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New Developments in the Technology of Automation-Related Joining Processes
Keynote-Papers New Developments in the Technology of Automation-Related Joining Processes Prof. Dr.-lng. H. J. Warnecke
1. Technology of joining processes 1.1 Assembly, a part of the production system Industrially produced, technical final products consist mainly of several individual parts which mostly have been manufactured at different times and in separate places. Assembly tasks thus result from the requirement to fit together certain individual parts, dimensionless substances and sub-assemblies into assemblies of final products of higher complexity in a given quantity or within a given unit of time. Assembly therefore represents a cross-section of the problems within the whole of production engineering, very different activities and assembly processes being performed in the individual branches of industry. Up to now, assembly work has been divided into two fundamental classes of operation /l/: actual assembly operations, such as handling checking, adjusting and joining; auxiliary assembly work, such as cleaning, removal of burrs, matching up, etc., in addition to which come the fields of transport, preparation for dispatch, storage, monitoring, repair work, refitting and maintenance.
3 Lybour
Fig. 1.2:
COllS
hurcm: Lnmr
Proportion of labour costs in production costs for three different precision engineering products.
Studies in the automobile industry show quite clearly the backwardness of automation in assembly in comparison with other production areas. Fig. 1.3 shows the development of costs in normal and flexible automatic assembly operations in the automobile industry.
Fig. 1.1:
Operational fields for assembly work
Joining is defined as the permanent connecting of two or more geometrically defined bodies or of geometrically defined bodies with a dimensionless substance (Cf. DIN 85931. In medium - and shortrun production, rationalisation measures have mainly been taken in the field of work structuring and work-station design. Automation moves were scarcely undertaken here, because
-
Solutions once found can only be applied to other products or companies with great difficulty, in contrast to parts manufacturing;
-
assembly, as the final production stage, must cope most extensively with continuously shifting market requirements with regard to timing, batch size, derivatives and product structure;
-
until now assembly automation has not seemed to be economically justifiable, or, has not been possible for lack of flexible, i.e. modifiable and rep r o g r m a b l e assembly facilities.
As a result of this situation, extensively rationalised parts manufacturing must be set against the increasing high proportion of assembly costs in relation to total production costs, which can amount 70% depending on the product and level of to 20% production.
-
Annals of the CIRP Vol. 35/2/1986
Fig. 1.3:
Evaluation of the assembly costs with manual and flexible automated assembly in the automobile industry
Over the next few years, company investment will be u s e d increasingly for rationalization measures, with special emphasis on the assembly sector. The survey of flexible automation of assembly work /1/ reveals that 25% of companies' total investments an? spent on assembly. However, automation does not represent the sole possible method of rationalization in the assembly sector. The cost-cutting potentials of the various rationalization strategies are assessed differently from branch to branch. Averaged-out over all branches, the following costcutting potentials (related to the current assembly times) are expected for the next five years:
-
mecnanisation / automation
17.5%
product design
17%
introduction of new production and joining technology
12%
-
organizationlstructuring of work
10.5%
453
1.2 Product structure The outstanding significance of product design in line with the requirements of assembly work as a rationalization strategy over the next few years was confirmed by a Delphi inquiry. It was established that by 1987 the assembly-oriented design will have the same priority as is at present given to design in the field of numerical control / 3 / . The greatest cost-cutting potentials here are expected by
-a
reduction in expenditure for joining and assembly work due to a reduction of the number of components 1e.g. outset technology)
- the introduction of new joining technology - product design suitable for automatic assembly Measures applied to the overall design of a product can achieve the greatest cost-cutting effects. However, such measures can only be efficiently carried out on a long-term basis, involving the redesigning of products or types of products, as the expenditure required to modify an existing product normally exceeds the amount to be saved bv such rationalization measures.
Fig. 1.5:
Areas of application of automation means in assembly work
Re-structuring will be necessary in branches which intend t o increase the level of automation in their assembly work in future. The most favourable cycle time for the application of programmable assembly systems ranges between 30 seconds and 3 minutes. Viable integration of flexible automation is not possible without organizational restructuring of the entire assembly system. The possibilities for the application of flexibly automated systems depend especially on /5/:
-
factors related to the product (e.g. number of pieces, variants, product design)
-
factors related to the company 1e.g. amortization time required)
-
factors specific to the branch (e.9. life-cycle of the product)
A fundamental factor influencing the application of flexible assembly systems is the annual volume of the product to be assembled. Automated assembly is at present most advanced in the field of electrical engineering [Fig. 1.6)
Fig. 1.4:
Steam pressing iron designed for automatic assembly /4/
1.3 Assembly structure Automatic assembly systems range from plants for workpieces weighing only a few grams to plants for workpieces weighing over 100 kg. The method of selecting automation is basically determined by /l/:
- the number of pieces to be assembled - each the number of product variants to be work station, and -
per time unit assembled at
the complexity of the product to be assembled.
The characteristic features and subsequently the area of application for an automatic assembly system are fundamentally influenced by:
Fig. 1.6: Frequency distribution of number of units/ annum of the companies' best selling products
-
2. Assembly processes
the design of the assembly station(s) the design of the means of interlinkage
Fig. 1.5 shows areas of application of various principles of assembly stations and methods of interlinkage.
454
Fig. 2.1 shows the distribution of various areas of operation at 355 companies, based on a representative sample /l/.
Problems involved in joining pairs of threaded parts:
Fig. 2.1:
Distribution of operations in the area of assembly and probable changes in these operations (random test at 306 companies)
Half of thestandardtime of assembly is required for actual joining work (bolting, rivetting, soldering, etc.). The area of delivery/handling accounts for 1/5 of assembly operations.
