subroutines, that of the firing cycle (paragraph (iv)), for which a machine code program is used for achieving fast execution. Further details, including flow charts, are given in [6.26~ 6.271. Function (vi) is included in the control program only when the machine is running continuously, in which case the blow energy is used as a feedback signal and hence the method of control can be called feedback control. When the hammer is used for exerting only occasional blows, then function (vi) is omitted, (although the blow energy is computed and displayed on the VDU); the input parameters of the next blow cannot be determined on the basis of the average energy of previous N blows since by the time that next blow comes some of the machine conditions (heat expansion) may have changed. In that case~ the computer controls solely the gas/air pressures according to a table stored in memory which relates these and the blow energy and the method of control is called

supervisory control. Finally, a program was also developed for simulating the charging conditions arising with the old timer controlled system, in which the charging times were kept constant for all levels of the blow energy. This method of control is called open loop control and it is used as a basis to which performance improvements are related. (h) Performance of computer controlled Petro-Crop. The test vehicle used for establishing the performance of the new system was a Petro-Crop System GP (General Purpose) cropping machine, already described in Section 6.4(b). It was chosen from among several alternatives because it allowed both single and continuous operation at a high cycling rate. The following summary of test results relates to the first 250 blows in continuous running conditions. All percentage scatter figures given exclude random scatter of about + 2.0% which was eliminated by smoothing: (1) With the open loop control the blow energy drifted to reach a mean level of about + 5.0% above its set value. Around this mean level scatter was within a band of _+ 5.0. There were sudden energy rises and falls, probably caused by changes of the supply pressure, due to supply or load changes of the airline network feeding the machine. Including such changes the total scatter band was + 10%. (2) When operating with supervisory control, the charge pressures remained practical-

Development of Petro-Forge FormingMachines

167

ly constant but the blow energy drifted downwards by as much as 8.0% and this can be attributed to heating. (3) With feedback control the charge pressure first rose slightly but later levelled out. The blow energy remained constant within a band of _+ 2.0%. (4) Including scatter, as well as rounding errors of the A/D converter of the control system, but excluding the first 10 blows of the run, blow energy remained between + 3.7%. Larger errors in the first 10 blows were probably due to "dry" sliding surfaces in valves. As with the old timer controlled system, this effect was reduced by "priming", i.e., by firing the machine a number of times while the platen was locked by the latches. (5) The cycle time of the machine varied between 0.8 sec at low energies and about 1.3 sec near the top output level. (i) Automatic self-diagnosis of faults. The micro-computer control system has adaptive capability ensuring that when some machine parameters change the blow energy is kept at the pre-set level. This adaptive capacity works only as long as the output variation to be compensated remains between pre-determined limits. Once these are exceeded a fault is registered and a process of diagnosing and rectifying it is instituted. In specific terms, a drop of blow energy due to, say, heat expansion is compensated automatically. However, if the compensation exceeds a certain level, in the extreme, by the platen guides seizing up, the system will recognise that a fault has arisen, pin point it by interacting with the operator and instruct him, through messages displayed on the VDU, how to correct it. In its simplest and crudest form, automatic self-diagnosis is effected by checking the satisfactory working of every single component of the system. This requires a large number of transducers and is therefore costly. The principle adopted for Petro-Forge hammers permits diagnosis without any additional transducers/proximity indicators, or at worst with only a small number beyond those which are required for normal operation of the system. According to the functions performed, there are five subsystems in the control system as follows: 1. Air/gas charging; 2. Back-pressure setting; 3. Pressure measurement and associated signal conditioning; 4. Ignition; 5. Interlock circuits for feeding device and/or tool set. Subsystems 4 and 5 are tested automatically during every firing cycle. Unless they pass the test the machine will not fire and the VDU displays a "fault" message with a list of diagnostic routines. The operator can choose whichever subsystem he suspects, or alternatively the computer runs through all the subsystems in succession. As far as subsystems 1 to 3 are concerned, the principle on which their self-diagnosis is based can be summarised as follows: The valves contained in the pneumatic control circuit of the system, Fig. 6.23(b), have two states, on/off or open/closed. At each particular time instant of the firing cycle the state of the valves forms the "normal valve state pattern". A fault arises when this pattern is disturbed, say, by some valve sticking. The principle of automatic self-diagnosis adopted involves the deliberate introduction of a fault, by changing the state of one valve and investigating the consequences of this. If the consequences are as expected, the valve tested works satisfactorily. Each valve is tested in turn and when one is found which is faulty, this has to be repaired before the diagnostic procedure can continue. As far as the operator is concerned, this is all that he needs to know about the diagnostic routines. They can detect a total of 19 different faults which may arise in subsystems No. 1 to No. 5. The time taken for a complete diagnostic test is about 13-15 seconds~ provided all elements work satisfactorily and no fault is detected. The logic on which the diagnostic routines are based is complex; it is discussed in detail in relation to subsystem No. 1 in [6.27] (see also [6.26]). Only one feature of the method needs mentioning in the present survey. This is that as far as subsystem No. 1 is

168

S.A. TOBIAS

concerned no additional transducer/sensor is required; the pressure transducer S, in Fig. 6.23(b), already needed for the control of the charging process, is used also for self-diagnosis. (j) Development of computer integrated forming system. Petro-Forge hammers equipped with the new control system can be operated by a central processor with only limited involvement of an operator; his job is largely to ensure that the material is available and the dies have been installed and that the necessary safety precautions have been taken. The initial blow energy (single stage operation) or energies (multi stage operation), with which the hammer starts the production process, as well as the number of components to be produced, are set by the central processor. The micro-computer control system of the machine, having adaptive capabilities, adjusts the blow energy/energies to ensure that the component is formed and holds it/them within close limits unless variations in the flow stress of the material or its temperature necessitate energy compensation. Current work on the micro-computer control of Petro-Forge hammers aims at the development of a Computer Integrated Forming System (CIFS), a diagrammatic picture of which is shown in Fig, 6.25. This consists of a central processor (IBM PC with hard disk), as the central management organ, to which are linked a number of peripheral processors performing the commercial, technical and control functions relating to departments or individual machines.

CUSTOMER

I

"

--

I PROCESSOR

I

~

1

~

tt

f fS CUSTOMER

FIG. 6.25. Diagrammatic representation of computer integrated forming system (CIFS).

. Requests for quotations are received by the micro-computer (CBM 8096) running ESCOQU, a program for ocmputer aided estimating, costing and quoting. It works in conjunction with the central processor, responsible for the scheduling of the production program and the materials store supervisor (CBM 8096).

Development of Petro-Forge Forming Machines

169

FIc. 6.26. Most advanced stage of development of combustion actuated hammers. (a) Petro-Forge Mk.IIG. (b) Petro-Crop System GP Series G.

2. Firm orders received in response to quotations issued by ESCOQU are processed by CIRCON. This is a CAD of dies program, (running on a Tektronix 4054 computer) [6.28, 6.29, 6.30], which evolves from the geometric shape of the finish-machined component a forged shape, by adding forging allowances, draft angles, etc. It also calculates billet weight (dimensions) and decides whether the component should be produced by a single blow or in multiple stages and the blow energy required by each of these. These and other process parameters (material to be booked/ordered, dimensions of billets to be cropped and initial energy setting of cropping machine and of single or multi-station forging hammers, number of components required) are communicated to the central processor which stores them. CIRCON also generates an NC tape for machining the electrodes of dies so that these can be spark eroded or produced directly on a die sinking machine. 3. Production commences in accordance with the schedule determined and updated by the central processor. This also sets the initial blow energy of production machinery (cropping and single/multi station forging machines). Each of these is adaptively controlled by its own micro-computer (CBM 8032). 4. Finally, the finished components are packed and dispatched by a processor controlling shipping and the issuing of invoices. The CIFS is in an advanced stage of development. The network linking the various micro-computers to the central processor is operational. Some of the software, for instance CIRCON, is highly developed, having been tested extensively and used for designing dies for conventional forming machines. The Petro-Crop System GP (Fig. 6.14), the Slow Speed Petro-Forge (Fig. 5.12) and a Petro-Forge Mk.IID have been converted to micro-computer control. The Petro-Crop System SP (Fig. 6.18), the robot-fed double station installation (Fig. 6.5) and the Wolfson Three-Station Forging

170

S . A . TOBIAS

Machine (Fig. 6.3) are in the process of being converted. Artist's drawings of the Petro-Forge Mk.IIG and the Petro-Crop System GP, Series G, are shown in Fig. 6.26(a) and (b); these represent the most advanced stage of development of combustion actuated hammers. The CIFS is being developed around the available forming capacity--which is made up mainly from Petro-Forge machines. Nevertheless, the system will contain also conventional machines and it will be able to produce components by hot, warm and cold forming. It is intended that the principles evolved and the software produced should be of relevance also to plants consisting mainly of slow speed machinery.

7. THE EFFICIENCY OF HERF HAMMERS AND THE CLASHLOAD

7.1. Overall efficiency o f H E R F h a m m e r s

As has already been mentioned, the operating principle of work-restricted forming machines involves the conversion of the kinetic energy of moving masses just before impact into forming work. This conversion is inevitably accompanied by losses. Friction can be ignored when the losses are related to the kinetic energy available at the point on the total stroke at which forming commences. However, during the forming stroke a significant amount of energy may be lost into the foundation and the elastic structure of the dies and the machine. The former is dissipated, causing disturbances in other machines or a neighbourhood nuisance. The latter manifests itself by a stressing of components, resulting in a rebounding of the tup(s) and a substantial increase of the general level of vibration and noise. Conditions arising will be explained by examining two extreme conditions: (i) when the machine can be assumed to consist of two perfectly rigid masses (modulus of elasticity infinite), and (ii) when the machine can be represented by a mass-less spring or, as the second order approximation, by a spring-mass system. Considerations based on (i) lead to the blow efficiency denoted by "Ob, which measures how much of the kinetic energy will become available for forming. Considerations based on (ii) lead to the efficiency o f energy transfer (forming efficiency), denoted by -q,,, which measures how much of this available kinetic energy is in actual fact converted into forming work. Hence the overall efficiency of the machine is given by the product

n = nb rl,,.

(7.1)

It will be seen that the blow efficiency is of minor importance, being relatively high because of the design configurations adopted in practice; it is of interest mainly in relation to the design of the machine suspension. The efficiency of energy transfer is dominant since it is dependent on the "hardness" of the forming blow. 7.2. B l o w efficiency [7.1] Assume that the hammer consists essentially of two perfectly rigid masses whose movement is confined to a vertical axis. The upper moving mass will be denoted as m r and the lower as m/, and the corresponding impact and recoil velocities by Vpo, Vf>, and vz,z, vtT, respectively. On the basis of the principle of the conservation of momentum and considering that residual kinetic energy losses are at a minimum when upper and lower masses have a common velocity after completion of deformation, i.e., vr,~ = vtz, the momentum equation can be written as ml, vl,,, + m~vt~, = (ml, + mt.)vlz.

(7.2)

Development of Petro-Forge Forming Machines

171

The blow efficiency "qb is defined as the ratio of the useful work available for forming E,, to the total kinetic energy at impact El, this being made up of the sum of the kinetic energy of the upper mass Epo and that of the lower mass E1o, both at the instant before impact. Thus

"qb-

E, Ei

E,, Er ° + Er ° - 1 -

(7.3)

(me + mf)vfl2

(mr~Vpo2 + mfVfo2)

.

