Computer Aided Design of Multi-Stage Cold Forging Process: Load Peaks and Strain Distribution Evaluation

Computer Aided Design of Multi-Stage Cold Forging Process: Load Peaks and Strain Distribution Evaluation P. Bariani, E. Benuzzi, Institute of Applied Mechanics, University of Padua/ltaly; and W. A. Knight (2). Department of Industrial and Manufacturing Engineering, University of Rhode Island, I
SUMMARY This paper describes some developments of an interactive program aimed at assisting the user in analysing for suitability the forming sequences for multi-stage cold forging of rotationally symmetric parts. The program capabilities include (i) the automatic analysis of sequences in forging solid as well as hollow parts and recognition of individual operations involved in each step, (ii) the evaluation of the load-peak distribution in the different forming stages.and (iii) the prediction of the strain distribution accumulated in the blanks and the finished part. The program is part of an integrated system consisting of a suite of interactive procedures whose purpose is facilitating all the engineering activities for the process planning functions involved in manufacturing components on automatic multi-station cold forging machines . The major implication of including the developed program in this CAD system is that of providing the user with the tools suitable to perform a complete producibility check and to identify the most appropriate forming sequence. KEY WORDS : CAD, CAPP, cold forging, forging sequence.

1. TNTRODCCTION

2. OUTLINE OF THE SYSTEM LNCORPORATLVG THE PROGRAM

The design of operation sequences for multi-stage cold forging operations is one of the most critical responsibilities of process planners, the mechanical properties of the finished product as well as the tooling and operating costs being greatly affected by preforming steps.

As mentioned above, the program is part of an integrated CAD system for cold forging on automatic multi-station machines. The flow of information within the system is illustrated in Fig.1 and the four modules encompassed by the system have been designed to assist the user respectively in:

Although a comprehensive computer-assisted process planning system for cold lorging has not yet been developed, d number of CAD procedures [or cold forging process design have recently been proposed I1 I - I4 I . They consist of modules to assist the user in generating complete forging sequences for classes of components according to design rules which are incorporated in the programs and formulated on the basis of precedence and grouping relationships and process limitations relevant to each operation.

- calculating the load peaks at the forming stations and the total

Alternative sequences are generated for the same finished part by adjusting certain design rules and input parameters such as wire or billet diameter. Identifying in these alhernatives the most appropriate forging sequence requires the skills of experienced planners. In fact, a considerable amount of information has t o be handled and processed concerning :

selecting and sequencing the forming operations, effective strain accumulated within the worked part, generating the drawings of the tool sets to produce the part, and then performing the complete timing of the machine chosen for the job. The sequence-design module enables the user to interactively generate appropriate sequences of the forming operations. It is based upon a simple classification of the finished part geometry and workplans for each class. The workplans have been developed on the basis of sequencing and grouping rules formulated from known processlimitations of the available operations. Geometric description of the workpiece utilizes simple volumetric elements, thus enabling easy

(i) the producibility of the parts on the machines (e.g. in terms of number of forming stations, maximum cut-off length and diameter, maximum combined press load and maximum press load per station, etc.), (ii) balancing of load peaks and tool Me in the different stages,

(iii) die and operating costs and (iv) mechanical properties of the finished part as affected by the forging steps. The paper describes some developments of an interactive program aimed at assisting the user in analysing for suitability the forming sequences for multi-stage cold forging of rotationally symmetric parts. The program capabilities include: (i) automatic analysis of sequences in forging solid as well as hollow parts and recognition of individual operations (e.g. dumping, reducing, forward and backward extrusion, can extrusion, hollow extrusion, upsetting, etc.) involved in each step;

(ii) evaluation of the load-peak distribution in the different forming stages. On the basis of part material and friction coefficient data as well as blanks geometry the program calculates the maximum forming load required to perform the relevant operations, and (iii) prediction of the strain distribution accumulated in the blanks and the finished part. At the time of writing the prediction is provided only for solid parts. The program is part of an integrated system consisting of a suite of interactive procedures whose purpose is facilitating all the engineering activities for t h e process planning functions involved in manufacturing components on automatic multi-station cold forging mdchines I 3 I I 6 I . The major implication of including the developed program in this CAD system is that of providing the user with the tools suitable to perform a complete producibility check and to identify the most appropriate forming sequence. In the first part of the paper an outline of the system incorporating the program is provided. The latter part concentrates on the description of structure, working principle, present capabilities and limitations of the program, some application examples ending the paper.