-
accurate positioning of industrial robot tolerance levels in freely interchangeable system random positioning of the screw in the screwing device positional tolerance of threaded hole tolerance of work-piece fixture and positioning positional tolerance of sorting devices (conveyors) twisting of the work-piece in the screwing device, due to positional error handling surfaces of component parts (threaded sleeves, threaded plates, spark-plugs, etc. weight of screwing unit simultaneous tightening of several screws for highquality screwed joints (cylinder head, etc.) fault in the component part (dimensions, partial fault in the thread, undefined surface, structural defect) additional washers problems when joining flexible parts.
Fig. 2.3 shows the most important grounds for automation and the aims of development.
At present, the most frequently used method of connection in the assembly sector is bolting (Fig. 2.2) / 6 / .
Fig. 2.3:
Fig. 2 . 2 :
Distribution of frequency of various joining methods
Important joining techniques for assembly automation
In the following, the results of research work in the 3 above-mentioned areas are presented. 3 . Technology of joining processes
At present little is known about the following areas:
-
assembly of flexible parts
- joining of parts with several points of contact
-
soldering
In the future a great deal of rationalization potential assembly is expected in the future which can be implemented to a large extent through automation. One prerequisite for automated assembly is fundamental research in certain specific areas. The following joining processes are of particular importance:
-
joining of flexible parts screwing joining with multiple contact
The following problems are involved in the assembly of flexible parts:
-
- large tolerances
3.1 Automatic assembly of flexible unfinished parts
-
support of large joining forces and moments
- great deformation of workpieces even with small or forces -- moments minimal joining clearance minimal gripper clearance
-
preparation and feeding of parts joining of two ends of a hose loinins of workpieces with nonlinear material characteristics- great changes in joining behaviour, even when temperature fluctuation is negligible often indefinable deformations to workpieces
-
3.1.1 Automation problems
One problem which remains to be solved in automatic assembly is the joining of "non-rigid" workpieces. The term "non-rigid" describes the behaviour of bodies under the influence of forces (tension and pressure) and of moments (torsion and bending). The exertion of even minimal moments of force will cause great deformation. In order to be able to solve those joining problems, the first step to be taken is to determine the joining parameters and to examine their mutual interdependence (Fig. 3.1).
Problems involved in joining with several points of contact (Insertion of component parts):
-
bent legs tolerance levels in dimensions of framework tolerance between legs and body of component part - inaccurate drill holes in printed circuit boards
..
Ldrk.", Trpr.lu.
L Fig. 3.1:
I
i
1
l
1
Joining parameters for the joining ot flexible hollow cylinders
In the following paragraphs the tolerance problem will be dealt with in more detail by way of an example.
455
3.1.2 Tolerances
One important parameter in automatic joining of flexible hollow bodies is the deviation in position and orientation between the basic part and the one to be joined. The numerous reasons for tolerances include:
-
It can be seen from this that high availability is achieved at low speed, but then the joining times are excessive. The best availability over a wide range of workpieces was achieved with the "tilt" joining strategy. The joining forces Fx, Fy, Fz, which result from the joining method "tilting movement" are shown in figure 3.4.
accuracy of industrial robot accuracy of gripper system accuracy of preparation and infeed facilities workpiece tolerance
Deviations in position and orientation between the basic part and the one to be joined are eccentricities, angles of error, or combinations of both. There are three possible methods of compensating these deviations: o Compensation with passive systems Passive tolerance compensation depends on the compliance of the systems involved in the joining process:
-
-
workpiece (basic and joined part) gripper industrial robot device between gripper and industrial robot with predetermined rigidity preparation and infeed devices
In the case of flexible parts, the rigidity of the workpieces is very small compared with other components. o Compensation with the aid of active sub-systems o Compensation with the aid of joining strategies 3.1.3
Joining strategies I
The joining strategies tried and suitable for flexible parts are shown in Figure 3.2.
-1-
o
0.1
1.0
1.3
1.0
a.5
'
I
I
1.0
1.5
1.1
LO
3.0
M"lnq TIM-
Fig. 3 . 4 :
Joining forces involved in the joining method "ti1ting movement" (see rubber hose in Figure 3.3)
3.2 Calculation of screw-joining parameters 3.2.1
Introduction
The establishment of screw connections is one of the most important and frequent assembly operations ( 6 ) . The basic problems arising in this context have already been examined in considerable detail by the IPA I 7 1 and others (Figures 3.5, 3.6). Fig. 3.2:
Joining parameters for the joining of flexible hollow cylinders
The individual joining strategies were examined using a wide variety of workpieces. The joining parameters shown in Figure 3.1 were measured. In Figure 3.3 the parameter "path speed" is listed with reference to its availability for different joining strategies. The clamping jaws used were combing jaws.
?I
Fig. 3 . 3 :
456
II
m
.I
,I
U
Sam .pa
nmn
Fig. 3.5:
Parameters influencing the positioning accuracy during screwing operations with industrial robots
Fig. 3.6:
Positioning error because of random position of bolt in screwing tool
M
-c
Availability of an assembly system for different joining strategies (for a rubber hose)
For the sake of simplicity, the analyses carried out to date ( 8 , 9, 10) have firstly ignored the existence of chamfers in the parts to be joined and secondly reduced the problem to one or two dimensions. The influence of relative movement during the self-acting position error compensation after connection of both parts has thus been completely neglected.
rc
3 . 2 . ~ Simulaticn of a screwing operation
In older to analyze tne jcining operation realistically a three-dimensional model with chamfers on both parts is required. For this purpose a finiteelement-model of a bolt (Figure 3.7) and associated threaded hole was produced.
Fig. 3.8:
Computer simulated screwing operation with rotation of the bolt. The position in space of the Finite-Element-Grid points are changed.
Points 2 and 3 are to be seen as functions of the compliance of the tool suspension facility. Requirements for screwdriving unit
3.2.3
Fig. 3.7:
Computer simulated screwing operation with bolt M6 x 15 (DIN 9 3 3 )
The sketch in Figure 3.9 shows the basic possibilities for compensating for lateral shift between the threaded hole and the spindle position. Case 2 is the most desirable because in case 1 the straightening bolt needs to move the entire mass of the screw spindle. In case 2 the force F depends primarily on the compliance of the screw nut guide.
