Sheet Metal Forming

part. The punch forces differ because the area of 
contact with the sheet at a given instant in the 
penetration length is not the same. Shear angles/
Fig. 1.11
 
Burr depends on material blanked; Mat 2 and Mat 3 are more ductile and homogeneous than Mat 1. Source: Ref 1.5
Fig. 1.10
 
Effect of tool wear and blanking clearance on part edge quality as predicted by simulations on 0.58 mm (0.02 in.) thick 
copper alloy. Source: Ref 1.6
chamfers on punches are also used for easy 
stackup of the slugs. The slugs, when bent, be-
come neatly stacked over one another. Figure 
1.6 shows the various punch shapes, and Table 
1.1 lists the advantages and disadvantages of 
the different shapes.
Die Geometry (Ref 1.3)
Minimal die land length allows the slug to 
fall away. Fewer slugs in the die matrix can

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Chapter 1: Blanking / 7
prevent slug jamming and pulling. Land length 
should not exceed four times material thickness 
for effective slug removal. When full tapers are 
sharpened, the dimensions of the die change a 
little; in terms of slug removal, they perform
the best. Increased taper is designed to prevent 
soft part material from sticking in the relief 
area, which minimizes slug stacking. Figure 
1.12 shows the various die geometries with
respect to die length and taper. The figure on 
the right in Fig. 1.12 shows the shear angles
on the die (instead of having the shear angle
on the punch) to achieve lower forces. This 
would enable the part to be flat instead of bent, 
which occurs when the shear angle is on the 
punch.
Punch Stagger (Ref 1.3) 
It is a common practice to stagger punches to 
minimize impact and snap-thru shock. Punch 
staggering (Fig. 1.13) reduces the initial shock 
but does not reduce the total shock. Staggering 
punches implies that different groups of punches 
come in contact with the blank material at differ-
ent instants of time by having different punch 
lengths. If the shear zone length of the blanked