Substituting vg from equation (7.2) introduces the condition for the maximum efficiency of blow which a system of the type considered can attain and hence

(mpVp° + mfvf°)2 xlhm~x = 1 -

(7.4)

(rnp + mf)(mpVpo 2 + myVfo2) "

Two special cases can be identified: (i) The momenta of the upper and lower moving masses are equal and opposite at the instant of impact, i.e., mpVpo = -mfVfo then rlb,n~ = 1. This means that the total kinetic energy of the moving masses can be converted into plastic work and, according to equation (7.2), upon completion of the deformation the moving parts are at rest; there is no residual kinetic energy to be absorbed and no possibility of transmitted shock. This condition arises in counterblow machines (see Section 2.2, Fig. 2.3(c)) in which the motion of the two tups is synchronized, as for instance in the case of the Counterblow Petro-Forge (Sub-Section 5.4). The same end effect can be achieved also without synchronization by adjusting the drive pressures of the upper/lower bolsters to ensure a balancing of their momenta, as in the case of CEFF machines (Section 4.2). (ii) The velocity of one of the tups (usually the lower) is insignificant at the instant of impact, i.e., Vfo = 0 and 1 "l~hm"lx

-

1 + (mJmf)

1 -

1 + Ix

(2.1)*

In this case, a predetermined fraction of the initial kinetic energy remains after impact, which will be transmitted into the foundation. The loss of efficiency is generally not very large, though it might nevertheless have a considerable nuisance effect. In hammers the mass ratio Ix = rnr/m f is recommended to be about V20, this value having been adopted for Petro-Forge Mk.I and Mk.II machines. Thus, with these the energy loss would be of the order of 5%. With single-acting counterblow type H E R F machines, such as Dynapak or PetroForge (see Section 2), the conditions arising are more complex since in these the machine frame is supported on soft springs. The theoretical model of such machines is shown in Fig. 7.1. With this, the previously discussed two cases arise for extreme values of the spring stiffness Ks. When the suspension stiffness is very low, i.e., K~ ~ 0, then the model approximates to that discussed in case (i): The efficiency of the blow will be "qb,~,x --- 1.0. When the suspension stiffness is very high, i.e., K,. ~ ~ , then case (ii) is approximated and the efficiency of the blow will be given by equation (2.1). For the general case, the differential equations of motion of the system shown in Fig. 7.1 are derived on the basis of the following assumptions [7.1]: The structure is in equilibrium until at time t = 0 energy is released by the application of the step function force p A = const., which is the product of the mean effective driving pressure p and the effective piston area A. This force drives the piston/platen downwards and simultaneously, an equal but opposed force p A will lift the frame upwards. Damping in the suspension * This equation was originally presented on p. 110.

172

S . A . TOBIAS i

PISTONAREAA , \

MASSOF PLATEN- . - - - _ _ _ _ .

MEANEFFECTIVEPRESSURE, p

,J

PLATEN DISPLACEMENT,yp

~

FRAMEDISPLACEMENT,yf

MASS OF FRAME

SUSPENSIONSTIFFNESS, Ks

FIG. 7.1. Theoretical model of single-acting counterblow machine.

with stiffness K~ will be neglected. The equations of motion in terms of the displacement yp of the platen mass mp and that of the frame Yt, having a mass mf lead to the following expressions: Platen displacement

yp = (pA/m,, + g) (/2/2).

(7.5)

£~ = [(pA + mpg)/KJ[1 - cos, (Kjmt)t ].

(7.6)

Frame displacement

By differentiating these: Platen velocity yp = Vp = ( p A / m p

+

g)t.

(7.7)

Frame velocity yt.= vf = - [ ~ A

+ mpg)/K~.][, (KJmf)sin,

(KJmf)t].

(7.8)

Multiplying equations (7.7) and (7.8) with ml, and m r, respectively, yields the platen and frame momenta. Equations (7.5) to (7.8) are valid until platen and frame collide at time t = to at a value ofypo - Yfo = Y, where Y is the working daylight. Equation (7.5) states that the platen is descending with the constant acceleration (pA/mp + g) which for p = 0 is reduced to the gravitational acceleration g. Equation (7.6) states that the frame performs a simple harmonic motions, with a frequency f = 2~vv K~./mf Hz, about a datum position which is vertically displaced from the equilibrium position by the distance (pA + mpg)/K~., equal to the amplitude of oscillation. The platen and frame motions of a Petro-Forge Mk.IIF will now be investigated for two values of the spring stiffness K~,, corresponding to frame suspension frequencies of 3 and 8 Hz. For each case, conditions arising for the maximum and minimum blow energies, Epmax and Epmin, and the corresponding mean driving forces (pA)m,x and (pA)m#, will be considered. The data used is summarised in Table 7.1.

Development of Petro-Forge Forming Machines

173

TABLE 7.1. DATA FOR INVESTIGATING THE EFFECT OF SUSPENSION STIFFNESS ON FRAME AND PLATEN MOTIONS

Suspension frequency Suspension stiffness

f K,

= =

3 Hz 1279 kN/m

8 Hz 9096 kN/m

Frame mass Platen mass Platen stroke (day light) Max blow encrgy

mr rnt, Ep .......

= = = =

36(10 kg 180 kg 250 mm 27.6 kJ

3600 180 250 27.6

Min blow energy Max driving pressure Min driving pressure

Em,,n

=

1.725 kJ 110 kN 6.9 kN

1.725 kJ 110 kN 6.9 kN

Y

(pA) ...... = (PA)mi,, =

kg kg mm kJ

The platen/flame displacements calculated with this data and equations (7.5) and (7.6) are presented in Fig. 7.2(a) for the suspension frequency o f f = 3 Hz and Fig. 7.2(b) for the f = 8 Hz case. In both the figures, the parabolic curves represent the stroke-time function of the platen, as described by equation (7.5), and the wave, or part of it, gives the motion of the frame, as given by equation (7.6). The co-ordinates of the intersection between these two curves correspond to the time instant t (abscissa) and the distance above the equilibrium position h (ordinate) at which platen and frame collide. Full and chain-dotted curves represent conditions arising for the minimum and maximum blow e n e r g i e s , Epmin and Epmax , respectively. Finally, the blow efficiency for each collision point, as calculated by using equation (7.4) is given in the bottom line. Considering the 3 Hz suspension (Fig. 7.2(a)) first, note that, for the minimum blow energy Epmi,, platen and frame collide just slightly after the frame has passed 1/4 of a cycle of oscillation, with a blow efficiency of - 9 8 % . For increasing values of Ep the collision point shifts towards the origin and the efficiency increases and reaches - 1 0 0 % for Epm,x. In this latter case, practically all the kinetic energy of the moving parts will be converted into forming work; the only ground reaction arising will be that which occurs when the frame settles back onto its springs under the influence of gravity, at the completion of forming. With the 8 Hz suspension (Fig. 7.2(b)), for Epmincollision takes place after the frame oscillation has passed its peak amplitude, moving in the same direction as the frame, though at a much lower velocity. Under these conditions, the blow efficiency is 94% which manifests itself by part of the platen kinetic energy being used for imparting an impulse to the frame as a result of which it will not just settle on its springs, but oscillate with an increased amplitude. This condition is clearly undesirable since the next forming cycle can start only after these oscillations have subsided due to friction. Increasing blow energy shifts the collision point toward the origin and when it coincides with a peak (or trough) the frame is stationary at the instant of impact; the blow efficiency will be given by equation (2.1) which in the example under discussion yields 95%. In the case of Fig. 7.2(b) a further increase of the blow energy leads to an improvement of the blow efficiency to reach - 9 8 % a t Epmax. To sum up, the importance of the blow efficiency lies not so much in the energy losses which can arise, which are relatively small in comparison with those due to the efficiency of energy transfer, to be discussed next. Its significance lies in the nuisance value of a low level of efficiency causing two effects: Firstly, a large ground reaction, and secondly, a prolonged oscillation of the frame following completion of forming. For minimising the two effects, the frame suspension frequency should be as low as possible, preferably so low that the time taken by the platen stroke to at the lowest blow energy Epmi, , should be 1/4 of the cycle time or less. A more refined analysis of conditions arising, which takes into consideration the variation of p as a function of the piston stroke, is given in [7.2].

174

S.A. TOBIAS 3 Hz SUSPENSION

8 Hz SUSPENSION ?so i

\

,\

i's°i YP ° I yf~ "~

(

\

~'E p~ax

;,0o

?\

vg~moxl

~-eprn,n

I ,

=~s

I

i

i

s**

....3.'" j_._ 0

,o

~o ,o

....

,oo ,~0

TIME-f ms¢c l a°,°,,

[ BLOW EFFICIENCY

,o

~100%

,v98%

20 /-0 60 80 11} 12,0 I,Ocycte I TIME-t msec [ 98%

(a)

94%

(b)

Fro. 7.2. Calculated platen and frame displacement of Petro-Forge Mk. IIF hammer for maximum and minimum blow energy. (a) Soft suspension, f = 3 Hz, (b) Hard suspension, f = 8 Hz.

7.3. The efficiency of energy transfer [7.3, 7.4, 7.5] Not all the energy available at the beginning of the forming stroke Ei is converted into forming work E,. The process is accompanied by losses, some of the initially available energy being used for elastically straining the dies and other components, and the energy thus lost will be denoted by Es. The efficiency of energy transfer (forming efficiency) can therefore be expressed as

E,, n,, =

Ei =

E i - Es Ei

-

1

E, E~

(7.9)

where E, is the useful work that actually goes into the forming process. A first order approximation of E, is easily found by assuming that the machine can be represented by the equivalent system shown in Fig. 7.3. In this m represents the mass of the moving tup(s) which has/have the impact velocity vi, giving an impact energy of E~ = Vz m v~2. The billet to be formed rests on an equivalent spring which takes up all the strain energy absorbed by the machine, including the tup(s) and dies. The distributed mass of this spring is neglected and its stiffness is denoted by Km. While the workpiece is formed the machine structure is strained; the equivalent spring undergoes an equivalent deflection s, producing an equivalent structural load P(s) = Kms. This can be taken to be equal (but opposed) to the forming load F deforming the billet. The energy stored in the equivalent spring is E, = 1/2 K,,s L. Thus, according to equation (7.9), the efficiency of energy transfer can be expressed as

,q,~ = 1

Km s" 2 Ei - 1

2 P max 2 KmEi

(7.10)

where Pmox is the terminal structural load at the end of the total equivalent elastic deflection s .........

Development of Petro-Forge Forming Machines

MOVINGMASS---~

BILLET

,

175

m, vi ']1_L h-"

FORMINGSTROKE

~._~_ s.,,-MACHINEDEFLECTION "7..~" EQUIVALENTSPRINGK,m ¢1////////////,/~

FI6. 7.3. Equivalent system of impact forming machine.

