A servo motor driven multi-action press for sheet metal forming

Int. J. Mach, Tools Manufact. Vol.31, No. 3, pp.345-359, 1991. Printed in Great Britain

A SERVO

0890-6955/9153.00 + .00 Pergamon Press plc

MOTOR DRIVEN MULTI-ACTION SHEET METAL FORMING

S. YOSSIFON,

PRESS

FOR

D. MESSERLY, E. KROPP, R. SHIVPURI and T. ALTAN (Received 10 October 1990)

Abstract--The goal of the current study is to adapt a servo-motor driven multi-action press for forming of sheet metal parts under precise velocity control and variable blankholding force. The Engineering Research Center for Net Shape Manufacturing [ERC/NSM] has developed a computer controlled multi-action press with two independently driven punches for research in cold forming of complex parts, using physical modeling techniques. Recently, the same press was modified and upgraded for use in sheet metal forming. As a result of its special design and construction, this press is highly controllable and can perform a wide range of sheet metal forming processes. Tests were conducted in order to obtain the actual press characteristics. Axisymmetric deep drawing experiments were performed with 1100-O annealed aluminum material using all three actions of the press. In these preliminary experiments, the crucial role of blankholding force control on final product quality and drawability was established.

1. INTRODUCTION

Ar XHE Engineering Research Center for Net Shape Manufacturing a computer controlled multi-action press with two independently driven punches was developed for systematic study of the mechanics of complex deformation process using physical modeling techniques [1, 2]. The press has two pneumatic cylinders for die clamping, and two counter acting punches mounted on top and bottom platens (Fig. 1). The punches are driven by a.c. servo motors manufactured by The Compumotor Division of Parker-Hannifin [3], and worm gear ball screw jacks manufactured by another supplier, Nook Industries [4]. The rotary input end of the screw jack is connected via a coupling to tlhe servo-motor drive, and the linear output end is connected to the punch. All the functions of the press are controlled by a personal computer. The press is capable of providing any desired punch stroke-time behaviour with up to 5 in. per second punch velocity. Because punch forces generated in the current press are small (1.5 tons), plasticine was chosen as material for process simulation. The structure of the press consists of a four post, three platen die-set with an effective die space of 15 in. by 15 in. The computer control system of the multi-action servo press has the capability to measure load-tiptoe data. The punch displacement at each load step can be calculated. Signal conditioners are used to accept the inputs of the various types of transducers. The entire press system is controlled by a PC through interface programs written in the C language. The command inputs include the direction and magnitude of motor rotation, motor velocity and acceleration, and the clamping plate position. The details of the press design and operation have been published earlier by Pale and Shivpuri [2]. The authors ran initial simulation experiments with a three legged "spider" made of plasticine (Fig. 2) using all three actions of the press. The nonsymmetrical geometry of the spider illustrates the special features and capabilities of this press. As a restilt of its special design, the press was found suitable also for investigating sheet metal forming processes.

