descent of the upper cutting edge causes cracks to form in the material. These cleavage planes occur adjacent to the corner of each cutting edge.
5.Continued descent of the upper die causes the cracks to elongate until they meet. Here then is the reason for the importance of correct clearance. If the cracks fail to meet, a bad edge will be produced in the blank.
6.Further descent of the upper die causes the blank to separate from the strip. Separation occurs when the punch has penetrated approximately 1/3 of the strip.
7.Continued descent of the upper die causes the blank to be pushed into the die hole where it clings tightly because of the compressive stresses introduced prior to separation of the blank from the strip. In other words, the material at C in step 3 was compressed and it acts like a compressed spring. The blank, confined in the die hole, tends to swell, but it is prevented from doing so by the confining walls of the die block. Conversely, the material around the punch tends to close in and, therefore, the strip clings tightly around the punch.
Figure 3.2 Mechanics of shear-enlarged views of clearance between cutting edges of a shearing die (step 1) and material undergoing shear (steps 2 to 8).
8.The punch has now penetrated entirely through the strip and the blank has been pushed entirely within the die hole. Observe that the edge of the blank and the edge of the strip have identical contours except that they are reversed. The strip will cling around the upper punch with approximately the same pressure as the blank clings within the die hole and a stripper will be required to remove it.
3.3.1 Sheared Edges
It now becomes necessary to understand exactly what occurs when sheet material is cut between the cutting edges of a punch and die. Figure 3.3 shows the cut edge of a blank with correct clearance c applied, enlarged many times to reveal its contour. Observe the following:
The top corner is defined by a small radius R. The size of this radius depends upon the thickness and hardness of the strip and on the sharpness of the punch and die members.
A smooth, straight, burnished band goes around the periphery of the blank. The extent of this band, distance D, is approximately 1/3 the thickness T of the blank when the die is properly sharpened and when the correct clearance has been applied.
The remaining 2/3 of the edge is called the breakoff. The surface is somewhat rough and tapers back slightly. The extent of the taper, distance B, is the amount of clearance between cutting edges. If burrs are produced in cutting the blanks, they occur on this breakoff side of the blank. Burrs are produced when improper clearance has been applied and also when cutting edges become dull. The other side of the blank, which has the radius and smooth, shiny band, is called the burnished side of the blank.
Figure 3.3 Enlarged view of sheared blank edge.
The location of the burnished side and of the burr side of the blank is very important for performing secondary operations such as shaving, burnishing, and the like. In addition, the burr-side position can influence the functioning or the appearance of the finished stamping.
In blanking, the burnished band goes completely around the blank and the breakoff taper extends completely around the blank on the opposite side. This is not the case for blanks produced in cut-off or progressive dies. In such dies, the burnished side may alternate from side to side in a number of positions. Careful study is needed to ensure that no burr will interfere with the function or appearance of the stamping.
Shearing of material occurs in a continuous action. However, to understand the process, it will be necessary to “stop” the action in its various stages and to examine what occurs.
3.3.2 Clearance
Clearance generally is expressed as a free space between two mating parts. In closed contours, clearance is measured on one side.
a) Insufficient Clearance
The inset at A in Figure 3.4a shows the four effects of insufficient clearance:
•Radius R is smaller than when correct clearance is applied.
•A double burnished band D is formed on the blank edge.
•The breakoff angle B is smaller than when correct clearance is applied
•Greater pressure is required for producing the blank.
Referring to Figure 3.4a:
1.This figure shows cutting edges of a punch and die in partial penetration. It is obvious that cracks have appeared at the punch. Die sides will not meet when extended because the clearance is insufficient.
Figure 3.4 Enlarged views of blank edge sheared: a) with insufficient clearance A and material undergoing shear with insufficient clearance (1 to 3); and b) with excessive clearance A and material undergoing shear with excessive clearance (1 to 3).
2.Continued downward descent of the punch causes elongation of the cracks. The uncut area between them will be broken in a secondary fracture.
3.At the bottom of the stroke, the secondary fracture has occurred. A second burnished band has been produced on the blank edge and on the strip edge. The characteristic contour shown in inset A has been formed.
b) Excessive Clearance
The inset at A in Figure 3.4b shows the four effects of excessive clearance:
•Radius R is considerably larger than when correct clearance has been applied.
•Burnished band D is narrower.
•The break-off angle B is excessive.
•A burr C is left on the blank.
Referring to Figure 3.4b:
1.This shows the cutting edges of a punch and die in partial penetration. Cracks have begun to form at opposite sides
2.Continued downward descent of the punch causes elongation and widening of the cracks. Their alignment is fairly good
3.At the bottom of the stroke, separation has occurred, leaving the characteristic blank edges shown in the inset at A.
When a die is provided with excessive clearance, less pressure is required to effect cutting of the material. For this reason, more clearance is often specified for blanking the heavy gages of stock to reduce pressure on the press.