The position of these two element grids in relation to each other can be selected at will. When simulating the docking process first the position of the first-contact points was calculated and then the bolt was given a rotational movement. To calculate the interaction thus produced on ?he complicated shape of the contact surface, a knowledge of the normal vector at the centre of gravity of the surface in question is required. To determine these vectors the thread geometry was transformed into a flat plane using the references:
Fig. 3.9: The normal vector in projection plane is now easy to calculate:
Compensation of the positioning error of the screwing unit during automatic screwing with industrial robots - Schematic diagram
A simplified equation shows
Transformed back into initial position the factual normal vector is: where
f E
I 1;
In this way, the change in the joining parameters caused by the rotation of the bolt can be determined for every point in the grid. These are: 1. Size and direction of righting movement (Figure 3 . 8 ) 2. Size and direction of farces and moments in screwdriving tool 3. Tensions in threaded sides
=
= = =
lateral displacement of the nut elasticity module of the nut guide moment of resistance length
As can be seen, the reaction force of the bolt can be minimized by using an extended guide length in keeping with the simultaneous requirement for a compact screwdriving unit.
457
3.2.6 F u t u r e Prospects
Both aspects are fulfilled by tt.e c r o s z - ' , e c t L c r i d : conplia7,ce element shown ir. Figure 1 . 1 c .
Among tr.e advantages of NCC as a p u r e l y passive elenenc i s tke possi~il;ty sf -ntegratir.g a ranqr cf a&iirior.al finctions, far exnnple:
Cortrol cf interns; pressure evacuatlng the Interlcr of the eiements thr >trips ir the case cf :ncl;ned screwlng can be t i q h t e r e d "package-wise" : c avo13 a positionrrlated change in the bclt 13cation throuqh its
3y
?'wl el 1gh.t
surveillarcr - f inrprnal pressure wito nr eccloicd i n t r i o r ar.y cnange in the shape of t h r elrmrnt i i l i L a u s e a c h a n g e in pressure. e . g . rxtrrs-on 3 u r i n j successtul screwinq 2pe:at;sn
Fig.
3.lC:
Extenslor. cf accive cornp:iar.ce s y s t e m b y simple means. Tne electrical resistance a t four positions staggered a l 9C degrees on t h e perlmeter can be scanned a s a gradual "short circuAt" of the irdividuai y t ~ l p s .In :r.e c a s e of deforrnt-on the lengtt-.of the currcnt pdtt. at t t e point 3 f Teasurernect cb.ar.qc.1 in a q t e p formatior.. The robcr can be guided in thc directiqn of t t . e least ~ e s i s t a n c e .
Screwing conpllance elenert Cross-section sf F E - M o d e l
The deformation behaviour v i t h an axial displacemnt of x = 6 m i s s h o w n in F L p r e 3.11. Tt.e tran6ve:se forces arising are in the reg-on ot 80 K and with d corsion of 3 degrees a torque 3 f approx. 50 Nm can be transferred ( c n l c ~ ~ a t i oby n means of finite element method). I n i t i a l tests Tade with a prototype s e t U F ac the :PA StLttgdrt have conf:rned the res'ilts 2.f t h e s e theore t i ca 1 as s ~ m pitsns .
3 Insertion of odd Yonponects w i t i assembiy r o b o t s 1 . 1 Insertion Techn31oq-f
I n t i g h 3 ~ 0 l ~?CB-P13ductloc e thc d d t o r n a t l c insertion of componrnts - s state 3 f tne art. Standard c o n p c e n t s ~ J C TA S
- axial ~
radial and 3 1 P canponents
can w inserted i i i ~ hspecialized machines. T h p remaining "cdd co>poner.tr", ap?roximately 2 0 Y of t'r.e total, are a l n ; o s t exclusively inserted manually. As a ~ ~ : . s e q ~ e n cthe e , assenbiy cost of the remaining 20\ 3 f compaonents represents a much nigner propsrtion. Ihe high fl~xibilicyrequired for irregular component insertion m a k p . i assembly robots an ideal to31 to perform this task. Assembly r o b o t s 'with differfit basic structures can be b s e d IFiqure 3 . 1 2 1 .
i.i.5
Sumrary
In order to solve a screwing problem the admissible limit v a l u e s for t h e joining parameters ran be calculated by s n u l a t i n g tne screwing p:oces6 first a n d t:hese can then be used f o r optlmal design of the "nearcollet compliance" i NCC) . T h e advantages of the described NCC element over compliant screw splndle attachments (remote-centre COmplldnCe-RCC! a r e d S fOlloWS:
minimal hardware expenses lov mass movement q o c d torsional rigidity !no generation 3 f so-called soft screw situati3ni
Flg. 3 . 1 2 :
Basic StructJres ?f assembly robots f o r PCB-asserbly
However, the flexible insertion uith assembly robots has the dravback of a signlflcantly I r , w ~ r i n s ~ r t i o n rate (see F l q u r e 3 . 1 3 1 .
integrated compliance in directlor. of screwing axis Ino additional spindle s u s p e r . s i o c r e q u i r e d i large dlsplacerenz p o s s i b l e with small dimens-ons no ur.cor.trolled d:splacener.t o f tt-.e screwing s p - n d l e by dead weight in case of tor-vertical 5crew:ng action
Fly.
458
j.:::
Trsprtion rate? of i n ~ e r t ~ s rrndchines . and assembly robots l f o r OED-Compoirntsi
Odd components can be categorized according to the parts presentation into
-
tape-able, stacks. non-st
3.3.3
Insertion methods
Several methods have been evaluated to be found in literature, references /11/12/13/14/ and the most promising laboratory tested. The results have led to the development of a new method and tools by the IPA. The existing methods either did not satisfy requirements concerning reliability, flexibility and quality (component damage) or could not be introduced in production economically. The developed method meets the requirements made in 3.3.2 without using vision systems. It is based on a special gripper system with a compliance which can be "frozen" and centered. The compliance movement in x , y and p (rotation) direction is without friction. Herewith the following assembly steps can be performed: 1. Gripping of the component 2. Insertion in a gage block with chamfered holes and correct insertion span. 3. Insertion into the printed-circuit-board.
