Die Design Fundamentals. Vukota Boljanovic
href="#ulink_4fbba556-ffaa-50f1-8969-8e9349cbdac7">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.
The first step in the actual production of stampings is to order standard-size sheets of the proper size and thickness from the mill. These are then sheared into strips, as described above. The widths of the strips into which the sheets are to be cut is specified by the die design department. Therefore, let us go over the steps taken in determining strip width so that your understanding will be complete.
Figure 3.5 Typical part drawing.
Figure 3.5 shows a part drawing of a typical representative stamping to be produced in a pierce and blank die.
3.4.1 Blank Layout
Figure 3.6 shows the two possible ways of running the strip of a typical representative stamping (Figure 3.5) through the die. The blanks may be positioned the wide way, necessitating a wide strip, or they may be run the narrow way, permitting the use of a narrower strip. These are called “blank layouts” and it is important that you understand exactly what is meant by the term. A blank layout shows the way in which the designer proposes to produce the blank. For both the wide-run and narrow-run layouts, two holes are to be pierced at the first station and the part is to be blanked out at the second station. It is customary to show small piercing punches in solid black. Section lines are applied through larger piercing punches and through blanking punches, as shown. The strip width and the feed are given directly on the blank layout.
Figure 3.6 Blank layouts necessitating either wide or narrow strips.
Now let us go through the first steps taken in the production of blanks in cut-off dies. Two of the sides of such blanks are originally sides of the material strip, and no scrap bridge is produced as in blanking dies. Figure 3.7 shows a representative stamping having the parallel sides typical of blanks suitable for production in cut-off dies.
Figure 3.8 shows two blank layouts for producing the stamping (Figure