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Sheet Metal Forming

347

2. APPLICATION OF MULTI-ACTION PRESSES

The use of multi-action presses is well known for many years. Recently, press designs and controls have been improved and the range of applications has been increased. For example, a typical hydraulic multi-action press has independent motions of the main ram, blank holder and the drive cushion [5]. Such presses can be used for forming complex parts such as stainless steel cooking utensils and automative body parts. A hydraulic press with four motions for high speed work was recently introduced by Lagan Company [6] and can be used for deep drawing of complex parts for high speed work. The press has a new control system which allows programmed changes of the press and blankholding forces as well as the ram speed during the actual stroke to achieve optimum drawing conditions. Another hydraulic press, with advanced design, is the 250T four action hydraulic press, which is used for complex deep drawing operations. The press has four separate actions, two on top and two on bottom [7]. Trend Industries, a custom stamping and fabricating firm (U.S.A.), produces deep drawn steel housings for a diaphragm-type air cylinder with this type of press where first draw, rew~,rse draw, redraw and trim are combined into one operation. All four actions are independently programmable for force, stroke and velocity. The use of a multi-action press to produce complex parts eliminates the need to anneal (in separate operation manufacture, alternate heating and cooling cycles work harden the metal, and frequently annealing is needed). Also, less time is required for set-up, compared to the time required to install three or more die stations in a single action press. Rhodes Company [8] has introduced a modern mechanical press with variable speed, powered by a d.c. moto;r which permits the speed of the machine to be changed during the press stroke. The variable speed feature of the drive enables the crank to be turned in either direction at speeds as low as 3 rpm. Based on the above references and discussions, the useful features of a sophisticated press, designed for complex drawing operations, can be summarized as follows: (a) multi-action capability; (b) precise computer control of the ram position, and the ram speed; (c) blankholder force control; (d) monitoring of forces, pressure and stroke position; (e) combination of mechanical and hydraulic drives in generating multi-action capability as discussed by Altan et al. [9]. The crucial :role of the blankholder force [BHF] control was studied by Romer et al. [10]. A special experimental set-up was designed and built in order to vary the BHF throughout the drawing stroke. The set-up consists of a special linkage mechanism driven by two horizontal screws. It was found that the limiting drawing ratio (LDR) can be increased by 15-20% with a variable BHF. Recently, Manabe et al. [11] have developed a new variable BHF method for deep drawing of sheet material. The BHF is controlled in order to prevent the punch load from exceeding a critical value which causes tearing in the drawn material. It was found that, in designing cups, the cup height and the LDR achieved by BHF control are larger than those obtained with constant BHF'. The importance of the fluid pressure acting on the flange area in hydroforming process has been studied by Yossifon and Tirosh [12]. The authors proved experimentally and theoretically that different pressure paths, varying during the process, lead to different strain distributions in the drawn parts. The effects of BHF on wrinkling ~tnd fracture limits for different sheets are discussed by Richards [13] and the acceptable BHF range is illustrated. 3. MODIFIED DESIGN OF THE MULTI-ACTION PRESS

The modification of the servo-motor driven multi-action press, located at the ERC/NSM, took into account the up-to-date design and control features of other modern multi-action presses for sheet metal forming [14]. The modification also includes the option of using the variable BHF during the working stroke.

348

S. YOSSiFON et al.

The objectives of the present study were to: (i) Modify the existing multi-action press, built for billet forming or forging studies, for forming sheet metal parts. (ii) Design special die sets for forming parts under precisely controlled BHF and ram speed conditions. (iii) Investigate the effect of the variable BHF during the forming operation upon part quality and LDR. (iv) Investigate the relationships between ram speed, BHF and draw ratio, in the first draw, redraw and reverse draw operations with multiple action tooling. In order to use the multi-action press for steel metal forming, modifications were done to the mechanical, pneumatic, data acquisition and control systems. Figure 3 shows the cross-section of the modified press with conventional deep drawing die set. The specifications of the modified multi-action press are shown in Table 1. The significant modifications of the press and the control include: (i) The top servo motor was transferred from the clamping plate to top plate, Figs 1 and 2. Through this redesign, the interaction between the blankholder force and the

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349

Sheet Metal Forming

TABLE 1. SPECIFICATIONSOF THE MULTI-ACTIONPRESS Number of punch motions (in addition to blank holder) Maximum punch load Maximum punch velocity Punch velocity profile Maximum punch travel

Two opposing and independent

Blank holder BHF profile

Variable, air activated User specified/PC control 4.0 tons

2.5 tons 1.25 in/s

(31.75 mm/s)

User specified/PC control

3.8 in.

Maximum BHF (200 psi inlet pressure) Press height Minimum shut height Daylight Effective die set space

(96.5 mm)

(2007 mm) (203 mm) (457 ram) (381 x 381 mm)

79.0 in. 8.0 in. 18.0 in.