Fig. 3.14:
Examples for stackable ODD-Components
With regard to the insertion process, odd components must be categorized into those with
-
"pin guided" insertion and those to be gripped at the component body.
Most large components belong to the second category. Examples are
-
block-capacitors relays coils
...
The insertion process for these componentswas analysed and methods and tools have beendeveloped to accomplish this task with assembly robotseconomically.
The gage block has chamfers which center the component pins. Through the friction compliance movement this is possible without pin bending or component damage. After insertion in the gage block the compliance is frozen. Next, for the insertion into the PCB the most critical tolerances are eliminated and the remaining tolerances are within the diameter difference between holes and pins. 4. Future of Automatic Assembly Future technological developments are a crucial factor when assessing the possibilities for application of automatic assembly equipment and the resultant lay-off potential. The results of a Delphi-inquiry are shown in Fig. 4.1 and Fig. 4.2 and refer to the Federal Republic of Germany. It is expected that 10,000 assembly robots will be in operation by 1992. This forecast is confirmed by the fact that a halving of the purchase price for assembly robots is expected by this time. Product design in compliance with the requirements of assembly work is also given high priority .
3.3.2 System Tolerances The analysis of system tolerances shows that the insertion of components without pin guidance cannot be a simple pick & place operation (see Figure 3.151.
Fig. 3.15:
Fig. 4.1:
Delphi-forecast: industrial robotapplication in the assembly field of I
Fig. 4.2:
Delphi-forecast: industrial robotapplication in the assembly field of I1
Tolerance accumulation during insertion of ODD-Components
Standard methods applied in the assembly of mechanical parts like a remote center compliance in the gripper system cannot be used for component insertion, because of the following problems: components have several pins to be inserted simutaneously (undefined multicontact to the PCB). The tolerances in dimensions (pin configuration) are relatively high. The pins to be inserted are relativly flexible and formable The PCB holes as well as the pin-tip configuration of components do not allow the insertion method to be based on chamfers as microscope pictures show. From the analysis of the tolerances, 2 basic requirements for joining can be summarized:
-
The III.JS: extreme influences throuah c o m o n e n t tolerances (body to pin centerline) and the-poor degree of straightness of the pins have to be eliminated. The relative tolerances between the pins (poor insertion span accuracy) must be compensated.
459
It was further forecast that the number of modular assembly robots would increase annually by 26% up to 1987.
'91
C.G. Mogged
: An analysis of screw mating
requirements for automated assembly. "Thesis for the Bachelor of Science". Massachussetts Institute of Technology 1977
Additionally, new areas of application will be opened up by new assembly systems based on modular processes. 1 1 0 1 J. Uilberg;
: Contribution to the Automation
C. Uaier
of Screwdriving with the aid of Industrial Robots. CCRP Annals 34 11985) 1, S. 49 52
-
i l l / B.D.Hoffmann;
:
S.H. Pollack: 8. Weissmann
Vibratory Insertion Process: A New Approach to Now-Standard Component Insertion. Robots Conference Proceedings. 4 - 7 Juni 1984, Detroit, Dearborn IUSA): SME 1984 Michigan
s. /12/ Kenneth H.K.
8
-
10
: Robot System for Insertion of
Custom Leaded Components Into a P.C. Board /13/ Rooks Brian
Pig. 4.3:
Concept and features of flexible automatic assembly system
/14/ Warnecke,H.J.;:
:
Robot assembly gives flexibility in computer manufacture Industrial Robot, 12 (1985) Sept. S. 169 - 1 7 3 Robotic insertion of odd components into printed circuit boards Assembly Automation 5 (19851 4, s. 198 201
-
B i b l i o g r a p h y :
/1/
Abele, E. u.a.:
/2/
Lotter. B.
:
/3/
B881er, R.; Wolf E. u. Elser. K.
: Rationalisieren der Montage
Einsatzmoglichkeiten von flexibe1 automatisierten Uontagesystemen in der industriellen Produktion Montagestudie. Dtisseldorf: VDI Vcrlag 1984 Manual of Assembly Technology. Krausskopf-Verlag. Mainz 1982 Industrieanzeiger 106 (19841 71, S . 104 109
-
/4/
Schraft. R.D.;: BaBler, -R.
Die montagegerechte Produktgestaltung muB durch systematische Vorgehensweise umpesetzt werden. VDI-Zeitung, 126 (1984) 22, s. 843 - 849
151
Abele, E.; Bahler. R.; Wolf, E.
I61
H.J. Warnecke;: Autmatisches Schrauben mit J. Walther Industrierobotern. wt-Zeitschrift fiir industrielle Fertigung 74 11984) 3, S . 137 - 140
: Entwicklungstendenzen bei
flexibel autmatisierten Montagesystemen wt-Z. Ind. Fertigung 74 (1984). S.
-
333
336
/-I/H.J. WarnecKe;: Investigations of the Screw
E. Abele, J. Walther, G. Fischer
/8/
Driving Process with SensorControlled Industrial Robots. CIRP Annuals 34 (1985) 1, S. 41 44
-
W. Kliemand; : Theoretische Untersuchungen K. Hopperdietz zum Einfldeln von Schrauben ftir die autmatische Montage mit Hilfe von Industrierobotern. Maschinenbautechnik 34 (1985) 3, S . 120 123
-
460
I would like to thank the members of CIRP who have taken part in the discussions about the problems and solutions involved in research and development in the field of assembly. In collecting the material for this paper and writing I was assisted by Dip1.-Ing. B. Frankenhauser, Dip1.-Ing. G. Fischer and Dip1.-Ing. E. Wolf, research fellows at the Fraunhofer-Institute for Manufacturing Engineering and Automation (IPA), Stuttgart.