It is clear that for each value of the impact energy Ei there is a terminal load structural P,,~,x for which the efficiency "qe = 0. This is the load which arises when all the impact energy is used for straining the machine structure. It occurs under practical conditions when the dies are allowed to clash without performing any forming operation. Putting "lqe = 0 in equation (7.10) gives the clash load as

Pc= ~ 2 K m E i

(7.11)

which substituted into equation (7.10) yields the efficiency of energy transfer as

E, _ 1 -

"qe- Ei

(7.12)

where the clash load Pc is dependent on the impact energy Ei as set on the machine, in accordance with equation (7.11). The clash load Pc as a function of the impact energy Ei for five values of the equivalent machine stiffness Km, is presented in Fig. 7.4. Note that in the derivation of equation (7.12) the manner in which the forming load F varies as a function of the stroke h has been left unspecified. Hence that equation is valid for all forming operations, is dependent solely on the ratio of the maximum load to which the machine structure is exposed, Pmax Fmax, and the clash load Pc corresponding to the impact energy set. There are two further conclusions of practical interest: (1) To the degree of approximation used, the efficiency of energy transfer is independent of the impact speed v,. It will be shown later that this is not correct to the second order approximation; high speed machines are more efficient than low speed devices. (2) Figure 7.4 shows that the clash load is affected relatively little by the impact energy Eg. For instance, taking a machine with an equivalent stiffness of K,,, = 5.257 MN/mm (1.34 × 104 tonf/in), if the impact energy is reduced from E~ = 13.6 kJ (10,000 ft lbf) to E~ = 1.36 kJ (1,000 ft lbf), then according to Fig. 7.4 the clash load drops from Pc = 12 MN (1,200 tonf) only to Pc = 3.8 MN (380 tonf). =

7.4. Efficiency of energy transfer as a function of forming stroke Equation (7.12) can be used for investigating in detail the efficiency of energy transfer for two simple idealised forming operations. These will be characterized by the manner

176

S . A . TOBIAS EOUIVALENT STIFFNES

73.36 MN/mm 14.59 MN/mm

z

~u

30

I

20

Iiii i /

10 I"

I J

,,

J

I

/

3

d

2 /

0"1

llL

I" 0"2

0"3

J

J 20

IMPACT ENERGY

f~" •

jv

I"

f

f J

i

rJ

j

J

j

1.46 MN/mm

I

0.50 MN/mm

j l

f

I

I

J

/i fl

0"5 0"7 1'0

I"

lJ j f

J

5

7

"

~f"

j<

5.257 MN/mm

I" f

JCIT

t

I

J

30

-

ff

5"0 7"0 10

20

I i

ii

! I

i

30

] 50

E i kJ

FIG. 7.4. C l a s h load as function of i m p a c t e n e r g y for series of v a l u e s of e q u i v a l e n t stiffness K,,,.

in which the forming load F(h) vanes as a function of the forming stroke h: (1) F varies linearly with forming stroke h. This type of load stroke characteristic arises when upsetting billets with a small diameter/length ratio to a reduction less than 50%. To consider a practical case, it will be assumed that for a particular machine the equivalent static stiffness of the structure is again g m -- 5.257 MN/mm (1.34 × 1 0 4 tonf/in). If the operation is carried out with an impact energy of Ei = 13.6 kJ (10,000 ft lbf) then according to Fig. 7.4 (equation (7.11)) the clash load is Pc. = 12 MN (1,200 tonf). Choosing now some value of F. .... = P m a x then the forming efficiency ~qe is obtained with the aid of equation (7.12). The corresponding stroke h is found by considering that E, = ~q,,Ei = I/2F,,,xh. The resulting data is plotted in Fig. 7.5. This shows the variation of the efficiency ~qe (chain dotted line), as well as that of the maximum forming load Fmox (full curve) as a function of the forming stroke h. The interrupted line in the figure gives the variation of the forming load when it is assumed that all the impact energy Eg is converted into forming work (efficiency 100%) in which case the forming load is obtained from F,,ax* = 2 Ei/h. As can be seen, for relatively long forming strokes, when the efficiency is almost 100%, the Fm,,x and the F, ..... * curves almost coincide. They deviate progressively as the strokes become shorter, i.e. the blows become harder, the Fm,x curve ending in the clash load Pc = 12 MN (1,200 tonf). The maximum forming load F m a x and the efficiency xl~ for a series of values of the impact energy Ei are shown in Fig. 7.6(a) and (b), respectively. The latter figure indicates that the forming efficiency increases with a decrease of the impact energy Ez. (2) F=constant over the forming stroke h. This type of load characteristic arises in extrusion processes. The forming efficiency -q~ is obtained in the same way as before, except that F m a x is now calculated from E, = "qeEi -- h F,,,x. Using the data of the previous example, the resulting graphs are identical with Fig. 7.5 (or Fig. 7.6), except for a halving of the abscissa scale. Clearly, the efficiency of extrusion operations is higher than that of upsetting. 7.5. Overload due to partial clashing of dies When using impact forming machines for hot/warm/cold operations, the ram/tup

D e v e l o p m e n t of P e t r o - F o r g e F o r m i n g M a c h i n e s

i

1

I

14

I

z

I'

J

I I

10

~

E i = 13.6 kJ

I !

12

o 0 _J

~

I00

I

B

Z

177

~e

V >. o

0

z

/

x

60

'\

I

~ \.z--

F~x (100% EFFICIENCY)

J

-~ o ;7 ul _z

I

0

E

2

2O

0 0

S

10

15

20

25

30

UPSETTING

2.5

5

7..5

10

12.5

15

EXTRUSION

FORMING S T R O K E - h mm FIG.

7.5. V a r i a t i o n of m a x i m u m f o r m i n g load F,,,,~ a n d f o r m i n g efficiency as function of f o r m i n g s t r o k e h for u p s e t t i n g and e x t r u s i o n for El = 13.6 kJ (10,000 ft lbf). 12 11 100

10 ~

9

u.

8

' '<

o

7

9O

IMPACT ENERGY

~

d

1/

IMPACTENERGY-.

--.

L1

6

-

Ei kJ

13.6 --1o.9 rr 8.2 ~E ~ --5.4 ~x 4 ~ | l ~ ; ~ 2.7 ~

.~

o z m

70

U. UJ

60

Z_ ~E ¢t"

so

o

4(1

3

t kd

--

~,~

1

30 20

1 o

O

I

0

2.5

S

10

I0 15 S

7.5

20

25

30

UPSETTING

10

1;'.S

15

EXTRUSION

0 0

S

10

15

20

25

30

UPSETTING

2.5

S

7.5

10

12.5

15

EXTRUSION

FORMING S T R O K E - h mm

FORMING S T R O K E - h mm

(a)

(b)

FIG. 7.6. V a r i a t i o n of m a x i m u m f o r m i n g load Fm,,x a n d f o r m i n g efficiency as function of s t r o k e h. (a) m a x i m u m f o r m i n g load. (b) F o r m i n g efficiency.

stroke should ideally be controlled by a stalling of its motion in the plastically deformed material. The alternative method of controlling component dimensions with stroke limiting clash surfaces, absorbing a small amount of excess energy, should be avoided; partial clashing of dies generates high excess loads.

178

S.A.

TOBIAS

It can be shown [7.4, 7.5] that the excess load ratio can be expressed as

aPm,,~ [ i i --'q~) R

/3~..... =

J~

+ 1

(7.13)

-

where AP,,,,,x = excess load, partial clash load, = impact energy for completing workpiece, Ei AEi = excess impact energy, R = AE/Ei energy ratio, = forming efficiency for completing workpiece with El. Vl,, Equation (7.13) is plotted in Fig. 7.7 for a series of values of the forming efficiency -q,,. Note that for a given excess energy ratio R, the excess structural load ratio AP,,,,x/Pm,,x increases with the forming efficiency. Moreover, since according to Fig. 7.6(b), for a constant forming stroke h, ~qe increases for decreasing impact energy Ei, the percentage excess structural load is much greater at low energy levels than at high ones, though in absolute terms the conditions are reversed. ~e

I -90%

=i

/

na
!-

4"00

o_ rl <

ff75

q ~ w o x w

0"50

0"25

0

'/~

0

0"05

.-

60 %

0"1

0"15

0 '2

0'25

E X C E S S E N E R G Y RATIO - AE}./E~

FIG. 7.7. E x c e s s l o a d as f u n c t i o n of excess e n e r g y r a t i o f o r series o f v a l u e s o f f o r m i n g e f f i c i e n c y .

In the derivation of equation (7.13) (Fig. 7.7) no assumption is made as to the manner in which the forming load varies; it is therefore valid for all forming operations. Thus, in general terms, if dimensional control with the aid of clashing surfaces cannot be avoided then the excess energy used should be as small as possible, irrespective of the level of energy required. 7.6. Determination of the equivalent machine stiffness The equivalent machine stiffness Km is found by considering that with the aid of equations (7.12) and (7.11) the strain energy absorbed by the machine structure during forming is

E,.= E i - E , = ( 1 - ' q e ) E i =

El-

2Kin

, 14,

which says that the square of the maximum structural load, (Pmax)2, is a linear function of the strain energy absorbed by the machine E , the slope of which is 2Kin.

D e v e l o p m e n t of Petro-Forge Forming Machines

179

The impact energy E i is known, either as a machine setting, or found by measuring platen velocity just before impact occurs. The useful (forming) energy E,, is determined by integrating the forming load stroke function F(h) over the deformation h; thus Es is known. Experiments carried out with a Petro-Forge Mk.IID have shown [7.4, 7.5] that when upsetting at a constant input energy Ei (= 13.6 kJ = 10,000 ft lbf) billets with a range of sizes, then (Pmux)2 plotted as a function of Es is indeed a linear function; all measured points lie close to a straight line passing through the origine. This tends to confirm the original assumptions on which the derivation is based. Half the slope of this line is Km =

5.257 MN/mm (1.34

x

10 4

tonf/in)

The clash load Pc of the machine is found by extrapolating that straight line up to an impact energy of Ei = 13.6 kJ (10,000 ft lbf) and it is found to be P~. = 12 MN (1,200 tonf). 7.7. Second order approximation of the efficiency of energy transfer [7.6] It has been pointed out that to the first order of approximation, the efficiency of energy transfer "qe is independent of the impact velocity vi. It does not depend on the dynamic characteristics of the machine structure or the impact duration or some other factors either; these parameters simply do not enter into equation (7.12). For a second order approximation, it is assumed that the machine structure can be represented by a single degree of freedom system excited by a force pulse simulating the forming operation. The assumed equivalent dynamic system of a single-acting forging machine is shown in Fig. 7.8(a). It consists of an equivalent mass M, representing the anvil, and an equivalent spring with stiffness K, representing the anvil and its support. The billet to be formed is resting on the anvil (M) and is deformed by the kinetic energy of the platen (m) being transformed into forming work.

(a)

~/////////////////////////. z

I"FI(t

0 J

--

Vz(t)----.

(b)

-

_z ~ 0 u.

0

l c To TIME

Tt -

t sec

FIG. 7.8. Equivalent system of impact forming machine to the second order approximation. (a) Model of machine/die system. (b) Model of process.

I'ITDR 2 5 : 2 - F

180

S . A . TOBIAS

Note that K is not identical with the Km used in the elementary theory. These two constants are determined differently. Km is found experimentally, as explained in Section 7.6. K is calculated by determining the spring stiffness of that part of the machine which supports the anvil. The reason for expecting a difference between these two machine parameters is the inertia loading caused by the anvil mass. Thus, for a Petro-Forge Mk.IID, K and K,, are found to be K = 1 . 0 M N / m m and K., = 5.257MN/mm. In addition the equivalent masses M and m are M=

3000 kg and m = 150 kg.