Annals of the CIRP Vof.36/1/1987

Fig. 1. The flow of information within the integrated system. geometric description and classification to be done automatically. The user is able to view the sequences appropriate to different choices of the wire diameter from which the part is produced. Detailed descriptions of the approach followed in developing the procedure and of its structure and capabilities are given in I3 I and I5 I respectively. The second module -the next section concentrates on presentation of this module- assists the user in analysing for suitability the forging sequences either generated by the previous module or traditionally prepared and then inputted into the system data-base. To this purpose, the distribution of load peaks at the machine stations and the effective

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"ancillary" operations, without any contribution to the load peak, surh as the operation carried out on the metal trapped at the die zone of the rod extrusion; operations which cannot be performed on multi-station cold forging machines.

strain accumulated in the blanks and the finished workpiece are automatically evaluated. The tooling design module -it has not yet been implemented- is devoted to generating the drawing of the set of punches and dies to be installed on a specific machine in order to process a part according to the sequence designed by the previous module. It is expected to assist he user in the critical task of drawing profiles of die and punch cavities and designing the assemblies (often shrink fitted) of which dies and punches consist. The module will enable integration of the process planning system with tool manufacturing. NC tapes can be prepared for machining both the components and the EDM electrode for some cavity profiles. The post-processor module completes the computer-based process planning system, its purpose being to assist the user in timing ejection and transfer operations for a specific machine and in designing the system for gripping blanks and their transfer between stations. Included in the capabilities of the module is automatic checking of the whole timing plan for collisions which can arise during the return stroke of the transfer mechanism. At the present stage, the module includes all the above post-processing capabilities for a set of I Iatebur automatic cold-forging machines and work is in progress to extend the range o f machines to include National machines. The approach followed in developing the module as well as its structure and capabilities are descri'sed in I 6 I and I7 1 respectively.

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As shown in Fig.1, each module accesses a specific region of the system data base. Accordingly, data files wThich are input for the individual modules are generated at the end of execution of the previous module when the system works as an integrated system. Alternatively, they can be directly inputted via an interactive graphical dialogue mode, when the modules are run a s stand-alone procedures.

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OUTI.INE OF THE PROGRAM

In the present section the automatic procedure developed by the authors to evaluate the load peaks at the machine stations and the effective strain distribution accumulated in the workpiece will be outlined. Details are provided concerning the steps involved in the procedure, its working principle as well as its present capabilities and limitations. 3. 1 The Program Stem and Workinv Principle. Independently of its use - as a module integrated into the system or as a stand-alone procedure - the program consists of the following steps:

(i) blank geometry input and classsification; (ii) volumetric-element correspondence identification; (iii)forming operation recognition; (iv)material and friction data input; (v) load-peak calculation, and (vi)effective-strain distribution evaluation. In step (i), the program accesses the data files describing the geometry of the blanks (hencelorth blanks include the billet and the finished workpiece besides the partially completed parts produced by each forming station) relevant to the proposed sequence. The blanks are then automatically classified and coded on the basis of the inner basicshape to distinguish solid blanks, blanks with cavity (single or double, stepped and non-stepped) and with a through bore. Finally, the volumes of the single blanks are compared to evaluate the maximum deviation from the volume constancy. In the second step, the program analyses automatically the pairs of blanks relevant to adjacent stations. For every pair, the blank geometry is split into elementary volumetric elements and the correspondence between them is then established. In finding the corresponding elements, the maximum deviation from the volume constancy calculated at the previous step is taken into account. At this stage, the piercing operation and the relevant geometric parameters are identified. The third step of the program is devoted to the automatic recognition

of the forming operations performed at each station of the machine along with the relevant deformation data. To this end, the deformation involved in the corresponding volumetric elements is processed by a "pattern recognition" logic based upon more than twenty decision tables such as that on Fig. 2. The operation responsible for the deformation is identified among a number of individual operations including, besides the no-operation case relevant to identical corresponding elemcnts,

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146

operations which can give a contribution to the load peak, such as rod, can and hollow extrusion, upsetting, etc;

Fig. 2. Corresponding volumetric elements and decision table At step (iv), the program accesses the data files to calculate the material current flow-stress and friction resistance at the material-tool interface. The strength coefficient and the strain-hardening exponent of the power law which approximates the material true stress-true strain curve are stored together with the shearing resistance, the latter being used to calculate both shearing and piercing load peaks. In the same data file, the friction coefficient values are arranged according to the workpiece material, the lubrication and forming operation and -for operations such as the hollow extrusion- to the particular toolworkpiece interface.

The load peaks at the machine stations are then calculated on the basis of the recognized operations, the relevant deformation data along with the material flow stress and friction coefficient. The forming operations considered by the program in its current stage of development and which can provide a contribution to the load peaks are the following

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shearing, piercing, indentation, dumping, reducing, heading, upsetting of solid and annular elements , rod extrusion, forward and backward, can extrusion, forward and backward, hollow extrusion, forward and backward.