15 x 15 in.

top punch force is prevented. This modification insures that all the motions of the press are controlled independently. As a result of this modification, an extension rod was needed between the output of the worm gear ball screw jack and the top punch. (ii) Top and bottom screw jacks were replaced to increase the punch forces. These screw jacks convert the rotational motion to the linear motion. Using the same servo motor with a different lead ratio screw jack, the output punch force was increased to 2.5 T and the maximum velocity decreased to 1.25 ips. In the first set-up, Fig, 3, the top punch was used as the ram and the bottom punch was used as an ejector. (iii) A rigid blankholder was mounted on the lower surface of the clamping plate, through three identical 1 in. diameter rods. Two Parker Hannifin pneumatic cylinders [15] of 5 in. diameter and 10 in. stroke were attached to the base plate to provide the necessary BHF. (iv) The modified pneumatic circuit of the press is shown in Fig. 4. The circuit basically consists of the two pneumatic cylinders, driven by a four port/two directional

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solenoid air valve with integrated speed adjustment. The cylinders are rated at 200 psi, permitting a theoretical BHF of 4 tons (including the 250 lb clamping plate weight). An air compressor or compressed air tank is used to generate the necessary line pressure of 200 psi. For a BHF up to 1.75 tons there is the option of using the 80 psi inlet pressure from the shop air. A special electro/pneumatic device made by Wabco [16] is connected close to the inlet of the cylinders to control the clamping pressure. The air supply to the convertor valve of the controller is obtained from a separate compressed air tank. This Wabco E/P relay convertor valve provides graduated pneumatic pressure output directly proportional to analog voltage command d.c. signal input of 1-5 V. This fast response valve provides the maximum air pressure within 0.64 s. (v) The data acquisition system is described in detail elsewhere [1, 2]. The modified press has the capability to measure the punch forces and the pressure in the cylinders during deformation• A 5000 lb load cell made by Sensotec [17] was installed in the extension rod between the screw jack and the punch to measure the forming load. Currently, the punch displacement is measured indirectly from the time-displacement relationship• In the near future an LVDT will be mounted to measure the punch displacement directly. A pressure transducer, range 0-150 psi, made by Omega [18], was connected to the air pressure line to record the clamping pressure vs time. These data can be converted to BHF vs stroke• (vi) The entire press system is controlled from a PC-AT compatible computer• The programs are written in the C programming language• The press has three axes of motion: (1) blankholder motion, (2) servo motor driven punch, and (3) servo motor driven ejector• The working cycle of the press for deep drawing consists of the following

steps:

(a) (b) (c)

Center the blank on the die surface after lubrication on both sides• Move both punches to an established initial position• Move the blankholder down. The BHF is regulated by the air pressure controller. Move the ram down. This can be done with either one or two steps in the ram stroke, fast approaching and slow forming• The ram force and the air pressure are recorded• (d) Move the top punch back to the home position with a constant velocity. (e) Return the clamping plate to the top position• (f) Eject the product with the bottom punch or ejector. (g) Return the ejector to its home position• Three programs were written to control this deep drawing process• Each program performs the above steps with a different level of automation. The fully automatic program is used for data acquisition• As soon as the ram starts to move down, the computer begins to collect data from the load-cell and from the pressure transducer• The data are stored in a data file, and then retrieved into a Lotus 123 file. This information is then processed to plot the ram force and BHF. A major obstacle encountered thus far has been the occurrence of slgmficant noise in the transducers' signals• To eliminate this noise caused by the motor controller, the loadcell and the pressure transducer were shielded and grounded. The press modification is still being carried out. Part of the modifications, discussed above, will be completed in the near future. Right now the pressure controller is being tested. The control programs are being modified to control the air pressure from the computer• 4. TESTING OF THE MODIFIED PRESS Three experiments were conducted to evaluate the capabilities of the modified triple action press. 4.1. Smiling punch forces in the press In order to obtain the stalling punch force, a rubber cylinder was placed between the punch and the base, Fig. 5. The rubber cylinder was 1.5 in. in diameter × 1.5 in.