During the forming operation, while the billet height is reduced from hh to hf- (Fig. 7.8), two equal and opposed forces F(h) are generated which act on both the anvil (M) and the platen (m). The form of the force function F(h) is determined by considering the conditions arising from t = 0 onwards; i.e. the time instance when the platen touches the billet (Fig. 7.8(a)), until the forming operation is completed at t -- T,. Initially, from t -- 0 until t = To the billet is deformed elastically until the stress in the material reaches the flow stress. During this time interval the forming force varies non-linearly with the forming stroke (billet deformation) h as h

F,(h) = Eb Ah h ; - - h

(7.15)

where Eh = modulus of elasticity of billet material, Ah = initial cross-section area of billet. From time t = T,, until t = T, the billet is deformed plastically and in this regime the forming force can be represented by an expression due to Siebel (see [2.3] p.44 or [7.7] p. 113)

[ where (~ A D c ho

= = = = =

cD il

(7.16)

flow stress of billet material, instantaneous billet area, instantaneous billet diameter, coefficient of friction, billet height at t = To, when forming commences.

Fl(h) and F2(h) as a function of the time are depicted in Fig. 7.8(b). Under normal forming conditions the work done by the elastic component Fl(h) is very much smaller than that done by the plastic forming force F2(h) and then the former can be neglected. The elastic part of the total forming work becomes significant with hard blows, with coining, and particularly when the dies are allowed to clash. In the last mentioned case the plastic part of the forming work becomes negligible. Temperature and strain rate effects are ignored and hence the conditions assumed bear some resemblance to those arising in the cold upsetting of billets. For the sake of arriving at an explicit solution, from which practical conclusions of general validity can be derived, it will be assumed that both Fl(h) = fl(t) and F2(h) = f2(t) are simple linear functions of the time t, as indicated in Fig. 7.8(b). This is an approximation, though a good one, as is shown in [7.6]. With these assumptions the equations of motion for the anvil x(t) and platen y(t) are easily derived. Solving these, introducing the appropriate initial conditions and consider-

Development of Petro-Forge Forming Machines

181

ing that the efficiency of energy transfer is defined as the ratio of the work consumed in the plastic deformation of the billet to the input kinetic energy, leads to an expression which gives tie as a function of the system parameters. This is a complex function and hence only some of the important practical conclusions derived from it will be discussed. For this purpose the equivalent constants of a Petro-Forge Mk. l i D machine, specified by K,, for the first order and K, m, M for the second order approximation, will be used. 1. High-speed machines are intrinsically more efficient than low-speed machines. This will be seen from Fig. 7.9(a) which shows the variation of the forming efficiency as a function of the impact energy Ei, for three values of the mass ratio Ix = m/M. The figure contains also four chain-dotted lines which signify constant impact velocity conditions. It shows that when the impact velocity is increased then the forming efficiency also increases. For instance, for an impact energy Ei = 10 k J, increasing the impact velocity from vi 5 m/sec to vi = 10 m/sec results in increase of 13% from "qe = 79% to = 92%. Such an increase of the forming velocity must be accompanied, of course, by a decrease of the platen mass m and hence a drop of the mass ratio from Ix = K~.s5 to Ix = 1/15.38since otherwise the impact energy would not remain constant. =

2. An increase of the impact velocity results in an increase of the forming stroke (billet deformation) h and this increase is more pronounced at high energy levels. This follows from the previous conclusion as can be seen by replotting the data contained in Fig. 7.9(a) to give the forming stroke h as a function of the impact velocity v~ for three mass ratios Ix, the resulting graph being presented in Fig. 7.9(b). This figure contains also three lines (interrupted) corresponding to constant energy conditions and these prove the statement made. 100

Y 1/20

7~ I >-

114

1/6,67

1/20

15 ( 3/

I 10

5

IMPACT VELOCITY - v t m/see ~

E w

5c

z ¢~

10kJ

25

5

C IO

20

IMPACT ENERGY

(a)

-

Io

Ei kJ

IMPACT

VELOCITY

20

-

v I m/sec

(b)

FIG. 7.9. Inter-relationship between process variables. (a) Efficiencyof energy transfer as a function of impact energy. (b) Forming stroke as a function of impact velocity, for three values of the mass ratio.

3. An increase in the initial billet diameter Dh results in a drop of the forming efficiency and this drop is more pronounced at the low energy levels. The reason for this is that the energy available for forming is used for causing elastic straining first and after this has reached a certain level, plastic deformation. Increasing the billet diameter causes the amount of energy going into the elastic straining to increase and hence the remainder available for plastic deformation must decrease. Nevertheless, given a final billet height, the initial billet height should be chosen to be as low as possible, consistent with the amount of plastic working needed for producing satisfactory components. The theoretical variation of the maximum forming load as a function of the forming stroke h, to the first order approximation, has already been shown in Fig. 7.5 (curve marked Fmax); it is repeated in Fig. 7.10(a). Fm,x to the second order approximation is

182

S.A. TOBIAS Z

z

14

h

'=12

UPSETTINGOF STEELBILLETS MATERIAL: En8 STEELINITIAL bIAMETERS " • 28.6 mm ~ 38.2 mm-

I

o

31.8 mm

o

47.8 mm

= 'fr ~ ~z (r~ 0 1068" ~ "

0,,

4

~ x<

2

I

"

Db mm __

10

I

• o

(D < 0

8

7

6

n," 0

4

o

x <

0

0

o

o

lO

15

20

25

I

i

10

20

STROKE (a)

30

30 FORMING

FORMING

31 "8

~NCk~ ~'& 0 0 0

2 p,~.^

28"6

38.2 {2 L7"8

,-I I

I

~

INITIAL BILLETDIAMETERS

STROKE

- h mm

- h mm (b)

Flo. 7. l(I. Predicted maximum forming load as a function of forming stroke with experimental results. (a) First order approximation. (b) Second order approximation.

presented in Fig. 7.10(b). Both graphs are for an impact energy of Ei = 13.6 kJ and they contain experimental results obtained by the cold upsetting of steel billets in the diameter range of Db = 28.6 mm, = 31.8 mm, = 38.2 mm and = 47.8 mm, of varying height, therefore of varying billet volume and aspects ratios. Experimental results pertaining to each of these diameters are marked by individual symbols, as given in the figures. As far as the first order approximation is concerned (Fig. 7.10(a)), correlation between theory and experiments is not unreasonable, bearing in mind the simplifying assumptions adopted. However, the variation of each group of experimental results, corresponding to a particular billet diameter Dh, does show a regularity which cannot be attributed to scatter. The second order approximation (Fig. 7.10(b)) predicts the trend of the experimental results, though it gives values of the F,,~,, which are generally on the low side. This discrepancy may well have been due to strain rate effects which were neglected in the theory. The clash load, to the first order approximation, is dependent on the impact energy Ei and the equivalent machine stiffness K .... as described by equation (7.14) and Fig. 7.4. The second order approximation does not permit the derivation of an explicit expression of the clash load. However, it does allow the investigation of the effect of a variation of the parameters m, M and K. It can be shown [7.6] that the clash load is hardly affected by these parameters and is almost entirely dependent on the impact energy El. The important and interesting conclusion to which the second order approximation led, that is, that the forming efficiency is velocity dependent, was established also in [7.9]. This investigation was concerned with the analysis of a particular laboratory drop hammer, the anvil structure of which was represented by a two-degree of freedom system which during the forming operation became coupled through a non-linear force (forming load) to a single-degree of freedom system representing the platen. This equivalent system can therefore be considered as a third order approximation of conditions arising. However, because of the complexity of the resulting mathematical equations, it cannot be used for a general discussion of the problem of the efficiency of energy transfer. The same applies also to the investigation presented in [7.10].

Development of Petro-Forge Forming Machines

183

8. THE FORMING CAPACITY OF HERF HAMMERS AND CONVENTIONAL MACHINES

8.1. Comparison of HERF hammers with conventional forming machines A comparison of H E R F h a m m e r s with conventional machines is difficult because of the different manner in which their performance is specified and the different way in which they are used in production. HERF h a m m e r s are specified in terms of blow energy, i.e., the impact energy available for conversion into forming work. Drop hammers are classified on the basis of their tup weight. Friction screw presses are characterised by the permissible maximum forming load and generally the clash load as well. Crank and similar presses are described in terms of the maximum forming force they can exert over a given part of the stroke preceding the bottom dead centre. For hydraulic presses the maximum forming load is given. To make a comparison possible some c o m m o n denominator must be found and it may seem reasonable to use for the purpose the m a x i m u m blow energy, as it was proposed in [8.2]. H o w e v e r , this on its own is not enough, as the examples presented in Fig. 8.1(a) will show. The figure depicts the silhouettes of four hot forging machines, each capable of exerting blows with a m a x i m u m energy of 20-27 kJ (15.000-20.000 ft lbf), under continuous operating conditions. Figure 8.1(b) shows their speed characteristics.

PETRO-FORGE n = 6 0 rain 1 , Fm = 8 M N

SCREW n = 30

DROP

PRESS

rnin I , F

HAMMER

ECCENTRIC

PRESS

n ~ 6 0 thin1 , Fm = 1 0 M N

n = 4 0 r n ~ '~

= 3 MN

r/ I t I L..__

(a)

I I t _

(,') PETRO-FORGE

E

SCREWPRESS

15 \

/

ECCENTRIC PRESS

DROP HAP~ER

.-'/

J

IO

"

O 0

(b)

-

• 0.3

•

• 0.6

0.9

TUP STROKE

-

1.2

'

1.5

m

FIG. 8.1. Comparison of forming machines on basis of forming energy. (a) Relative size of forming machines with maximum continuous blow energy of 20-27 kJ (15.(}00-20.000 ft Ibf). Cycling rate n rain ~and maximum permitted forming load F,, under continuous running conditions. (b) Variation of tup speed as a function of stroke.

Note that the m a x i m u m permissible forming load F,, of these machines varies widely, between F m = 10 MN (1000 tonf) for the eccentric press and F,,, = 3 MN (300 tonf) for the friction screw press. The Petro-Forge can exert very high forming loads, in excess of the somewhat arbitrary Fm = 8 MN (800 tonf) given, though with a very low forming efficiency. The same applies also to the drop stamp for which a m a x i m u m permitted forming load is generally not specified. There is a substantial variation also in the cycling rate. This is high for the Petro-Forge, with the intention of achieving high productivity. It is high also for the eccentric press but for quite a different reason, this being that the die/billet contact time be minimized and with this die life be improved.

