May 2021 Volume 3

FORGING RESEARCH

in the optimization of the blank and tooling dimensions in a design of experience (DOE) of a cold forging process so a satisfactory part could be made. The design objective was to define the minimum and the maximummaterial conditions of the blank and appropriate punch nose dimension to ensure the forging would fill correctly at locations A and B (Figure 23a), enabling the finished component to be machined from the forging without a non-clean-up (NCU) spot even under the minimummaterial condition.

Furthermore, the simulations revealed two problems. One was that under the nominal material condition, the upper section of the workpiece was curved and underfilled. This might potentially leave NCU of the upmost OD corner. The other was that under the maximum material condition, the workpiece was too full, whereas under the minimum material condition, the workpiece was barely touching the tooling. In addition, the forging forces under the two conditions were substantially different (95.3 metric-tons vs. 296 metric-tons). These revelations led to a sensitivity study in which the dimensions of the blank and punch nose were varied. The simulation results from this study showed that the ID dimension at Point B was controlled mainly by the blank size, but the OD dimension at Point A was controlled by the diameter of the punch nose. After several iterations of simulations, the design was finalized by slightly increasing both the minimum material condition of the workpiece OD and the diameter of the punch nose. When the minimum material condition of the blank OD was increased from 138.887 mm to 139.192 mm, that of the blank wall thickness was increased from 11.041 mm to 11.532 mm, and the punch nose diameter was increased by 3.297 mm, the diameters at Points A and B from the simulation were 149.509 mm and 117.699 mm, respectively, both of which satisfied the design criteria. The deformed workpiece from the simulation, as shown in Figure 25, was significantly improved from the initial design shown in Figure 24b. After these modifications, the forging force under the minimum material condition was 255 metric-tons, narrowing the maximum force difference from 200 metric-tons to 40 metric-tons. Because of the increase in the minimum material conditions, the nominal material conditions shifted slightly when the maximum material conditions of all dimensions of the blank were kept constant. Based on the above finite element analysis, new blank nominal dimension and tolerances were proposed, as shown in Table 3, with a comparison of the initially designed blank dimensions. By following the proposed blank tolerances, quality products were made in production.

a

b

c

Figure 23. Forging, blank and tooling Factors considered in this DOE included the initial blank dimensions (Figure 23b) and punch nose dimensions (Figure 23c). The initial gap between the punch and die at location A was 11.753 mm. By varying these factors in the preset ranges, the models were able to display the influence of these factors on the forging geometry. In all models, the blank was assigned as a rigid-plastic material and all tools were assumed to be rigid material. The nominal, minimum and maximummaterial conditions of the blank are given in Table 2. Table 2. Three material conditions of the initial blank (in mm) The design required that the limit dimensions of the workpiece at the two critical points A and B were: 1) at the OD gage height of Point A, the workpiece diameter should not be less than 146.990 mm; and 2) at the ID gage height of Point B, the workpiece diameter should not be greater than 117.704mm.The first group of models was built using the nominal, minimum and maximum material conditions of the blank from the initially given dimensions and tolerances. Simulation demonstrated that the workpiece under the nominal (Figure 24a) and maximum (Figure 24c) material conditions satisfied the above requirements, whereas the workpiece under the minimum material condition (Figure 24b) did not.The diameters from the simulations at Points A and B under the minimummaterial condition were 145.034 mm (-1.956mm) and 119.088mm (+1.384mm), respectively.

Figure 25. Simulation result with the increased blank minimum material condition and punch nose modification

Figure 24. Simulation results under the nominal (a), minimum (b) and maximum (c) material conditions

FIA MAGAZINE | MAY 2021 81