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Abstract. Experimental and computational procedures for studying deflections, flit, and alignment characteristics of a sequence of stamping dies, housed in a transfer press, are pre- sented. Die loads are actually measured at all the 12 die stations using new load monitors and used as input to the computational procedure. A typical stamping die is analyzed using a computational code, MSC/NASTRAN, based on finite element method. The analysis is then extended to the other dies, especially the ones where the loads are high. Stresses and deflections are evaluated in the dies for the symmetric and asymmetric loading conditions. Based on our independent die analysis, stresses and deflections are found to be reasonably well within the tolerable limits. 41671
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However, this situation could change when the stamping dies are eventually integrated with the press as a total system which is the ultimate goal of this broad research program. INTRODUCTION Sheet metal parts require a series of operations such as shearing, drawing, stretching, bending, and squeezing. All these operations are carried out at once while the double slide mechanism descends to work on the parts in the die stations, housed in a transfer press [1]. Material is fed to the press as blanks from a stock feeder. In operation the stock is moved from one station to the next by a mechanism synchronized with the motion of the slide. Each die is a separate unit which may be independently adjusted from the main slide. An automotive part stamped from a hot rolled steel blank in 12 steps without any intermediate anneals is shown in Figure 1. Transfer presses are mainly used to produce dif- ferent types of automotive and aircraft parts and home appliances. The economic use of transfer presses de- pends upon quantity production as their usual pro- duction rate is 500 to 1500 parts per hour [2]. Al- though production is rapid in this way, close tolerances are often difficult to achieve. Moreover, the presses produce a set of conditions for off-center loads owing to the different operations being performed simulta- neously in several dies during each stroke.
Thus, the forming load applied at one station can affect the alignment and general accuracy of the operation being performed at adjacent stations. Another practical problem is the significant amount of set-up time in- volved to bring all the dies into proper operation. Hence, the broad goal of this research is to study the structural characteristics of press and dies combina- tion as a total system. In this paper, experimental and computational procedures for investigating die prob- lems are presented. The analysis of structural char- acteristics of the transfer press was pursued separately [3]. A transfer press consisting of 12 die stations was chosen for analysis. Typical die problems are exces- sive deflections, tilt, and misalignment of the upper and lower die halves. Inadequate cushioning and off- center loading may cause tilt and misalignment of the dies. Tilt and excessive deflections may also be caused COMPUTATIONAL PROCEDURES Linear static analysis using finite element method wasused to study the effect of symmetric and asymmetricloading for this problem. A finite element model ofdie station 4 was created using the graphical prepro-cessor, PATRAN, and the analysis was carried outusing the code MSC/NASTRAN. The code has a wide spectrum of capabilities, of which linear static anal- ysis is discussed here. The NASTRAN code initially generates a struc- tural matrix and then the stiffness and the mass ma- trices from the data in the input file. The theoretical formulations of a static structural problem by the dis- placement method can be obtained from the refer- ences [6]. The unknowns are displacements and are solved for the appropriate boundary conditions. Strains are obtained from displacements. Then they are con- verted into stresses by using elastic stress-strain re- lationships of the die material. The solution procedure began with the creation of die geometry using the graphical preprocessor, PA- TRAN. The solution domain was pided into appro- priate hyper-patches. This was followed by the gen- eration of nodes, which were then connected by elements. Solid HEXA elements with eight nodes were used for this problem. The nodes and elements were distributed in such a way that a finer mesh was cre- ated at the critical region of the die-sheet metal in- terface and a coarser mesh elsewhere. The model was then optimized by deleting the unwanted nodes. The element connectivities were checked. By taking ad- vantage of the symmetry, only one quarter of the die was analyzed. In the asymmetric case, half of the die was considered for analysis. Although, in practice, the load is applied at the top of the die, for the pur- pose of proper representation of the boundary con- ditions to the computational code, reaction forces were considered for analysis. The displacement and force boundary conditions are shown for the two cases in Figure 4. As mentioned earlier, sheet metal was not modeled in this preliminary research.
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