184

S. A, TOBIAS

Except for a limited range of components, the four machines are not comparable or interchangeable also because of their differing performance characteristics (see Section 2, Figs. 2.1 and 2.2 and Fig. 8.1(b)) and the differing manner in which they are used. Petro-Forge, the most compact of the four, is generally employed for producing components with single blows in single impression dies. Drop hammers are almost invariably used in a multiple-blow mode. Although the forming efficiency from the third blow onwards is small, multiple blows increase the maximum component size that can be produced. With eccentric and similar presses the die area is large and multi-impression dies are the norm. However, the forming speed of crank and similar presses is low and therefore components with a small heat capacity (for instance, turbine blades) are better suited for a friction screw press or a H E R F hammer. The friction screw press occupies an intermediate position between the drop stamp and the H E R F hammer. The maximum forming load could also be used as a basis for a comparison but on its own it is also inadequate. Figure 8.2 compares the CEFF H E 55 hammer with a crank press and a hydraulic press [8.3]. The CEFF machine is capable of exerting very large forming loads (400 MN (40.000 tonf)) for a relatively small forming energy input, as the table in Fig. 8.2 shows. However, because of its small compact size, its die area is also small and this limits the range of components for which it is suitable. The hydraulic press would not be used for producing components normally manufactured on a crank press or a H E R F machine, mainly because of its very low forming speed. 10.23

7.62 m

m

E ¢0

3.0 m (=

FLOOR LI~E

E o.

t

CEFF HE 5 5 HAMMER

M E C H A N I C A L FORGING PRESS

S

WEIGHT

7 0 tonne

COS~

100 %

700 %

ENERGY

5 4 8 kJ

8 , 1 6 0 kJ

MAX. FORCE

4 0 0 MN

FORGING

VELOCITY

14 - 2 0 m / s e c

1 , 2 4 7 tonne

1 2 0 MN 0.06 -

1,5 mlse~

HYDRAULIC PRESS

6 , 5 7 7 tonne 3,400 % 8 1 6 , 0 0 0 kJ 5 0 0 MN i.03 - 0.15 m/sec

FI(;, 8.2. Comparison of forming machines on basis of maximum forming toad. "lable gives comparative weight, cost, energy, maximum forming force and forming velocities {8.2].

Clearly, no simple comparative assessment can be made between different forming machines; not only the machine specifications but also process conditions must be covered for a complete appraisal. Nevertheless, a simplification of the problem does serve a useful purpose and this can be achieved by using as a basis for comparison the maximum forming energy per blow and the maximum forming load, combined with the loss characteristic of the machine.

Development of Petro-Forge Forming Machines

185

8.2. Forming requirements and forming capacity [8.4, 8.5j The forming of any component in a single stage requires the application of a forming force F, which acting over a stroke h reaches a peak load of Fr, and demands forming energy J ,Fr

Er = [ i

F(h) dh

(8.1)

for its completion. The forming requirements of the component can be specified by Er, Ft. The component can be formed on a particular machine if its forming requirements ( E . Fr) are within the forming capacity of the machine. The forming capacity of a machine at some loading of the primary power source is specified by the useful energy E. it can deliver as a function of the peak forming load F. arising while the deformation takes place. The useful energy E. can be expressed as E , = E d -- E l -

where E. El-

Es

(8.2)

= energy per stroke available from the primary power source, = energy per stroke lost by friction in the drive of the machine, = energy lost during forming stroke by straining of machine structure.

Equation (8.2) is valid for all levels of E , but it will be taken to refer to the case when the primary power source is loaded to its nominal maximum value. E~ is a function of the forming load acting, as given by equation (8.1). Equation (8.2) can be written as Fu 2 E,, = ~f-E. - -2-K,.-

where "qf Kin

(8.3)

-- overall efficiency, including all electro-mechanical and other losses, but excluding elastic straining of structure, -- equivalent spring constant of structure, to the first order of approximation given by equation (7.14).

The forming capacity diagram of a machine is obtained by plotting the useful energy E, as a function of the peak forming load F,. The resulting curve encloses an area; points lying within this area represent operations, specified by the forming requirement ( E , Fr), which can be carried out with that machine.

Y

Y

Y

I

I

re o ._J w U. LU CO

0 Fu

Fumax=Pc

F,~ urnax

P E A K FORMING L O A D - ~

(a)

Fumax: Pc

T F° umax

MN

(b)

(c)

FI(;. 8.3. Typical forming capacity diagrams. (a) Hammer, (b) Screw press, (c) Crank press, (after [8.4]).

186

S . A . TOBIAS

The forming capacity diagram for hammers (both conventional and HERF) is of the type shown in Fig. 8.3(a). The maximum forming load F,,,,,,,.~ = P,. arises when the dies clash (Section 7.3). In that case the useful forming energy E,, = 0 since all available energy is used for straining the machine structure (dies. platen, etc.). Thus, the clashing condition is represented by point X on the abscissa in Fig. 8.3(a). The maximum useful energy E ......... is available when the peak forming load F,, is very small (soft blows) since in that case practically no energy is used for straining the structure (E, = F~,......../2 Km = 0), the corresponding point being denoted by Y in Fig. 8.3(a). Between the origin 0 and point Y the useful energy E,, is limited by the useful stroke that can be accommodated within the total stroke length and hence E,, varies linearly. The forming capacity diagram of screw presses (Fig. 8.3(b)) with a rigid flywheel coupling is similar to that of hammers. However, screw presses are generally more flexible than hammers and therefore the useful energy out-put E,, drops more rapidly with increasing peak forming load F,. Screw presses with a slipping clutch have a peak forming load which is below their clash load. The corresponding diagram is obtained by raising an ordinate line at the nominal maximum load capacity (F*, ........., point X*) and ignoring that area of the diagram which lies to the right of this. Crank or similar presses are designed to exert a maximum forming load F,~....... represented by point X* in Fig. 8.3(c). The maximum useful energy E ......... that can be taken out of the machine is represented by point Y. arising at small forming loads. As the forming load increases, more and more work is used for straining the structure and hence the diagram follows the line between Y and X*. For very small forming loads the work that can be used is also restricted by the limited stroke available and hence the diagram is closed by the line OY. Crank and similar presses are even more flexible than screw presses. The determination of the forming capacity diagram of a machine requires data on the following two characteristics: (1) The maximum useful energy per stroke E .......... available when the peak forming load F,, is very small. E ......... can be found with the aid of the coefficient of the overall efficiency of the drive "qt, since E .......... = "qrEa; E j is contained in the machine specification. (2) The equivalent spring constant of the machine K,,, or (with hammers or friction screw presses) the clash load P,., since E .......... = F~, ......../2 K,,,. With few exceptions, this data is not contained in the sales literature of machines. It can be found only with considerable experimental effort, although estimates can easily be made [5.14, 5.15]. Figure 8.4 shows the forming capacity diagram of the Petro-Forge Mk.IID hammer. Figure 8.4(a) contains two curves, corresponding to the "'nominal maximum performance" (full line) and the "ultimate performance" (chain dotted line). The maximum energy ratings corresponding to these, in terms o f E .......... were presented in Table 4.1. The equivalent spring constant K , , and the clash loads P,. were determined in Section 7.6. It should be pointed out that, in practice, very hard blows, approaching the clash load, should not be applied. Thus, in a somewhat arbitrary manner, the practical load range of the machine extends to about 8 MN (800 tonf), consisting of the "'forging range" up to 4 MN (400 tonf) and the "'coining range" above that. (The maximum load actually measured with this machine was 9 MN (900 tonf) [7.2].) As explained in Section 5.2, in the energy range of the "'ultimate performance", hard, coining type blows, should be avoided. To take this restriction into account, it is assumed that for coining blows the boundary of the forming capacity diagram can be represented by the straight line BD (dotted). Thus, the forming capacity diagram of a Petro-Forge Mk.IID machine can approximately be represented by Fig. 8.4(b). 8.3. R e l a t i v e j b r m i n g

capacity

The forming capacity of two machines can be compared by superimposing their

Development of Petro-Forge Forming Machines FORGING R A N G ~ I N I N G

RANGE

FORGINGRANGE-I_

,

"\

25

z

l/

2°tlt f f - - i : ?I

',

4

n = 60

~" I

['--'COINING RANGE

|_

_

min

= 60

-;

,

8

187

i =,

Pc=12 Pc--17

PEAK F O R M I N G L O A D - Fu MN

-]

n

~//,

o

rain

5

]

1o

~5

8 PEAK FORMING L O A D -

(a)

F MN

(b)

Fit;. 8.4. Forming capacity diagrams of Petro-Forge Mk.IID HERF hammer. (a) Full line for "'nominal maximum performance", chain dotted line for "ultimate performance". (b) Forming capacity diagram combining practically useable areas of "'nominal maximum" and "'ultimate" performance ranges.

TABLE 8.1. SPECIFICATION OF FORMING MACHINES

Weingarten screw presses Nominal forming load Max. permanently permissible load Chlsh load P No of blows for max. forming load Driving power

Model PS180

Model PSS200

3.15 5.(1(I 6.3(I 38 15

4.(X) 6.30 8.00 38 22

Model RSSP 200/401)

Model RSSP 225/500

(MN) (MN) (kJ) (kJ)

4.00 8.00 25 17

5.00 10.00 35 24

(min l) (kW)

56 38

50 38

(MN) (MN) (MN) (min ~) (kW)

Berrenberg screw presses Nominal forming load Max. load for which press is calculated Max. flywheel energy Nett impact energy at nominal forming load (minus frictional losses) Max. number of blows Driving power Komatso-Maypres cold forming presses Max. lk~rmingload No. of strokes Energy capacity Driving power

(MN) (min l) (kJ) (kW)

Model MKN 300

Model MKN 45(1

3.00 50 17 15

4.50 45 27 22

forming capacity diagrams; the areas over which these overlap will represent forming operations which both can carry out, while areas which are not c o m m o n correspond to operations which only one can perform. Figure 8.5 shows such a comparison [5.14, 5.15] between a Petro-Forge M k . I I D , Weingarten and Berrenberg screw presses and K o m a t s u - M a y p r e s cold forming presses. The essential specification of the conventional machines is presented in Table 8.1. F r o m this the data required for determining their forming capacity diagrams, i.e., "Of, Eumax, Ed

MTDR 25:2-G

188

S . A . TOBIAS

n - 38 min 1

3O

~

30

~

17.2-

d i

ZO

20 ~

\ 0

5 I ~=63

~Nb

10

i

n - 50 rain 1

20] ~ ' ~

PEAK FORMING LOAD - Fu MN

.~

.\

I

30t ~

18.920

(e) MODEL MKN 300

rl . 50 mll~ 1

.~

3

0

1

-

~

,

~

~q0

( b ) M O D E L PSS 2 0 0

n - 45 talk 1

~L.~\

,

PEAK FORMING LOAD - Fu MN

"b---.-

PEAK FORMING LOAD - Fu MN

(C) M O D E L R S S P 2 0 0 • 4 0 0

1

I\1

330~-

'

( a ) M O D E L PS 1 8 0

,,AL

~)'\, ! \,

"~

'

PEAK FORMING LOAD - Fu MN

,

n - 56 mi~'lI

-

%&~.s

PEAK FORMING LOAD - Fu MN

(d) MODEL RSSP 224/500

PEAK FORMING LOAD - Fu MN (f) MODEL MKN 450

FI6. 8.5. Relative forming capacity of conventional machines and Petro-Forge Mk.ll, Weingarten friction screw presses: (a) Model PS 180, (b) Model PSS 200, assumed overall efficiency "qt = 0.42. Berrenberg friction screw presses: (c) Model RSSP 200/400, (d) Model RSSP 224/500, assumed overall efficiency "qt = 0.42. Komatsu-Maypres cold forming presses: (e) Model MKN 300, (f) Model MKN 450, assumed overall efficiency • lr = 0.6.

and Kin, c a n be estimated by a procedure explained in [5.14, 5.15]. The resulting forming capacity curve of each machine has been shaded in Fig. 8,5 while the line corresponding to the Petro-Forge Mk.IID is drawn in full. The figures also contain the cycling rate of the conventional machines. A cursory examination of the six figures contained in Fig. 8.5 shows that the maximum useful work output of all six conventional machines was in the vicinity of the "normal maximum" output of the Petro-Forge Mk.IID but well below the "ultimate" maximum. Moreover, as far as the peak forming load is concerned, only the Berrenberg Model RSSP 224/500 screw press can exert a force larger than the maximum coining load of 8 MN (800 tonf), considered to be the upper useful limit of the Petro-Forge. The conclusions can be summarised by the following table: FORMING CAPACITY of

FORMING CAPACITY of

Petro-Forge Mk.II

corresponds to

4-5 MN (400-500 tonf) screw press (Fig. 8.5(b) and Fig. 8.5(d))

Petro-Forge Mk.II

corresponds to

4.5 MN (450 tonf) crank press (Fig. 8.5(f))

to which the following riders should be added: (1) The Petro-Forge should perform better for severe coining blows than the friction screw press because of its higher stiffness. (2) The Petro-Forge should take higher coining blows than the crank or similar press. (3) The Petro-Forge has a higher cycling rate than all the machines discussed.