Sheet Metal Forming

351

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tall with a 65 shore A hardness. The punch was moved downward, compressing the rubber cylinder at a predetermined constant velocity. The top a.c. servo motor eventually stalled. The punch force was measured by a precision miniature load cell model 31 [17], mounted within the extension rod, Fig. 3. The compression of the rubber during punch travel was used to simulate the deep drawing process. Four experiments were conducted with different constant punch velocities. The measured and the expected stalling forces versus punch velocity are shown in Fig. 6. The measured results show that the stalling force increases as the punch velocity increases, while the expected stalling forces, based on the maximum value of the torque-speed curve of the a.c. servo motor (KH-260 model) and the ball screw jack (2.5 HLBSJ model), decrease moderately. The discrepancies between the measured and expected forces could be due to a variation in inertial forces and a different response of the selected servo motor to different load-time profiles resulting from the dynamic response of the rubber at different punch velocities. 360O

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4.2. Blankholder force (BHF) calibration The blankholder force is obtained by pushing the clamping plate down toward the workpiece flange area. The set-up used to measure the blankholder force is shown in Fig. 7. In order to calibrate the blankholder force, the air pressure was gradually increased. The force exerted on the load-cell (mounted between the blankholder and the base plate) and the air pressure were simultaneously measured using a strip chart recorder. Figure 8 shows the experimental results. For increasing air pressure, the graph shows a near linear relationship between the blankholder force and the air pressure in the cylinders• With decreasing air pressure, the curve is slightly higher, probably as a result of the friction forces in the air pistons and the guides of the clamping plate. 4.3. Blankholder stiffness The stiffness of the blankholder, i.e. the axial movement of the blankholder with varying air pressure, is a significant parameter in the deep drawing process. Increasing

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Sheet Metal Forming

353

the stiffness of the clamping plate, which is holding down the blankholder, results in an axial load that depends on the axial displacement of the flange (in wrinkling mode). This type of b]tankholder is called a "spring-type blankholder". It provides an axial loading proportional to the thickening or wrinkling of the drawn part's flange area. Yu and Johnson [19] have studied the effect of a spring type blankholder on the flange wrinkling instability by assuming constant wall thickness of the drawn cup. A similar study has been done by Triantafyllidis and Needleman [20]. They have concluded separately that as the blankholder stiffness increases, the onset of wrinkling is considerably delayed, and the number of "wrinkle waves" is greatly increased. This means that with a higher stiffness, the blankholding forces needed to prevent wrinkling type failure may be smaUer (less initial an axial force needed). Reducing the blankholding force also reduces the tension stresses in the radial direction and prevents premature rupture or tearing in the product. A screw jack was mounted between the blankholder and the load-cell (placed on the die surface). After pushing the blankholder down with a constant blankholding force, the screw jack was adjusted upward against the blankholding plate, Fig. 9(a). The lateral force vs the lateral displacement was recorded up to the sliding point of the clamping plate. Figure 9(b) shows the experimental results for four different air cylinder

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354

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presures. The curves are nearly linear and almost parallel after 0.001 in. displacement. The tangents on the graphs indicate the stiffness of the blankholder. The stiffness values ranged between 5 x 106-7 x 106 N/m. The measured stiffness is lower than the calculated stiffness of the clamping plate, assuming free movement in the guides, but higher than the calculated stiffness of the air pistons. During an actual deep drawing process, due to the thickening of the outer zone of the flange area throughout the drawing, the actual blankholder forces will increase slightly as seen in Fig. 9. 5. EVALUATION OF PRESS CAPABILITIES

The redesigned computer controlled multi-action press, discussed in this paper, will be used in the investigation and development of sheet forming processes such as: (i) conventional deep drawing with rigid or rubber pad blankholder and ejector; (ii) conventional deep drawing with blankholder assisted by fluid and ejector; (iii) redrawing process using the second punch; (iv) First draw, redraw and reverse draw in one cycle with controlled BHF; (v) deep drawing of polygonal shapes under controlled BHF. The first step in the verification and evaluation of the press was to use a conventional deep drawing die set with rigid blankholder, as shown in Fig. 3. The die and the punch were made from 4340 alloy steel with 40 Rc hardness. In this process all of the three actions of the press are used. The essential die set dimensions are summarized in Table 2. An annealed aluminum blank (Al. 1100-O) of 0.5 mm thickness and 90.0 mm diameter was used. A Swift-type equation (or=oro (~+e)n) was utilized to represent the effective stress-strain (cry) relationship. Where fro and to are the material constants, n is the work hardening exponent. The material properties are listed in Table 3, where R is the average normal anisotropy and AR is the planar anisotropy. The local deformation pattern in the workpiece can be determined by using a circular grid pattern of 2.70 mm outer diameter, electrochemically etched on one side of the blank surface. Care is taken to insure that the etching is not excessive. A set of experiments was conducted with four different constant BHF in order to evaluate the effect of the BHF on the strain distribution and on the punch force profile for fully drawn parts. The experiments were conducted with Valvoline Moly grease lubricant. The punch velocity selected for the current study was constant and equal to 6.25 mm/s. Experimental strain measurements were made for each of the specimens after forming, using a Nikon Measurescope with 5 × magnification and digital position read