1. Technology of joining processes 1.1 Assembly, a part of the production system Industrially produced, technical final products consist mainly of several individual parts which mostly have been manufactured at different times and in separate places. Assembly tasks thus result from the requirement to fit together certain individual parts, dimensionless substances and sub-assemblies into assemblies of final products of higher complexity in a given quantity or within a given unit of time. Assembly therefore represents a cross-section of the problems within the whole of production engineering, very different activities and assembly processes being performed in the individual branches of industry. Up to now, assembly work has been divided into two fundamental classes of operation /l/: actual assembly operations, such as handling checking, adjusting and joining; auxiliary assembly work, such as cleaning, removal of burrs, matching up, etc., in addition to which come the fields of transport, preparation for dispatch, storage, monitoring, repair work, refitting and maintenance.
3 Lybour
Fig. 1.2:
COllS
hurcm: Lnmr
Proportion of labour costs in production costs for three different precision engineering products.
Studies in the automobile industry show quite clearly the backwardness of automation in assembly in comparison with other production areas. Fig. 1.3 shows the development of costs in normal and flexible automatic assembly operations in the automobile industry.
Fig. 1.1:
Operational fields for assembly work
Joining is defined as the permanent connecting of two or more geometrically defined bodies or of geometrically defined bodies with a dimensionless substance (Cf. DIN 85931. In medium - and shortrun production, rationalisation measures have mainly been taken in the field of work structuring and work-station design. Automation moves were scarcely undertaken here, because
-
Solutions once found can only be applied to other products or companies with great difficulty, in contrast to parts manufacturing;
-
assembly, as the final production stage, must cope most extensively with continuously shifting market requirements with regard to timing, batch size, derivatives and product structure;
-
until now assembly automation has not seemed to be economically justifiable, or, has not been possible for lack of flexible, i.e. modifiable and rep r o g r m a b l e assembly facilities.
As a result of this situation, extensively rationalised parts manufacturing must be set against the increasing high proportion of assembly costs in relation to total production costs, which can amount 70% depending on the product and level of to 20% production.
-
Annals of the CIRP Vol. 35/2/1986
Fig. 1.3:
Evaluation of the assembly costs with manual and flexible automated assembly in the automobile industry
Over the next few years, company investment will be u s e d increasingly for rationalization measures, with special emphasis on the assembly sector. The survey of flexible automation of assembly work /1/ reveals that 25% of companies' total investments an? spent on assembly. However, automation does not represent the sole possible method of rationalization in the assembly sector. The cost-cutting potentials of the various rationalization strategies are assessed differently from branch to branch. Averaged-out over all branches, the following costcutting potentials (related to the current assembly times) are expected for the next five years:
-
mecnanisation / automation
17.5%
product design
17%
introduction of new production and joining technology
12%
-
organizationlstructuring of work
10.5%
453
1.2 Product structure The outstanding significance of product design in line with the requirements of assembly work as a rationalization strategy over the next few years was confirmed by a Delphi inquiry. It was established that by 1987 the assembly-oriented design will have the same priority as is at present given to design in the field of numerical control / 3 / . The greatest cost-cutting potentials here are expected by
-a
reduction in expenditure for joining and assembly work due to a reduction of the number of components 1e.g. outset technology)
- the introduction of new joining technology - product design suitable for automatic assembly Measures applied to the overall design of a product can achieve the greatest cost-cutting effects. However, such measures can only be efficiently carried out on a long-term basis, involving the redesigning of products or types of products, as the expenditure required to modify an existing product normally exceeds the amount to be saved bv such rationalization measures.
Fig. 1.5:
Areas of application of automation means in assembly work
Re-structuring will be necessary in branches which intend t o increase the level of automation in their assembly work in future. The most favourable cycle time for the application of programmable assembly systems ranges between 30 seconds and 3 minutes. Viable integration of flexible automation is not possible without organizational restructuring of the entire assembly system. The possibilities for the application of flexibly automated systems depend especially on /5/:
-
factors related to the product (e.g. number of pieces, variants, product design)
-
factors related to the company 1e.g. amortization time required)
-
factors specific to the branch (e.9. life-cycle of the product)
A fundamental factor influencing the application of flexible assembly systems is the annual volume of the product to be assembled. Automated assembly is at present most advanced in the field of electrical engineering [Fig. 1.6)
Fig. 1.4:
Steam pressing iron designed for automatic assembly /4/
1.3 Assembly structure Automatic assembly systems range from plants for workpieces weighing only a few grams to plants for workpieces weighing over 100 kg. The method of selecting automation is basically determined by /l/:
- the number of pieces to be assembled - each the number of product variants to be work station, and -
per time unit assembled at
the complexity of the product to be assembled.
The characteristic features and subsequently the area of application for an automatic assembly system are fundamentally influenced by:
Fig. 1.6: Frequency distribution of number of units/ annum of the companies' best selling products
-
2. Assembly processes
the design of the assembly station(s) the design of the means of interlinkage
Fig. 1.5 shows areas of application of various principles of assembly stations and methods of interlinkage.
454
Fig. 2.1 shows the distribution of various areas of operation at 355 companies, based on a representative sample /l/.
Problems involved in joining pairs of threaded parts:
Fig. 2.1:
Distribution of operations in the area of assembly and probable changes in these operations (random test at 306 companies)
Half of thestandardtime of assembly is required for actual joining work (bolting, rivetting, soldering, etc.). The area of delivery/handling accounts for 1/5 of assembly operations.
-
accurate positioning of industrial robot tolerance levels in freely interchangeable system random positioning of the screw in the screwing device positional tolerance of threaded hole tolerance of work-piece fixture and positioning positional tolerance of sorting devices (conveyors) twisting of the work-piece in the screwing device, due to positional error handling surfaces of component parts (threaded sleeves, threaded plates, spark-plugs, etc. weight of screwing unit simultaneous tightening of several screws for highquality screwed joints (cylinder head, etc.) fault in the component part (dimensions, partial fault in the thread, undefined surface, structural defect) additional washers problems when joining flexible parts.