Development of Petro-Forge FormingMachines

189

(4) These conclusions are conservative estimates; the up to date Mk.IIF or Mk.IIG Petro-Forge hammers have a higher energy output and a stiffer structure than the Mk.IID. (5) The effects of strain rate and of component cooling have been neglected. (6) The relative assessment presented is valid only for components which, with a Petro-Forge, can be formed in a single blow. The argument presented can be extended also to components which on a Petro-Forge cannot be formed in a single blow. Such comparisons must encompass the total machine effort for the HERF installation and for the conventional set-up. The general conclusion to be drawn is this: For components which on a H E R F hammer can be produced in a single blow, the economic case for high speed forming is very strong. However, if with the HERF machine additional forging stages are required, for instance, preforming for removing scale, or trimming of flash, the attractiveness of the process is progressively diminished. Such additional forming stages can be carried out on other HERF hammers, as in the case of the Transfer Forging Machine shown in Fig. 6.4, or on a conventional crank press, because of the limited die space available on high speed machines. The more forming stages required, the greater the relative attractiveness of a conventional crank or similar press. It also follows that improvements in the total forming process - - introduction of atmosphere controlled heating and closed die forming - - are likely to strengthen the position of HERF.

9. MACHINEADVANTAGES/LIMITATIONSOF PETRO-FORGE From the very beginning of its conception, Petro-Forge was closely tied to HERF and was considered to fall into the category of Dynapak, CEFF and similar machines. When summarizing its machine advantages/limitations it is, therefore, reasonable to consider these in relation to those of pneumatic-hydraulic HERF hammers. Following that the potential of the Petro-Forge drive unit for the actuation of machines operating at conventional speeds will be outlined.

9.1. Advantages of HERF machines The favourable features of HERF hammers are claimed to be as follows: 1. Small compact size and consequent low capital cost. 2. Lower installation, erection and building costs. 3. Accurate and repeatable control of blow energy. That HERF hammers are very much smaller than equivalent conventional forming machines has never been disputed. As already mentioned, for the same mass ratio, the weight of a high-speed machine should be only about t/9th of that of a conventional machine with the same forming capacity; often it is substantially smaller. Evidence as to the size advantage of HERF machines has already been provided with Fig. 8.1 and Fig. 8.2, though it was explained that comparisons with different types of machines might be misleading. Further examples are shown in Fig. 9.1 and Fig. 9.2 in which a HERF hammer is compared with a conventional hammer. The former shows the relative sizes of a USI machine and a steam hammer of "equal capacity" [9.1] and the latter compares the size of a Petro-Forge Mk.IIF with that of a 20 cwt drop hammer, both exerting a maximum blow energy of 20 kJ (15,000 ft lbf). According to [1.1] (p. 192), as a rough estimate, for the same blow energy, HERF equipment is about 25 to 50% cheaper than conventional machines. This is true only for large machines, as can be seen from Fig. 9.3 which shows the machine cost in terms of £/kJ (£/1000 ft lbf), 1964 prices, for pneumatic-hydraulic HERF and conventional equipment, as a function of the maximum blow energy ([8.2], see also [5.4] p. 706). More specific data on the cost advantage of a particular type of H E R F machine in relation to presses is presented in Table 9.1 (after Lahr [9.2]). This is divided into

190

S . A . TOBIAS

(a)

(b)

Fl(;. 9. I. Comparative size of USI machine and "'equivalent" steam hammer (after [9. l]).

_IL

.3

1=

o .=

FLOOR LEVEL

(a) .

•

.

-

.

,

.

•

.

o

•

.

.

(b) FIG. 9.2. Comparative size of Petro-Forge Mk.IIF and drop hammer. (a) Petro-Forge Mk.IF-10K, (b) 1.0 tonne (20 cwt) drop hammer.

three column sections, each giving the specification of a USI (pneumatic-hydraulic) H E R F hammer and that of the equivalent press, both from the same manufacturer. Note that the maximum allowable tonnage of the H E R F machines is above that of the press and the same applies also to the blow energy. The sixth row lists the weights and shows

Development of Petro-Forge Forming Machines

191

TABLE 9.1. COMPARISONOF USI HERF MACHINESWITH USI FORGING PRESSES[9.2]

HERF Model designation Max. forming load (MN) Max. blow energy (k J) Cycling rate (min ~) Power pack (kW) Machine weight (tonne) Pump unit weight (kg) Prices 1964 (US $)

500C 12 68 10 45 5.9 2200 34,500

Price ratio press/HERF

Press

HERF

Press

HERF

Press

B11000 10 57 75 45 45.4

2000C 50 204 7-8 91 14.5 4309

F13000 30 170 50 151 235.87

E 16000 60 340 35 303 612.4

110,000

69,950

3500C 100 408 5 opt 8 91 or 182 24 4309/6804 112.000 117,500

3.2

q

1600 ;

i

815,000

7 to 7.28

$

t

$

e------e HYDRAULIC HOT FORMING PRESSES MECHANICAL PRESSES . = " NIGH ENERGY RATE MACHINES DROP HAMMERS / / / / , PNEUMATIC HAMMERS

1400

.,3

315,000 4.5

1200

J

~o

1000 ( I,-

800

)o

O

O

,'~\\"t ×\\',~

600 4OO

zo I

ZOO

0-

1oo o(

)o

400000

STROKE ENERGY~ft rl~~) 1 0

I

200 BLOW

I

4100

300 ENERGY

-

I

500

600

kJ

FIG. 9.3. Capital cost of HERF and conventional hot forming machines in terms of 1964 prices.

that the HERF machines are very much lighter, even when the power-pack weight is included, the weight ratio being below 1:10 and decreasing further with size. The only feature which is more favourable for the press is the cycling rate, being about 7 times that of the HERF device. Finally, the bottom row gives the relative prices, that of the HERF machine being taken as 1.0. Note that the cost advantage of the HERF machine increases substantially with size. It is also generally agreed that HERF machines have lower installation costs than conventional machines. The built-in vibration isolation system of semi-double acting machines, or the balanced moving masses of double acting devices, prevents shock and vibration being transmitted into the foundation and hence deep, massive concrete foundation blocks are not required. Medium sized machines do not need any special foundations at all and can be placed directly on a concrete floor of appropriate load carrying capacity. Thus, the layout of production facilities is easily re-arranged. The small and compact size also reduces the cost of buildings needed for HERF machines. A substantially lower ceiling level is adequate and crane or other handling facilities for maintenance can be of a smaller load carrying capacity than for conventional forming machines. It must, however, be pointed out that the advantages due to the smaller machine size are relevant only for a limited range of workpieces. What is important is the total machine capacity needed for producing a particular component.

192

S.A. TOBIAS

Little if anything is gained by moving the production of a component from a press, using multi-impression dies, to a HERF hammer when with the latter auxiliary forming machines are required for "cheesing" the billet and trimming the workpiece; pneumatichydraulic HERF machines can be made to pay only for a selected range of components. The claim that HERF machines allow a more accurate and repeatable control of the blow energy is valid for some devices but it is due to their sophistication and does not follow from their high impact speed. Some conventional power-assisted hammers perform equally well from this point of view. To complete the picture, process advantages accruing from high forming speeds must also be considered. These aspects fall outside the scope of the present paper and will be dealt with in a separate publication [1.4]. At this stage, let it suffice to state that the range of applicability of HERF bulk forming equipment seems to be much greater than was originally realised. Research, done mainly in connection with Petro-Forge, has shown that the biggest potential of HERF is not necessarily in hot forming. High forming speeds offer advantages also with warm and cold forming, powder compaction and forging, and in particular with cropping, etc., provided that fast cycling machines, suitable for these processes, become available. The original pneumatic-hydraulic HERF machines cannot compete in these areas because of their low cycling rate. 9.2. Limitations of HERF machines The unfavourable features of HERF machines can be divided into two categories: 1. Limitations of particular designs. 2. Limitations caused by high impact speed. Difficulties encountered with particular designs have been discussed in S~ction 4.4 and hence a brief summary of the most important aspects will suffice. The main complaints against pneumatic-hydraulic devices (Dynapak, CEFF, USI, etc.) were their long cycle time, resulting in low output and making restriking of the workpiece in one heat impossible, a long "dead time" (between pressing the firing button and the start of the working stroke), long dwell and ejection times (during which the hot workpiece heats the die) and a slow recocking speed. Some of the pneumatic-hydraulic machines were not sufficiently reliable and needed frequent maintenance and this more than anything else (such as short die life) gave the process a bad reputation. Although little is known about the precise reasons for the lack of reliability of some designs, it can be taken for granted that high tup speed was at least a contributory factor. A machine limitation which can be directly attributed to the high impact speed was the difficulty of keeping bolts and nuts tight. The precautions that must be taken for achieving this are understood but are apparently difficult to maintain under production conditions. On the most recent Dynapak, die holding bolts are being replaced by conventional wedges (see Section 4.1). 9.3. Advantages/limitations of Petro-Forge as a HERF machine Petro-Forge can claim all the machine advantages attributed to pneumatic-hydraulic HERF machines and a good many more. The capital cost of Petro-Forge is well below that of an equivalent pneumatic-hydraulic HERF machine because combustion actuation does not require a special and expensive high pressure compressor and/or hydraulic power-pack. Basic energy costs are about the same (Section 5.6) but maintenance costs are likely to be significantly lower. The mechanical action of its drive unit ensures a high cycling rate, a minimal "dead time" and very short dwell/recocking/ejection times. Restriking of the workpiece in the same heat is feasible and bounce-free operation can be ensured. Its blow energy can be controlled within wide limits and maintained within a close tolerance. One of the major assets of Petro-Forge is that it is supported by a vast amount of research. This paper has surveyed only part of the machine R & D that has been generated. Specialised problems, such as research on liquid fuel combustion [9.3, 9.4], on the design of the "Super-Charged Machine" (see Section 5.1) [5.11], of critical

