Sheet Metal Forming

Effect of Punch-Die Clearance in
High-Speed Stamping (Ref 1.3)
Traditionally, clearance of approximately 5% 
material thickness per side is used as punch-die 
clearance. However, a clearance of approxi-
mately 10 to 12% instead of 5% has many ad-
vantages. During impact, when the punch first 
touches the sheet, the part material bulges from 
under the punch point and compresses into the 
die. Using greater clearances produces the op-
posite effect; that is, the part stretches under 
tensile stress. This is predominant in materials 
thinner than 0.5 mm (0.02 in.). Hole character-
istics differ with different clearances. Lower 
clearances achieve lower burr. The burr length 
increases with increase in clearance but starts to 
decrease and reaches a minimum value at a 
higher clearance close to the engineering clear-
ance. Using a higher clearance also gives a 
higher tool life. While using engineered clear-
ance, there should be a means of slug control. 
Otherwise, there would be a problem of slug at 
withdrawal of the punch. Some of the solutions 
for slug pulling are Jektole punches with side 
vent holes, air blowoff, or different punch point 
configuration and die designs. Jektole punches 
have a hole along the length of the punch for air 
to pass through, which would enable easy slug 
removal (Fig. 1.14). The slug becomes stuck to 
the punch because of the vacuum formed be-
tween the punch and slug, and Jektole punches 
prevent that from occurring. Jektole punches 
may not be effective for heavily applied lubri-
cant and lightweight slugs. Overentry of the 
punch into the die may cause excessive wear 
and slug pulling.
Punch Misalignment (Ref 1.1) 
The punch and die assembly can align well 
on the bench during die set-up. But when the 
press is up and running, the deflective forces in 
the press and tooling can cause the punch to 
drift and misalign. Punch vibrations also cause 
misalignments and greater stresses in the tool. 
These vibrations must be dampened for a longer 
punch life. An effective guiding system for the 
press and tooling can help reduce both the 
problems.