In the process of metal stamping, the gap between punch and die not only has an important impact on the quality of blanking parts, but also affects the die life, blanking force, unloading force and pushing force. Therefore, clearance is a very important parameter in the design of blanking die.
1. Influence of clearance on quality of blanking parts
The cross-section quality of blanking parts mainly refers to the size of collapse angle, the proportion of smooth surface (bright zone) to plate thickness, the bevel size of rough surface (fracture zone) and burr. When the gap is appropriate, the shear cracks at the upper and lower cutting edges coincide. At this time, the smooth surface accounts for 1 / 3 ~ 1 / 2 of the plate thickness, and the collapse angle, burr and slope of the cutting section are very small, which can fully meet the requirements of general blanking.
The gap is too small. When the crack at the punch edge is larger than the reasonable clearance, it is staggered by a certain distance. The material between the upper and lower cracks will be sheared for the second time along with the blanking process, and then the punch will be included into the die hole. In this way, a second smooth surface is formed on the cutting section of the blanking part, a rough surface is formed between the two smooth surfaces, and the extruding burr appears on the end face. This kind of extruding burr is higher than the burr with reasonable clearance, but it is easy to remove, and the inclination and collapse angle of the rough surface are small, so it can be used as long as the middle tear is not very deep.
When the gap is too large, the crack at the punch edge is staggered by a certain distance from the reasonable gap. The bending and stretching of the material increase, the tensile stress increases, and the plastic deformation stage ends earlier, resulting in the reduction of the smooth surface of the cross section, the increase of the collapse angle and slope, and the formation of thick and large elongated burr, which is difficult to remove; at the same time, the warpage of blanking parts is serious, which affects the normal production.
2. Influence of clearance on blanking parts
The results show that with the increase of clearance, the blanking force decreases to a certain extent, but when the single side clearance is within the range of 5% ~ 20% of the material thickness, the reduction of blanking force is not more than 5% ~ 10%. Therefore, under normal conditions, the clearance has little effect on the cutting force. The clearance has a significant effect on the unloading force and pushing force. With the increase of clearance, the unloading force and pushing force will decrease. Generally, when the single side clearance increases to 15% ~ 25% of the material thickness, the unloading force almost drops to zero.
3. Influence of gap on the life of metal stamping die
The blanking die often fails in the form of blunt cutting edge and chipping edge. When the punch and die are blunt, the edge of the die will be rounded and abnormal burr will appear on the blanking part. When the punch edge is blunt, burr is generated at the edge of the blanking part; when the concave die edge is blunt, the burr is generated at the edge of the punched hole; when both the convex and concave edges are blunt, the manufacturing edge and the edge of the orifice produce burr.
Due to the bending deformation of the material, the reaction force of the material on the die is mainly concentrated on the edge of the punch and die. When the clearance is too small, the vertical force and side pressure will increase, and the friction force will increase, which will aggravate the wear of the die edge; then the metal debris generated by the secondary shear will aggravate the wear of the edge side; when the material is unloaded and pushed after blanking, the sliding friction between the material and the punch and die will cause the side wear of the edge again, making the wear of the side of the edge larger than that of the end face.
4. The clearance value of stamping die for blanking hardware
The clearance of each side between punch and die is called single side clearance, and the sum of two sides clearance is called double side clearance. Unless otherwise specified, blanking clearance refers to double-sided clearance.