Development of Petro-Forge Forming Machines

193

components [9.5] or that of energy absorbers [6.18], and some aspects of automation [6.5, 6.6, 6.7, 6.8, 6.9, 6.10, 6.11, 6.12] had to be omitted. With current work on computer control and the self-diagnosis of faults and the computer aided integration of the CAD/CAM of forming dies with the operation of the forming plant (CIFS, Section 6.5), Petro-Forge has reached a degree of sophistication which goes well beyond that of any other forming system. Petro-Forge machines have been designed to be more versatile and have a wider range of applicability than pneumatic-hydraulic H E R F hammers of a comparable size. However, this feature, as well as the small size and the ease of rearranging production lines, are of interest mainly to the small jobbing forger. They are of lesser importance in mass production when machines are engaged on the manufacture of the same range of components for very long periods. The main limitation of Petro-Forge is the absence of realistic production experience. Machines have been used in simulated production runs and exposed to accelerated life testing, involving more than 180,000 blows of exceptional severity, arising only rarely under normal workshop conditions. Nevertheless, the life of a machine which is capable of cycling at a rate of 60 blows per minute must be measured in terms of millions of blows and experience does not extend anywhere near that far. There is no denying, PetroForge has not been exposed to the wear and tear of a real production environment. 9.4. The potential of Petro-Forge for forming at conventional speeds With hindsight, associating Petro-Forge closely with H E R F was a mistake. No doubt, the advantages of combustion actuation and of the mechanical action of Petro-Forge, coupled with the small compact size and low cost of H E R F hammers, as well as the process advantages due to high forming speeds, seemed to constitute a most attractive, unbeatable package. This belief was reinforced by an extensive market survey [9.6], carried out by a highly reputable organisation and completed in 1970, which came to the conclusion that there was "a total of 14,200 Petro-Forge applications within the potential user-companies in the UK" and that "the total potential market for the Petro-Forge machine", mainly for cropping and hot forging, to a lesser degree for blanking and other purposes, was "approximately 1,420 applications". So much for the reliability of market surveys! It has been pointed out (Section 5.3) that lowering the impact speed of Petro-Forge is achieved simply by increasing the platen/ram/piston assembly mass. Dropping the nominal impact speed range of the current Mk.IF and Mk.IIF models from 1.9-15 m/sec (6.25-50 ft/sec) by 20% to 1.5-12 m/sec (5-40 ft/sec) requires an increase of the moving mass by 36% and this can be achieved without significant design changes. Such a change would have little effect on the process advantages that can be attributed to high forming speeds. A larger drop would require more substantial design changes. Petro-Forge drive units could be used in the manner hydraulic cylinders are used in power-assisted hammers. As a matter of fact they would be eminently suitable for retro-fitting conventional steam hammers, with the aim of reducing running costs. An alternative design would involve a development of the current geometry by a stretching of the horizontal dimensions of the die area, possibly also making the machine up-stroking. The resulting geometry would be similar to that of the "Slow-Speed Petro-Forge" discussed in Section 5.3 (see Fig. 5.12). The large die area would allow the accommodation of a larger die bolster, possibly incorporating multi-impression dies. The cycling rate of presses is increasing with the aim of reducing contact time and thus lengthening die life. Friction screw presses are gaining in popularity since they have similar characteristics to hammers (Section 2.1, Fig. 2.1 and Fig. 2.2) and as such are well suited to some components. Their impact speed is also creeping up. The maximum speed of H E R F hammers has been dropping (Table 4.1) and this has led to some benefits (lower stress wave levels, prevention of bolts and nuts shaking loose), though at the expense of machine size. From the machine side there is probably an optimum balance between maximum impact speed and machine size (capital cost) which needs to be

194

S . A . TOBIAS

matched to the process requirements. The establishment of such optimum process conditions is probably the most important research topic in the broad field of forming machine design which needs exploring.

9.5. Conclusions Petro-Forge is one of the off-spring of the "white-hot technological revolution", promised by the Labour Party during the 1965 general election [5.13] which was supposed to regenerate British industry. It was the first major advanced technology project financed by the then Ministry of Technology, launched with a great deal of publicity, arousing enormous industrial interest, leading to lengthy negotiations with a number of companies and licensing agreements which, in the end, came to nothing. These aspects of the project lie outside the scope of this paper. They have been discussed at some length in [5.13], which gives some insight into the fate of the "white-hot technological revolution" from which so much was expected and so little gained. There are lessons to be learnt from that phase of the economic history of the UK. A post mortem awaits some bright economic historian to elucidate them and write them up in a Ph.D. thesis. All through that period and ever since, Petro-Forge has suffered from its greatest limitation, that is, that no machines are working under industrial conditions. The question, 'fif the system is any good why hasn't some company taken it up" has been asked ad nauseam and no satisfactory answer was ever provided. This, no doubt, is one of the reasons why so far the major use of Petro-Forge has been that of a research vehicle for generating new knowledge and for training future generations of research workers. Acknowledgements--Many individuals have contributed to the development of Petro-Forge at one time or other; the names of academic staff and research students are contained in the References. A special mention must be made of some of the technical staff who spent a major part of their working life on the project. Early devices were designed by the late C. G. Price; all other machines from the Mk.IIC onwards by M. Bird. Dies and automatic transfer mechanisms were designed by M. Chapman. Machine development w a s carried out by C. Shaw, P. M. McKeown, J. Parkins (retired), joined more recently by D. Millard. Electronic development was the responsibility of W. A. Hewitt. A. C. Burgess prepared all photographic work. Most of the illustrations contained in this paper are the work of the late J. Smith and his successor S. Phillips. REFERENCES [1.1] [1.2] [1.3] [1.4] [2.11 [2.2] [2.3] [4.1] [4.2] [4.3] [4.4] [4.5[ [4.61 [4.7] [4.8] [4.9]

I4.10] [4.11l [4.121

E.J. BRUNO(ed), High Velocity Forming of Metals. 2nd edn, pp. 73-184. American Society of Tool and Manufacturing Engineers (ASTME), Dearborn, Mich. (1968). R. DAVIESand E. R. AUSTIN, Developments in High Speed Metal Forming, pp. 184-270. The Machinery Publishing Co. Ltd., Brighton (1970). M . C . NOLAND, H. M. GADBERRY, J. B. LOSER and E. C. SNEE~AS, High-Velocity Metalworking, A Survey, NASA SP-5062, Office of Technolot, v Utilization, NASA, Washington (1967). S. A. TOBIAS, Survey of process R & D with Petro-Forge machines (in preparation). K. LANGE, Lehrbuch der Umformtechnik, vol. I, p. 309, Springer-Verlag, Berlin (1972). K. LANe,E, Theory und Grundlagen des Gesenkschmiedens, Industrie-Anzeiger, No. 58, 67 and 77. 1963, pp. 1392, 1549 and 1739. English translation: Theory and basic principles of drop forging, Metal Treatment, May, June and July, 1965, pp. 184, 210 and 264. H . W . HALLER, Handbuch des Schmiedens, p. 44, Carl Hanser Verlag, Munchen ( 1971 ). S.R. CARPENTER, High energy forming. Aircraft Engng, 32, Feb. 1 (1960). E. W. FEDDERSEN, Production application of high energy rate, A.S.E. Trans., 69 (1961). W, G. MANG, Dynapak, a new dimension in high energy rate forming, Sheet Metal Industries, 39, No. 424, August (1962). W, G. MANG, The status of the pneumatic-mechanical acuator in metalworking, Proc. ].s'l Int, Cotl[~ High Energy Forming, University of Denver, pp. 4.3.1-4.3.31 (1967). W . G . MANG, Analysis of pneumatic-mechanical actuator, A.S.T.M.E., Tech. Paper MF68-546 (1968). M . J . GALLAGHERand J. A. YOBLIN, Recent developments in HERF forging, S.M.E., Tech. Paper MF70-229 (1970). Sales Literature, Dynapak for high energy rate forming, Fielding & Platt Ltd., Glouccster, England. ANON, CEFF high-speed forging machine, Metal Treatment. June 1960, pp. 240-242. J . K . MUREK and R. C. MILLER, CEFF metal forming machines and their practical use in production, A.S.T.M.E., Tech. Paper MF68-547 (1968). Sales Literature, CEFF high velocity impact forging machines, Weingarten-CEFF Corp., San Diego, Cal. ANON., High energy rate forging machine, The Engineer, Feb, 23, p. 390 (1963). ANON., Controlled impact forges, Product Engng, March 5, pp. 68-70 (1962).

Development of Petro-Forge Forming Machines

[4.131 [4.141 [4.151 [4.16] [4.17] [4.181 [4.19] I5.1] [5.21 [5.3] [5.4] [5.5] [5.6] [5.7] [5.8] [5.9] [5.101 [5.11] [5.12] [5.13] [5.14] [5.15] [5.161 [5.17] [6.1] [6.2[ [6.3] [6.4]

[6.5] [6.6] [6.7] [6.8] [6.9] [6.10] [6.11]

