Sheet Metal Forming

part is used as stagger, it allows the next group of 
punches to contact the blank material prior to the 
first group snapping through. Hence, the snap-
thru energy from the first group of punches is 
absorbed to drive the next group of punches. 
This is particularly important in high-speed 
stamping.
Factors Affecting Punch Life
Punch materials and coatings
play an im-
portant role in influencing punch life. Punch 
material and coatings are selected depending on 
the blank material, forces involved, and the ex-
pected tool life.
Table 1.1 Properties of various blanking punch shapes (Fig. 1.11 and 1.12)
Punch and die end shapes
Advantages
Disadvantages
Flat
…
Large punch force generated
Single shear
Lower punch force because shear is local instead of 
entire tool periphery
Reduce reverse tonnage
Unilateral displacement of punch due to 
asymmetry
Double shear, reverse shear, conical
Lower punch force
Unilateral displacement of punch is avoided
Can control negative tonnage/snap-thru
Deformed slug
Chipping can occur in reverse shear
Conical, chamfer at edge
Used when deformed slug is required; easy for slug 
stacking
Punch forces may not be reduced because en-
tire periphery of the punch comes in contact 
with the blank at the same instant
Convex and concave dies
Lower punch forces
Slug remains flat, important when it is the useful part
…
Source: Ref 1.3
Fig. 1.12
 
Die geometries and sheared dies used in blanking. Adapted from Ref 1.3, 1.8

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8 / Sheet Metal Forming—Processes and Applications
Clearance.
Punch wear is greatly influenced 
by the clearance between punch and die. Smaller 
clearances induce greater punch forces. There is 
an optimum clearance for which punch forces 
are the minimum, based on the properties and 
thickness of the material blanked.