Fig. 2.3 shows the most important grounds for automation and the aims of development.
At present, the most frequently used method of connection in the assembly sector is bolting (Fig. 2.2) / 6 / .
Fig. 2.3:
Fig. 2 . 2 :
Distribution of frequency of various joining methods
Important joining techniques for assembly automation
In the following, the results of research work in the 3 above-mentioned areas are presented. 3 . Technology of joining processes
At present little is known about the following areas:
-
assembly of flexible parts
- joining of parts with several points of contact
-
soldering
In the future a great deal of rationalization potential assembly is expected in the future which can be implemented to a large extent through automation. One prerequisite for automated assembly is fundamental research in certain specific areas. The following joining processes are of particular importance:
-
joining of flexible parts screwing joining with multiple contact
The following problems are involved in the assembly of flexible parts:
-
- large tolerances
3.1 Automatic assembly of flexible unfinished parts
-
support of large joining forces and moments
- great deformation of workpieces even with small or forces -- moments minimal joining clearance minimal gripper clearance
-
preparation and feeding of parts joining of two ends of a hose loinins of workpieces with nonlinear material characteristics- great changes in joining behaviour, even when temperature fluctuation is negligible often indefinable deformations to workpieces
-
3.1.1 Automation problems
One problem which remains to be solved in automatic assembly is the joining of "non-rigid" workpieces. The term "non-rigid" describes the behaviour of bodies under the influence of forces (tension and pressure) and of moments (torsion and bending). The exertion of even minimal moments of force will cause great deformation. In order to be able to solve those joining problems, the first step to be taken is to determine the joining parameters and to examine their mutual interdependence (Fig. 3.1).
Problems involved in joining with several points of contact (Insertion of component parts):
-
bent legs tolerance levels in dimensions of framework tolerance between legs and body of component part - inaccurate drill holes in printed circuit boards
..
Ldrk.", Trpr.lu.
L Fig. 3.1:
I
i
1
l
1
Joining parameters for the joining ot flexible hollow cylinders
In the following paragraphs the tolerance problem will be dealt with in more detail by way of an example.
455
3.1.2 Tolerances
One important parameter in automatic joining of flexible hollow bodies is the deviation in position and orientation between the basic part and the one to be joined. The numerous reasons for tolerances include:
-
It can be seen from this that high availability is achieved at low speed, but then the joining times are excessive. The best availability over a wide range of workpieces was achieved with the "tilt" joining strategy. The joining forces Fx, Fy, Fz, which result from the joining method "tilting movement" are shown in figure 3.4.
accuracy of industrial robot accuracy of gripper system accuracy of preparation and infeed facilities workpiece tolerance
Deviations in position and orientation between the basic part and the one to be joined are eccentricities, angles of error, or combinations of both. There are three possible methods of compensating these deviations: o Compensation with passive systems Passive tolerance compensation depends on the compliance of the systems involved in the joining process:
-
-
workpiece (basic and joined part) gripper industrial robot device between gripper and industrial robot with predetermined rigidity preparation and infeed devices
In the case of flexible parts, the rigidity of the workpieces is very small compared with other components. o Compensation with the aid of active sub-systems o Compensation with the aid of joining strategies 3.1.3
Joining strategies I
The joining strategies tried and suitable for flexible parts are shown in Figure 3.2.
-1-
o
0.1
1.0
1.3
1.0
a.5
'
I
I
1.0
1.5
1.1
LO
3.0
M"lnq TIM-
Fig. 3 . 4 :
Joining forces involved in the joining method "ti1ting movement" (see rubber hose in Figure 3.3)
3.2 Calculation of screw-joining parameters 3.2.1
Introduction
The establishment of screw connections is one of the most important and frequent assembly operations ( 6 ) . The basic problems arising in this context have already been examined in considerable detail by the IPA I 7 1 and others (Figures 3.5, 3.6). Fig. 3.2:
Joining parameters for the joining of flexible hollow cylinders
The individual joining strategies were examined using a wide variety of workpieces. The joining parameters shown in Figure 3.1 were measured. In Figure 3.3 the parameter "path speed" is listed with reference to its availability for different joining strategies. The clamping jaws used were combing jaws.
?I
Fig. 3 . 3 :
456
II
m
.I
,I
U
Sam .pa
nmn
Fig. 3.5:
Parameters influencing the positioning accuracy during screwing operations with industrial robots
Fig. 3.6:
Positioning error because of random position of bolt in screwing tool
M
-c
Availability of an assembly system for different joining strategies (for a rubber hose)
For the sake of simplicity, the analyses carried out to date ( 8 , 9, 10) have firstly ignored the existence of chamfers in the parts to be joined and secondly reduced the problem to one or two dimensions. The influence of relative movement during the self-acting position error compensation after connection of both parts has thus been completely neglected.
rc
3 . 2 . ~ Simulaticn of a screwing operation
In older to analyze tne jcining operation realistically a three-dimensional model with chamfers on both parts is required. For this purpose a finiteelement-model of a bolt (Figure 3.7) and associated threaded hole was produced.
Fig. 3.8:
Computer simulated screwing operation with rotation of the bolt. The position in space of the Finite-Element-Grid points are changed.
Points 2 and 3 are to be seen as functions of the compliance of the tool suspension facility. Requirements for screwdriving unit
3.2.3
Fig. 3.7:
Computer simulated screwing operation with bolt M6 x 15 (DIN 9 3 3 )
The sketch in Figure 3.9 shows the basic possibilities for compensating for lateral shift between the threaded hole and the spindle position. Case 2 is the most desirable because in case 1 the straightening bolt needs to move the entire mass of the screw spindle. In case 2 the force F depends primarily on the compliance of the screw nut guide.