195

Sales Literature: High-energy rate forging machines, U.S. Industries Inc., Production Machine Division, Chicajzo, I11. M. OVANEand H. SEzloucm, High energy rate forming, (In Japanese), Precision Machine Tools, 31 (1965). TADASl OIKAWAand HIROYASUYAOUTI,High speed-high energy rate forging machine, 1st report, (In Japanese), J.S.W. Techn. Rev., 19, 1-7 (1966). Sales Literature: Hi-Fomac, pneumatic mechanical machines, Kobe Steel Ltd., Kobe, Japan. Final report on the influence of high rate forging on die life, Ministry of Technology Contract, Reference F/TP/01, D.F.R.A. (1970). A. THOMASand K. DENHAM, Die performance in high-energy rate forging, Proc. 12th Int. M.T.D.R. Conf., Manchester, pp. 153-160, Macmillan (1972). S. M. J. ALl, B. W. ROOKSand S. A. TOBIAS,The effect of dwell time on die wear in high speed hot forging, Proc. Inst. Mech. Engrs, 185, 83/71, pp. 1171-1186 (1970--71). Provisional Patent Specification, High Energy Head, Repco Ltd., Commonwealth of Australia. 17 February (1977). P. R. BOADLEand B. M. HADAWAV,A Novel System of Deriving Energy by Combustion of Propane and Air for Use in HERF Processes, Australian Conference on Manufacturing Engineering, 17-19 August, pp. 277-278 (1977). U. P. SOORISmN, Present State, Problems and Development Prospects of High Speed Forging, (In Russian), Kuzn Stamp Proizvod, 23, 18-19 (1981). L . T . CHAN, F. BAKrtXARand S. A. TOBIAS, Design and development of Petro-Forge high energy rate forming machines. Proc. Inst. Mech. Engrs. 180, Pt. I, (1965-66). S. A. TOBIAS, A survey of the development of the Petro-Forge system, Proc. Int. Conf. on Manuf. Technology, Ann Arbor, Mich., p. 883, American Society of Tool and Manufacturing Engineers (ASTME), Dearborn, Mich. (1967). L . T . CHAN and S. A. TOBIAS,Performance characteristics of Petro-Forge Mk.I and Mk.II machines, Proc. Inst. Mech. Engrs, 184, Pt. I (1969-70). L . T . CHAN and F. BAKHTAR,Some basic consideration in the design of Petro-Forge high energy rate forming machines. Proc. Inst. Mech. Engrs, 184, Pt. I (1969-70). S . A . TOBIAS,The Petro-Forge forming system, S.M.E. Technical Paper MF 70-184 (197(I). S . A . TOBIAS, Das Petro-Forge Umformsystem, Fertigung, No. 1, 3-14 (1971). A. D. SHEIKH,M. K. DAS and S. A. TOBIAS,The development of the slow-speed Petro-Forge machine, Int. J. Mach. Tool. Des. Res., 11, 13-29 (1971). M. S. CETINER, Automation of high energy rate forming machines, Ph.D. Thesis, Department of Mechanical Engineering, University of Birmingham, May 1974. I. MARLAND, A. J. ORGAN and S. A. TOBIAS, Design and development of a compressed-air driven, counterblow high energy-rate forming machine, Proc. Inst. Mech. Engrs, 180, Pt. I (1965-66). S. A. TOBIAS,Petro-Forge--A cautionary tale for aspiring inventors, In Twelve Notable Projects (Edited by J. C. Levy and W. E. J. Farvis) pp. 153-223, Report for SERC, City University (1982). M. K. DAS and S. A. TOBIAS, The forming capacity of HERF hammers in relation to that of conventional machines, Proc. 5th Int. Conf. on High Energy Rate Fabrication, Center for High Energy Forming, Denver, Col., USA (1975). M, K. DAS and S. A. TOBIAS,Die Kapazit~it yon Hochenergieumformmaschinen im Vergleich mit der von konventionellen Maschinen, Fertigung, No. 6, 161-168 (1977). M, M. SADEK and S. A. TOmAS, Research on noise generated in impact forming machines at the University of Birmingham, 1971-1976, Proc. 17th Int. M.T.D.R. Conf., Birmingham, pp. 257-273, Macmillan (1977). S. A. TOBIAS, Research on the computer aided prediction of hammer noise at the University of Birmingham, 1977-1983, Workshop for Advances in Impact Forging, Michigan Technological University, Houghton, Mich, 1984, (in the press). J. STOETER,Messen der Beriihrzeiten beim Gesenkschmieden, Schmiedetechnische Mitteilungen, 48, 143-144 (1958). S . A . TOBIAS, Automation of high energy rate processes, Proc. 1st Int. Conf. Center for High Energy Forming, University of Denver, Estes Park, Col. 4.1.2-4.1.32 (1967). S . A . TOBIAS, Die Automatisierung von Hochleistungs-Umform-verfahren, lndustrie-Anzeiger, No. 3 and 12 (1968). S . A . TOBIASet al., Micro-computer control of the Wolfson transfer forging machine (in preparation). K . O . OKPERE, Automation of a Petro-Forge installation, Ph.D. Thesis, University of Birmingham, (1972). B.W. RooKs, K. O. OKPEREand R. M, H. CnENO, Asynchronous sequential control of a billet handling system for feeding a hot forging machine, The Industrial Robot, 1, No. 2, Dec. [1973). B. W. RooKs, O. K. OKPERE, A. W. KWlATKOWSm and S. A. TOmAS, Hardware and software requirements for automation of a Petro-Forge hammer, Proc. 17th Int. Mach. Tool Des. Res. Conf., Birmingham, pp. 37-44, Macmillan (1977). W. C. K. WoNo, Identification of hot forging process characteristics for optimisation of production economics, Ph.D. Thesis, University of Birmingham (1976). A . W . KWIATKOWSKI,B. W. ROOKSand R. M. H. CHENG, On-line control of a work-restricted forging process, Proc. C.I.R.P. Seminar on Manufacturing Systems, University of Llubijana, June (1972). A. W. KWIATKOWSKLB. W. ROOKSand R. M. H. CnENO, Adaptive control of work-restricted forging machines, Manufacturing Systems, 2, 23 (1973). W. C. K. WONG, B. W. ROOKSand A. W. KWIATKOWSKI,Experimental identification of a hot forging process, Proc. Int. Conf. on Production Technology, Session 2A, Sydney, Australia, pp. 65-72 (1974).

196

[6.12] [6.131 [6.141 [6.15] [6.16] [6.17] I6.181 [6.191 [6.201 [6.21] I6.221 [6.231 I6.24] [6.251 [6.261 [6.27] [6.28] [6.29] [6.301 [7.11 [7.2] [7.3] [7.41 I7.51 [7.6] [7.71 [7.81 [7.9]

[7.11)] I8.11 I8.2] [8.31 [8.4] ]8.5] [9.11 19.21

S . A . TOBIAS A. W. KWIATKOWSKI and S. K. BISWAS, Identification of a work-restricted forging process for the optimisation by adaptive control, Proc. A.S.M.E.. Paper No. 75-WA/Prod-24 (1976). C. E. N. STURGESS,The industrial potential of high-energy rate cold forging, Proc. 5th Int. Conf. Center Jbr High Energy Forming, University of Denver, Vail, Col. (1975). C. E. N. STURGESS, B. W. ROOKS and W. HEWm', Application of an industrial robot to a cold forging process, Proc. 3rd Conf. Ind. Robot Tech.. University of Nottingham (1976). M. K. DAS and S. A. TOBIAS, The Petro-Forgc H.E.R.F. cropping system, S.M.E., Technical Paper MF72-143. Int. Eng. Conf. Chicago, April (1972). M. K. DAS and S. A. TOBIAS, Das Petro-Forge--Abschersystem, b2,rtigung, No. 2.29-35 (1973). M. K. DAS and S. A. TOBIAS, Recent advances in high speed cropping, Proc. Int. Cold-Forging Congress, Brighton, Oct.. pp. 459--490. (1975). Metallurgia and Metal Forming. February, pp. 47-54 (1976). A. J. OR6AN, Bar cropping in high energy rate forming machines, Ph.D. Thesis, University of Birmingham (1968). A. IBANES RUIS, Impact effects and the absorbtion of surplus energy in high speed cropping, Ph.D. Thesis, University of Birmingham (1977). S. K. MAITI, M. K. DAS and S. A. TOBIAS. The dynamics of a HERF cropping machine, Int. J. Mach. Tool. Des. Res., 15, 1-24, (1975). ANON., Petro-Forge cropping systems, Technical Note No. 11, Engineering Design Research Development Ltd. C. L. CHUAH, On-line billet volume control in high speed cropping, Ph.D, Thesis, University of Birmingham (1983). A. J. ORGAN,C. G. PRICE, S. A. TOBIAS and M. K. DAS, Improvement in billet production. British Patent No. 1,379,896. C. K. CltooN~, Some aspects of thick tube cropping, M.Sc. Thesis, University of Birmingham (1980). C. K. CHOONG, M. K. DAS and S. A, TOBIAS, A survey of methods for producing tubular billets by shearing, Proc. 23rd Int. Con~ Maeh. Tool Des. Res.. Manchester, Macmillan (1983). C. LEE, Microcomputer control of high energy-rate forming machines, Ph.D. Thesis, University of Birmingham (1983). C. LEE and S. A. TOBIAS, Microcomputer control of Petro-Forge machines, Int. J. Mach. Tool. Des. Res., 25, in the press (1985). G. B. Yu, Computer-aided design and manufacture of axisymmetric forging die cavities, M.Sc. Qualifying Thesis, University of Birmingham (1983). G. B. Yu and T. A. DEAN, A practical computer-aided approach to mould design for axisymmetric die cavities, Int. J. Mach. Tool. Des. Res.. 25, 1-13 (1985). G. B. Yu and T. A. DEAN, A CAD/CAM package for axisymmetric forging dics, Proc. 25th hit. Conf. Mach. Tool Des. Res., Birmingham, Macmillan scheduled for publication (1986). A. J. ORGAN, Dynamic behaviour and energy capacity of work-restricted forming machines, Proc. 9th Int. Conf] Mach. Tool Des. Res., Manchester, pp. 213-228, Pergamon Press (1969). A. M. C. CHAHERLI, Dynamics of Petro-Forge HERF machines, Ph.D. Thesis, University of Birmingham (1971). F. W. DORENBOS and J. M. PULSULICH, Pneumatic-mechanical high velocity forming High Velocity F}~rming (Chapter 6), Ed. F. W. Wilson, Prentice Hall (1964). M. K. DAS and S. A. TOBIAS,The efficiency of energy transfer and the determination of the clash load in impact forming machines, Proc. 4th Int. Con l] on High Energy Rate Fabrkation, Center for High Energy Forming, Vail, Co[., USA, pp. 3.1.1-3.1.18 (1973). M. K. DAS and S. A. TOBIAS, Energic und Aufpral[kraft beim Hochgeschwindigkcitsumformen, Fertigung, No. 3, 81-88 (1975). S. VAJVAYEE,M, M. SADEK and S. A. TOBIAS, The efficiency and clash load of impact forming machines to the second order of approximation, Int. J. Math. Tool. Des. Res., 19, 237-252 (1979). W. JOHNSON and P. B. MEEt,OR, Engineering Plasticity, p. 113, Van Nostrand Reinhold Company, London (1973). S. VAJPAVEE,Dynamical and acoustical studies of impact forming process, Ph.D. Thesis, University of Birmingham (1978). V. GREGORIAN, M. M. SADEK and S. A. TOBIAS, Noise generated by a laboratory drop hammer and its interrelation with the structural dynamics and process parameters, Int. J. Mach. Tool. Des. Res., 16, 301-318 (1976). C. J. HOOKE, The design of mechanical hammers for cold working and coining. Proc. 7th hit. Conj. Math. Tool Des. Res., Birmingham, pp. 81-89, Pergamon Press (1967). T. A. DEAN, Private Communication. Sec also M. G. JONES, Le Fromage a Haute Energie, Mecanique Matdriaux Electricitd, No. 281, Mai (1973). F. BAKHTARand E. R. AUSTIN. Relative economics of conventional and high strain-rate forming, Int. J. Mach. Tool. Des. Res., 5, 139-154 (1965). J. K. MUREK, Private Communication (1982). O. KIENZLE, Kenngr6ssen fiJr Werkzeugmaschincn zum Gesenkschmicden. Werkstattstechnik, 55, 509-514 (1965). H. MEYER-NOLKEMPER, Eigenschaften und Vcrhalten von Gesenkschmiedemaschinen, Wt-Z ind Fertg, 62, 274-279 (1972). 8. A. SKEEN, High-Velocity Forging. Part I. Principles of operation and advantages, Machinery, October, 117-123 (1964). E. J. LAHR, High energy rate forging, an appraisal, Internal Report, Mechanical Presses, Martteet & Weight Ltd. (1971).

Development of Petro-Forge Forming Machines

197

[9.3] A.F. AL-OMAR,Combustion problems in Petro-Forge, Ph.D. Thesis, University of Birmingham (1969). [9.4] A . F . AL-OMARand F. BAKHTAR,Combustion problems in Petro-Forge, Proc. Inst. Mech. Engrs, 185, 68-71 (1970-1971). [9.5] R . O . YILDRIM,Impact dynamics of high energy rate forming machines, Ph.D. Thesis, University of Birmingham (1982). [9.6] ANON., Market Survey: PETRO-FORGE, Market Intelligence Section, Wickman Machine Tool Mfg. Co. Ltd., May (1970).