# Multi-Pass Stamping Forming a Concave Ring
## 2 Die Design and Process Parameter Selection  
The flanging process was divided into compression-type flanging and elongation-type flanging [19]. The flanging profile of the compression-type flanging is convex. The metal material in the deformation  
Figure 1: Concave ring.  
area was subjected to compressive stress which resulted in compression deformation and a thickness increase. In contrast, the flanging profile of the elongation-type flanging is concave. The metal material in the deformation zone was subjected to tensile stress which caused elongation deformation and a thickness reduction. The forming process of a concave ring has both compression-type flanging and elongation-type flanging. It bears compressive and tensile stresses at the same time. The inward flanging of the outer edge of the blank is compression-type. The outward flanging of the hole edge of the blank is elongation-type. The multi-pass stamping and bending process of the concave ring is shown in Figure 2. The contour of the upper-1 groove is composed of chamfers, arcs and straight lines. The 45\({}^{\circ}\) chamfer was used to avoid the upper-1 and lower die being unable to be closed due to the roundness error of the blank. The arc of the upper-1 groove is to guide the blank to bend in the desired direction. The inclined straight line is to make the bending angle of the blank reach 45\({}^{\circ}\) in the first pass. In the second pass, the bottom surface of the upper-2 was designed as a flat surface, so that the bending angle of workpiece reaches 90\({}^{\circ}\). The inner die is to ensure the forming shape inside the concave ring.  
To realize the above forming process, the designed stamping dies consist of six parts: upper (including first-pass upper die and second-pass upper die), inner and lower. The inner was designed in three parts: external inner, internal inner and middle inner. The exploded view of the stamping die assembly is shown in Figure 3. In order to facilitate the installation and removal of the inner, the middle inner is a cone wedge with a cone angle of 1\({}^{\circ}\). The external inner and internal inner were divided into six pieces. The division method of the external inner and internal inner is shown in Figure 4.  
Figure 3: Exploded view of concave ring stamping die assembly. 1—upper; 2—middle inner; 3—internal inner; 4—external inner; 5—blank; 6—lower.  
Figure 2: Stamping and bending forming process of concave ring.  
When putting the inner into the workpiece, first install the internal inner, then install the external inner, and finally insert the middle inner. In addition, there is a gap between the bottom of the middle inner and the workpiece. The inner is then inserted into the workpiece, and the installation drawing for the first pass is shown in Figure 5a. After the concave ring is formed, first pull out the middle inner, then take out the internal inner and the external inner in order. In the second pass, the workpiece is bent under the punching of the upper-2 as shown in Figure 5b.  
When the concave ring is bent, the length of the straight edge of the bent part must meet the following conditions [19]:  
\[h>2t \tag{1}\]  
where \(h\) is the length of the straight edge of the bent part of the bending piece; \(t\) is the thickness of the bending piece.  
Figure 4: Design of internal inner and external inner.  
Figure 5: Installation drawing of stamping die. (**a**) First pass; (**b**) second pass.  
The cross-section of the concave ring is shown in Figure 6. If \(h<2t\), the straight edge of the blank exceeds to be increased, and it will be removed when the processing is completed.  
According to the bending conditions and the requirements for forming the concave ring, the die parameters are shown in Table 1.