The position of these two element grids in relation to each other can be selected at will. When simulating the docking process first the position of the first-contact points was calculated and then the bolt was given a rotational movement. To calculate the interaction thus produced on ?he complicated shape of the contact surface, a knowledge of the normal vector at the centre of gravity of the surface in question is required. To determine these vectors the thread geometry was transformed into a flat plane using the references:
Fig. 3.9: The normal vector in projection plane is now easy to calculate:
Compensation of the positioning error of the screwing unit during automatic screwing with industrial robots - Schematic diagram
A simplified equation shows
Transformed back into initial position the factual normal vector is: where
f E
I 1;
In this way, the change in the joining parameters caused by the rotation of the bolt can be determined for every point in the grid. These are: 1. Size and direction of righting movement (Figure 3 . 8 ) 2. Size and direction of farces and moments in screwdriving tool 3. Tensions in threaded sides
=
= = =
lateral displacement of the nut elasticity module of the nut guide moment of resistance length
As can be seen, the reaction force of the bolt can be minimized by using an extended guide length in keeping with the simultaneous requirement for a compact screwdriving unit.
457
3.2.6 F u t u r e Prospects
Both aspects are fulfilled by tt.e c r o s z - ' , e c t L c r i d : conplia7,ce element shown ir. Figure 1 . 1 c .
Among tr.e advantages of NCC as a p u r e l y passive elenenc i s tke possi~il;ty sf -ntegratir.g a ranqr cf a&iirior.al finctions, far exnnple:
Cortrol cf interns; pressure evacuatlng the Interlcr of the eiements thr >trips ir the case cf :ncl;ned screwlng can be t i q h t e r e d "package-wise" : c avo13 a positionrrlated change in the bclt 13cation throuqh its
3y
?'wl el 1gh.t
surveillarcr - f inrprnal pressure wito nr eccloicd i n t r i o r ar.y cnange in the shape of t h r elrmrnt i i l i L a u s e a c h a n g e in pressure. e . g . rxtrrs-on 3 u r i n j successtul screwinq 2pe:at;sn
Fig.
3.lC:
Extenslor. cf accive cornp:iar.ce s y s t e m b y simple means. Tne electrical resistance a t four positions staggered a l 9C degrees on t h e perlmeter can be scanned a s a gradual "short circuAt" of the irdividuai y t ~ l p s .In :r.e c a s e of deforrnt-on the lengtt-.of the currcnt pdtt. at t t e point 3 f Teasurernect cb.ar.qc.1 in a q t e p formatior.. The robcr can be guided in thc directiqn of t t . e least ~ e s i s t a n c e .
Screwing conpllance elenert Cross-section sf F E - M o d e l
The deformation behaviour v i t h an axial displacemnt of x = 6 m i s s h o w n in F L p r e 3.11. Tt.e tran6ve:se forces arising are in the reg-on ot 80 K and with d corsion of 3 degrees a torque 3 f approx. 50 Nm can be transferred ( c n l c ~ ~ a t i oby n means of finite element method). I n i t i a l tests Tade with a prototype s e t U F ac the :PA StLttgdrt have conf:rned the res'ilts 2.f t h e s e theore t i ca 1 as s ~ m pitsns .
3 Insertion of odd Yonponects w i t i assembiy r o b o t s 1 . 1 Insertion Techn31oq-f
I n t i g h 3 ~ 0 l ~?CB-P13ductloc e thc d d t o r n a t l c insertion of componrnts - s state 3 f tne art. Standard c o n p c e n t s ~ J C TA S
- axial ~
radial and 3 1 P canponents
can w inserted i i i ~ hspecialized machines. T h p remaining "cdd co>poner.tr", ap?roximately 2 0 Y of t'r.e total, are a l n ; o s t exclusively inserted manually. As a ~ ~ : . s e q ~ e n cthe e , assenbiy cost of the remaining 20\ 3 f compaonents represents a much nigner propsrtion. Ihe high fl~xibilicyrequired for irregular component insertion m a k p . i assembly robots an ideal to31 to perform this task. Assembly r o b o t s 'with differfit basic structures can be b s e d IFiqure 3 . 1 2 1 .
i.i.5
Sumrary
In order to solve a screwing problem the admissible limit v a l u e s for t h e joining parameters ran be calculated by s n u l a t i n g tne screwing p:oces6 first a n d t:hese can then be used f o r optlmal design of the "nearcollet compliance" i NCC) . T h e advantages of the described NCC element over compliant screw splndle attachments (remote-centre COmplldnCe-RCC! a r e d S fOlloWS:
minimal hardware expenses lov mass movement q o c d torsional rigidity !no generation 3 f so-called soft screw situati3ni
Flg. 3 . 1 2 :
Basic StructJres ?f assembly robots f o r PCB-asserbly
However, the flexible insertion uith assembly robots has the dravback of a signlflcantly I r , w ~ r i n s ~ r t i o n rate (see F l q u r e 3 . 1 3 1 .
integrated compliance in directlor. of screwing axis Ino additional spindle s u s p e r . s i o c r e q u i r e d i large dlsplacerenz p o s s i b l e with small dimens-ons no ur.cor.trolled d:splacener.t o f tt-.e screwing s p - n d l e by dead weight in case of tor-vertical 5crew:ng action
Fly.
458
j.:::
Trsprtion rate? of i n ~ e r t ~ s rrndchines . and assembly robots l f o r OED-Compoirntsi
Odd components can be categorized according to the parts presentation into
-
tape-able, stacks. non-st
3.3.3
Insertion methods
Several methods have been evaluated to be found in literature, references /11/12/13/14/ and the most promising laboratory tested. The results have led to the development of a new method and tools by the IPA. The existing methods either did not satisfy requirements concerning reliability, flexibility and quality (component damage) or could not be introduced in production economically. The developed method meets the requirements made in 3.3.2 without using vision systems. It is based on a special gripper system with a compliance which can be "frozen" and centered. The compliance movement in x , y and p (rotation) direction is without friction. Herewith the following assembly steps can be performed: 1. Gripping of the component 2. Insertion in a gage block with chamfered holes and correct insertion span. 3. Insertion into the printed-circuit-board.
