# Multi-Pass Stamping Forming a Concave Ring
## 4 Multi-Pass Stamping Experiment  
From the above simulation, it proved that stamping is suitable for producing concave rings. The concave ring stamping experiment was performed on a 100 T hydraulic press. The punching velocity was set to 100 mm/s. The blank was shown in Figure 10. The dies need to be installed before the experiment. First, the blank was placed in the groove of the lower. Next, the inner needs to be placed in the groove of the blank in sequence. Figure 11 shows the completion of the mold installation. After the first pass was completed, the upper-1 was replaced by the upper-2 for the second pass forming. The experimental process of the first pass and the second pass is shown in Figure 12.  
Figure 9: Strain field. (**a**) First pass; (**b**) second pass.  
cross-section of the experimental and simulated workpiece is shown in Figure 14a. The final workpiece is sectioned along the axial direction for measurement, as shown in Figure 14b. The measurement results are shown in Table 2. The height of the workpiece is 3.31 mm. The width is 10.82 mm. They are all within the allowable dimensional tolerance range. The dimensional error is less than 0.5%. The average processing efficiency of stamping the concave ring is 4.6 min per piece.  
The defects of the concave ring formed by multi-pass stamping are shown in Figure 15. In the first pass, there is a positioning error between the workpiece and the upper-1. If there is no 45\({}^{\circ}\) chamfer at the entrance of the upper-1 groove, the bending surface of the workpiece will rupture as shown in Figure 15a. The influence of the positioning error can be eliminated by adding a 45\({}^{\circ}\) chamfer at the entrance of the upper-1 groove. In addition, when the inclination \(\alpha\) of the upper-1 groove is too small, the force of the upper-1 on the workpiece is mainly in the vertical direction, which makes the bottom of the workpiece bend, as shown in Figure 15b. In the second pass, when the depth of the lower groove is less than the height of the concave ring, cracks will occur on the sidewall of the workpiece where it contacts the top of the lower groove, as shown in Figure 15c. There will be no cracks when the depth of the lower groove is the same as the height of the concave ring.  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **Height of the Workpiece (mm)** & **Width of the Workpiece (mm)** \\ \hline Design Value & 3.30 & 10.80 \\ Actual Value & 3.31 & 10.82 \\ Error & 0.3\% & 0.2\% \\ \hline \hline \end{tabular}
\end{table}
Table 2: Dimensional measurement results of the experimental workpiece.  
Figure 14: Results of the second pass. (**a**) Comparison of the cross-section of the experimental and the simulated workpiece; (**b**) dimensional measurement of the experimental workpiece.  
Figure 13: The workpiece after the multi-pass forming. (**a**) First pass; (**b**) second pass.  
Simulations and experiments show that it is feasible to form a concave ring by multi-pass stamping. The deformation of the workpiece during stamping is stable. The dimensions of the workpiece after stamping is within the allowable dimensional tolerance range. The multi-pass stamping process for processing the concave ring has a higher production efficiency. It can replace the spinning process for processing the concave ring.  
Figure 15: Cont.