Lastly, - only for solid parts - the effective-strain distribution accumulated into the the blanks at the different stations are evaluated. In calculating the strain, only the ideal contribution to the total effective-strain is taken into account as if the deformation were truly homogeneous in the blank sections, the contribution of the redundant deformation being neglected at all. 3. 2 The Program Cap&lities and Limitations In conformity with the capabilities of the other modules of the system, the program can deal with solid and hollow rotationally symmetric workpieces.

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Fig. 6 . Load-peak distribution for the sequence of Fig.4.

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The "pattern recognition" logic utilized to identify the corresponding volumetric elements of two adjacent blanks and then recognize the involved operation is successfully applied to parts with nearly any suhsidiary shape feature, the only substantial constraint being an axisymmetric geometry. The program can, in fact, handle parts with straight, stepped or flanged outer surface, solid or with single or double cavity, with and without a through bore, on the only conditions that the inner profile of the cavity is straight and obtained by a flat and/or conical punch and no boss protrudes from it. The shape classification for rotationally symmetric workpieces proposed by Wagener and reproduced on I8 I could be helpful to this regard. It is worthwhile to note that these shape limitations are essentially due to the lack of

Fig. 8. Effectivestrain distribution for the blank marked by arrow

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Regarding the prediction of effective-strain distribution, the current limitation of the program for hollow blanks is mainly due to the fact that - for this class of parts - the contribution of the redundant deformation to the total strain is not negligible. Moreover, in some cases such as in the material region below the punch during can extrusion, this redundant deformation prevails over the homogeneous deformation. Work is currently in progress to incorporate into the program some analytical models, thusly extending the capability of predicting the strain distribution to certain classes of hollow workpieces.

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Fig. 9. Forging sequence and main operations.

4.

APPLICATION EXAMPLES

In order to illustrate the capabilities of the program, examples are given of displays involved in analysing some forging sequences for two workparts. Figs. 3 and 4 show two sequences for the same solid part generated according to the selection of two different wire diameters. In the same figures the operations responsible for the load peaks -which are automatically recognized by the program together with secondary operations- are displayed above the respective blanks. The diskibution of the load peaks at the shearing station and the six forming stations are shown in Figs. 5 and 6 respectively for the sequences of Fig. 3 and 4. The displays of Figs. 7 and 8 concern the distribution of the effective strain into the respective finished parts. Fig. 10 illustrates the load-peak distribution calculated for the SAE 1015 steel bevel gear processed through the four-stage sequence reproduced in Fig. 9.

5. CONCLUDING REMARKS A computerized procedure has been developed to assist the user in analysing for suitability the forming sequences for multi-stage cold forging of rotationally symmetric workpieces. The major implication of utilizing the program as a module of an integrated CAD system for cold forging i s that of providing the process planner with the tool suitable to carry out a complete producibility check and to identify the most appropriate forging sequence. Benefits which may result from application of the program are the design of preform sequences consistent with the mechanical properties of the finished product and correct utilization of presses and tools.

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Fig. 10. Load-peak distribution for the sequence of Fig.9

6.

ACKNOWLEDGEMENTS

The authors wish to thank the CNR - Progetto Finalizzato Tecnologie Meccaniche for its grant for the work on which this paper is based 7.

REFERENCES

I1I Noack, P.,"Cornputer-Aided Determination of Operation Sequence

and Costs in Cold Forging of Rotation-Symmetric Workpieces", SME Technical Paper (MF73-141), 1973. I21 liebholz, M.,"Computer-Aided Production Planning in Cold Forging", Annals of the CIRP, Vol. 29/1,1980. i31 Davison, T.P. and Knight, W.A., "Computer Aided Process Design for Cold Forging Operations", Advanced Technology of Plasticity, Proc. 1st Int. Conf. on Technology of Plasticity, Vol.1, Tokyo, Sept, 19%. 141 Radawy, A.A. and al., "Computer-Aided Design of Multistage Forging Operations for Round Parts", J . of Mech. Working Technology, Vol.11, 1985, pp. 259-274. 15 I Davison, T.P., unpublished report, Department of Engineering Science, University of Oxford, 1983. 161 Bariani, .'I and Knight, W.A., "Computer Aided Cold Forging Process Design: Determination of Machine Setting Conditions", Annals of the CIRP, Vol. 34/1,1985. 171 Bariani, P. and Knight, W.A., "Prototype Post-Processor for a Computer-Aided Cold Forging Workplanning System", report No OUEL 1590/85, Department of Eng. Science, University of Oxford, 1985. I8 I Lange, K.,"Handbook of Metal Forming", McGraw-Hill, 1985