TABLE 2. DIE-SET DIMENSIONS Punch diam. (ram)

Die opening (mm)

Punch profile radius (mm)

Die profile radius (mm)

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5.0

5.0

TABLE 3. MATERIALPROPERTIES O"o (MN/m 2)

%

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0.22

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0.16

Sheet Metal Forming

355

out. The strains at the curved regions were measured using the Maglessi and Lee method [21]. Narrow strips of transparent adhesive tape were placed on the outside surface of each specimen, and the major and minor outer sizes of the deformed grids were marked. The strips were then removed from the cup for strain measurement. 6. R E S U L T S

AND

DISCUSSION

The measured true radial and tangential strain distributions for two different constant BHF values (4 kN and 6.8 kN) are shown in Fig. 10(a) and (b). The true strain distributions were plotted vs the initial blank radius. It is seen from these two figures that for the higher BHF, the absolute values of the true radial and the tangential strains at the far edge of the flange area are also higher. Figure 10(c) shows the experimental true thickness strain distributions which was obtained indirectly through the volume constancy assumption. Higher BHF leads to less thickening at the far edge, and eventually a thinner and longer cup. These experimental results are expected because as the BHF increases, based on the equilibrium equations, the radial stresses also increase. Figure 11 shews the punch force vs ram stroke for the two different BHF (4 kN and 6.8 kN). As expected, for the same lubrication conditions and the same geometry and material, higher forces were recorded for the higher BHF test. Table 4 shows the maximum strains and punch forces for the four different BHF tests. Figure 12 shows a picture of a fully formed cup drawn with a 6.8 kN blankholder force. The deformed grid pattern is seen very clearly. A sound product was obtained, with relatively ,;hort "ears". Recently, a computer program called SHEET_FORM was developed for simulating axisymmetric sheet metal forming operations such as stretch forming and deep drawing [22]. The program is capable of predicting the strain distribution, punch forces, andl the dimensions of the workpiece during the stroke and takes into account the bending = effects as a result of the die and the punch radius profile. The program will be used in the near future to study the effects of variable BHF on the drawn parts. The modified servo motor driven multi-action press will still be used for simulating

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forging processes, with the model material, plasticine. All of the modifications that were completed in this study contribute to expand the fundamental experimental capabilities of the ERC/NSM. 7. FUTURE WORK

In the near future, the multi-action press will undergo further modifications. The special air pressure controller will be installed for evaluation of the effects of variable BHF on the limiting drawing ratio, strain distribution and punch force in sheet metal forming processes. Two LVDT units will be installed for displacement measurement,

Sheet Metal Forming

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358

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TABLE 4. EXPERIMENTAL RESULTS FOR DIFFERENT BHF DRAWN CUPS BHF (kN) 2.5 4.0 5.4 6.8

Max. radial strain

Max. thickness strain

Max. punch force (kN)

0.32 0.32 0.353 0.444

0.13 0.12 0.ll 0.097

4.98 5.25 5.47 5.65

one for the top punch and the second for the blankholder. Special die sets will be designed and built for single stage operation, using all three actions of the press. The SHEET_FORM program will be modified in order to simulate drawing, redrawing, reverse drawing with variable BHF. A multi-action press driven by servo motors with high power and punch forces could be available in the near future. Fanuc company [23], produces a.c. servo motors up to 17 hp, with rated torque of 147 Nm. Recently, it was reported that an a.c. servo motor, with up to 70 hp capacity, was developed for high-power motion control applications [24]. With increasing servo motor power, it will be possible to develop special presses where high torque and excellent speed control will be available over a wide speed range. The ERC/NSM is planning to pursue these developments as part of a strategy to develop novel concepts and applications in metal forming machine tools. Acknowledgements--The authors appreciate the help of Mr Kevin Sweency in modifying the control and data acquisition of the press. Thanks to Ms Mary Hartzler for her technical assistance during the press modification. Also, the authors would like to thank Ms Joan Snyder for typing the manuscript.

