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In the press shop. the rear fioor panel was stamped by a single-action press with two nitrogen cylinders supplying power to the blank-holder. In order to provicle more metal for the cup area without adding anextra operation, the lower die sliape was pided into two portions, as shown in Fig. 15. The middle portion of the die shape was driven by a wedge mechanism and can move up and down relative to the rest of the lower die, which remains stationary. Since the modified die shape was designed to eliminate the split-defect, the location of the movable portion was chosen at the cup area. The movable portion was driven upwards through the wedge mechanism as the blank-holder closed. At the closure of the blank-holder, the movable portion lifted the sheet-blank to a particular height. providing the cup area with more metal than that in the original design. The movable portion was then forced to move downwards when it was contacted by the top die during the die-closing process. At die closure, the movable portion was retrieved to its position and a sound part was formed.
It is obvious that the mechanism for the movable portion of the lower die provides a better binder-wrap in the cup area before the punch moves down. However, the optimum amount of additional metal provided is yet to be determined. The optimum amount of metal is defined as that with which the split defect can be eliminated without causing unacceptable wrinkles. In the present study, a simple calculation which limited the average elongation of cross-sections at the cup area to a maximum value of 20% was performed to determine this amount. According to the calculation, the movable portion of the lower die should raise 300 mm at the closure of blank-holder to provide the proper amount of metal for the cup area.
6. Results and discussion
A modified lower die frame corresponding to the above calculation was manufactured to validate the optimum die design. As a result, rear floor panels with good quality were obtained using the modified dies during the try-out period. To quantify the quality, circle-grid analysis was conducted again for the formed part, the circles being scribed on the same area as they were in the original design. The major and minor strains around the cup wall where the split used to occur were measured and plotted in the forming-limit diagram, as shown in Fig. 16.As seen in this figure, all the measured strains were in the safe zone, which indicates that the first-draw operation with the modified dies is very stable and is not sensitive to process variation.
In order to make the validation more complete, circlegrid analysis was performed for the second-draw operation also. In addition to forming the ribs in the panels, as shown in Fig. 1. the second-draw operation also sharpened the corner radii at the root of the cup. Thus, the second draw would stretch the cup area a little. The major and minor strains measured in the first draw and the second draw at the cup area are shown in Fig. 17. It is seen clearly in Fig, 17 that the strains are higher in the second draw, as is expected. Although several strains are in the marginal zone, most of the strains stay in the safe zone. Since the number of points falling into the marginal zone is small and their strains are still close to the design curve, the second-draw operation can also be considered stable, As for the third and forth operations, only triming and simple flanging processes are involved. Therefore, these two operations do not cause any further deformation, and need not be analyzed.
References
[1]S.P.Keeler,SAE paper 650635(1965).
[2]Z.Marciniak and K. Kuczynski, Int. J. Mech. Sci. , 9(1967)609-620.
[3]R.Arrieux and M. Boivin,Ann. CIRP, 31(1)(1987)195-198
[4]B.R. Deis, SAE paper 900279(1990).
[5]S.P. Keeler and W.A. Backofen, Trans. ASM, 56(1963)25-49.
[6]W.G. Brazier and R.W. Thompson,SAE paper 850274(1985).
摘要 本文研究了客车后底板的冲压制造过程。使用圆栅格分析和3-D有限元方法，对产生拉深开裂缺陷的最初的冲模设计进行了分析。开裂缺陷是由于压边圈下大范围的金属限制了向杯状区域的流动。优化的冲模设计，包括一个分离的冲模面和一个楔形机构组成的凹模结构，目的是在不添加工序的情况下，向杯状区域提供额外金属、消除开裂缺陷。这种优化的冲模设计在第一次和第二次拉深的圆栅格分析结果中得到验证，获得了合格的拉深面板。42327