Fig. 3.14:
Examples for stackable ODD-Components
With regard to the insertion process, odd components must be categorized into those with
-
"pin guided" insertion and those to be gripped at the component body.
Most large components belong to the second category. Examples are
-
block-capacitors relays coils
...
The insertion process for these componentswas analysed and methods and tools have beendeveloped to accomplish this task with assembly robotseconomically.
The gage block has chamfers which center the component pins. Through the friction compliance movement this is possible without pin bending or component damage. After insertion in the gage block the compliance is frozen. Next, for the insertion into the PCB the most critical tolerances are eliminated and the remaining tolerances are within the diameter difference between holes and pins. 4. Future of Automatic Assembly Future technological developments are a crucial factor when assessing the possibilities for application of automatic assembly equipment and the resultant lay-off potential. The results of a Delphi-inquiry are shown in Fig. 4.1 and Fig. 4.2 and refer to the Federal Republic of Germany. It is expected that 10,000 assembly robots will be in operation by 1992. This forecast is confirmed by the fact that a halving of the purchase price for assembly robots is expected by this time. Product design in compliance with the requirements of assembly work is also given high priority .
3.3.2 System Tolerances The analysis of system tolerances shows that the insertion of components without pin guidance cannot be a simple pick & place operation (see Figure 3.151.
Fig. 3.15:
Fig. 4.1:
Delphi-forecast: industrial robotapplication in the assembly field of I
Fig. 4.2:
Delphi-forecast: industrial robotapplication in the assembly field of I1
Tolerance accumulation during insertion of ODD-Components
Standard methods applied in the assembly of mechanical parts like a remote center compliance in the gripper system cannot be used for component insertion, because of the following problems: components have several pins to be inserted simutaneously (undefined multicontact to the PCB). The tolerances in dimensions (pin configuration) are relatively high. The pins to be inserted are relativly flexible and formable The PCB holes as well as the pin-tip configuration of components do not allow the insertion method to be based on chamfers as microscope pictures show. From the analysis of the tolerances, 2 basic requirements for joining can be summarized:
-
The III.JS: extreme influences throuah c o m o n e n t tolerances (body to pin centerline) and the-poor degree of straightness of the pins have to be eliminated. The relative tolerances between the pins (poor insertion span accuracy) must be compensated.
459
It was further forecast that the number of modular assembly robots would increase annually by 26% up to 1987.
'91
C.G. Mogged
: An analysis of screw mating
requirements for automated assembly. "Thesis for the Bachelor of Science". Massachussetts Institute of Technology 1977
Additionally, new areas of application will be opened up by new assembly systems based on modular processes. 1 1 0 1 J. Uilberg;
: Contribution to the Automation
C. Uaier
of Screwdriving with the aid of Industrial Robots. CCRP Annals 34 11985) 1, S. 49 52
-
i l l / B.D.Hoffmann;
:
S.H. Pollack: 8. Weissmann
Vibratory Insertion Process: A New Approach to Now-Standard Component Insertion. Robots Conference Proceedings. 4 - 7 Juni 1984, Detroit, Dearborn IUSA): SME 1984 Michigan
s. /12/ Kenneth H.K.
8
-
10
: Robot System for Insertion of
Custom Leaded Components Into a P.C. Board /13/ Rooks Brian
Pig. 4.3:
Concept and features of flexible automatic assembly system
/14/ Warnecke,H.J.;:
:
Robot assembly gives flexibility in computer manufacture Industrial Robot, 12 (1985) Sept. S. 169 - 1 7 3 Robotic insertion of odd components into printed circuit boards Assembly Automation 5 (19851 4, s. 198 201
-
B i b l i o g r a p h y :
/1/
Abele, E. u.a.:
/2/
Lotter. B.
:
/3/
B881er, R.; Wolf E. u. Elser. K.
: Rationalisieren der Montage
Einsatzmoglichkeiten von flexibe1 automatisierten Uontagesystemen in der industriellen Produktion Montagestudie. Dtisseldorf: VDI Vcrlag 1984 Manual of Assembly Technology. Krausskopf-Verlag. Mainz 1982 Industrieanzeiger 106 (19841 71, S . 104 109
-
/4/
Schraft. R.D.;: BaBler, -R.
Die montagegerechte Produktgestaltung muB durch systematische Vorgehensweise umpesetzt werden. VDI-Zeitung, 126 (1984) 22, s. 843 - 849
151
Abele, E.; Bahler. R.; Wolf, E.
I61
H.J. Warnecke;: Autmatisches Schrauben mit J. Walther Industrierobotern. wt-Zeitschrift fiir industrielle Fertigung 74 11984) 3, S . 137 - 140
: Entwicklungstendenzen bei
flexibel autmatisierten Montagesystemen wt-Z. Ind. Fertigung 74 (1984). S.
-
333
336
/-I/H.J. WarnecKe;: Investigations of the Screw
E. Abele, J. Walther, G. Fischer
/8/
Driving Process with SensorControlled Industrial Robots. CIRP Annuals 34 (1985) 1, S. 41 44
-
W. Kliemand; : Theoretische Untersuchungen K. Hopperdietz zum Einfldeln von Schrauben ftir die autmatische Montage mit Hilfe von Industrierobotern. Maschinenbautechnik 34 (1985) 3, S . 120 123
-
460
I would like to thank the members of CIRP who have taken part in the discussions about the problems and solutions involved in research and development in the field of assembly. In collecting the material for this paper and writing I was assisted by Dip1.-Ing. B. Frankenhauser, Dip1.-Ing. G. Fischer and Dip1.-Ing. E. Wolf, research fellows at the Fraunhofer-Institute for Manufacturing Engineering and Automation (IPA), Stuttgart.