REFERENCES [1] A. J. PALE, R. SHIVPURIand T. ALTAN, Development of equipment and capabilities for investigation of the multi-action forming of complex parts, Engineering Research Center for Net Shape Manufacturing, The Ohio State University, Rept No. ERC/NSM-B-89-28 (July 1989). [2] A. J. PALE and R. SmVPURI, Development of a computer multi-action press for physical modeling of complex deformation, NAMRC XVIII Conference, Pennsylvania State University (May 1990). [3] Compumotor, Programmable Motion Control, Catalog No. 8000, Petaluma, California (1989). [4] Nook Industries, ActionJae, Catalog, AJ-87, Cleveland, Ohio (1987). [5] K. H. PETERS, Design features of the hydraulic press and its field of application, Sheet Metal Ind. 46, 221-226 (March 1969). [6] Lagan Presses, Quadruple ram presses, Sheet Metal Ind. 64, 138 (March 1987). [7] J. N. PENNINGTON,Single press operation forms complex deep drawing, Modern Metals 45, 88-91 (September 1989). [8] Presses Feature Modern Developments, Sheet Metal Ind. 65, 228 (May 1988). [9] T. ALTAN, S. OH and H. GEGEL, Metal Forming Fundamentals and Applications. American Society for Metals, Metals Park, Ohio (1983). [10] I. ROMER,F. FISCHERand F. BRUEN,On the influence of a variable hold-down pressure on the boundary drawing ratio, Stahl und Eisen 104, 1065-1072 (1984). [11] K. MANABE,H. HAMANOand H. NISHIMURA,A new variable blank holding force method in deep drawing of sheet materials, J. JSTP 29, 7405 (1988). [12] S. YOSSIFONand J. TIROSH, On the dimensional accuracy of deep drawing products by hydroforming processes, submitted for publication in Int. J. Mech. Sci. [13] P. N. RlCnARDS,Forming and drawing of sheet steel--part 2, Sheet Metal Ind. 58, 913-917 (November 1981). [14] H. RU~ER, Modern control of deep drawing presses, Sheet Metal Ind. 66, 221-223 (May 1989). [15] Parker Hannifin Corp., Pneumatic Cylinder Products, Catalog No. 0900-2, Des Plains, Illinois. [16] Wabco Electro/Pneumatic Devices, E/P or I/P Electric to Pressure Controls, Wabco Fluid Power Catalog No. SC/600, Lexington, Kentucky (July 1988). [17] Sensotec Corp., Sensotec Transducers and Instrumentation, Catalog No. 7000-989, Columbus, Ohio (1988). [18] Omega Engineering Corp., Complete Data Acquisition and Computer Interface Handbook and Encyclopedia, Stamford, Connecticut, 27 (1990). [19] T. X. Yu and W. JOHNSON,The buckling of annular plates in relation to the deep drawing process, Int. J. Mech. Sci. 24, 175-188 (1982). [20] N. TRIANTAFYLLIDISand A. NEEDLEMAN, An analysis of wrinkling of the swift cup test, J. Engng Mat. 102, 241-248 (1980).

Sheet Metal Forming

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[21] S. A. MAGLESSIand D. LEE,Development of multistage sheet metal forming analysis method, J. Mat. Shaping Technol. 6, 41-54 (1988). [22] S. SITARAMAS,G. KINZELand T. ALTAN,Process sequence design for multi-stage forming of axisymmetric sheet metal parts, Engineering Research Center for Net Shape Manufacturing, The Ohio State University, Report Number ERC/NSM-S-89-49 (1989). [23] Fanuc AC servo motor series, catalog No. ASV-12 (1989). [24] W. ERICI~SON,Intelligent drive for induction motors provides true servo accuracy, Information Quarterly, Mannesmann Rexroth, pp. 10-11 (January 1990).