Sheet Metal Forming

Factors Influencing Part Edge 
Characteristics
effect of Blanking Clearance on Part edge 
Quality.
In general, as the clearance between 
punch and die increases, the shear zone, roll-
over zone, fracture angle, and burr increase 
while the fracture zone decreases. Insufficient 
clearance produces secondary shear; that is, the 
cracks originating at the punch and die do not 
meet. Hence, a ring of material is further stressed 
to its shear limit, expending more energy. Ex-
cessive clearance causes large plastic deforma-
tion, large burr, and high fracture angle. Fur-
thermore, tool life is also lowered by improper 
clearance. Many applications demand a certain 
shear zone and fracture zone limit in the hole/
blank for good assembly/alignment. Based on 
the demands required by the specific application 
and tool life consideration, a specific clearance 
can be set between the punch and die. Advanced 
high-strength steels require greater clearances 
of approximately 10 to 14% compared to mild 
steels, which give good part edges at clearances 
of approximately 6% of sheet thickness (Ref 
1.3). 
The effect of punch-die clearances in the 
range of approximately 5 to 20% in blanking 
1.4 mm (0.06 in.) DP590 steel is shown in Fig. 
1.8 (Ref 1.4).
The effect of tool wear on part edge qual-
ity 
is significant. Tool wear leads to the forma
-
tion of burrs and increases burr length. Burr 
length is generally an important criterion in the 
industry to evaluate part quality. Burr length in-
dicates when the tool should be reground to ob-
tain the sharp die-and-punch radius. It has also 
been observed that the effect of tool wear is 
more pronounced at higher blanking clearances. 
This has also been shown experimentally by 
Makich et al. (Ref 1.5) (Fig. 1.9).
Tool wear can be estimated using finite-
element simulations by theoretically increasing 
the punch and die corner radii in simulations.