Chapter 1: Blanking / 3
results in an increase of the cutting force up
to the maximum load. At this time, the cross
section is not reduced and shearing has not
started.
•
Step 3:
The increased pressure
at the cutting
edges prevents the material from flowing,
and shearing starts. Due to a decreasing
cross section, the blanking force decreases
despite the strain hardening of the material.
•
Step 4:
Fracture starts after the formability
limit of the material is exceeded. As soon as
the
initial cracks meet each other, slug and
skeleton are completely separated. The cut-
ting force decreases rapidly during this
phase.
A way to reduce the maximum force on the
tool would be to change
the design of the punch
such that it is stepped or has an angle on the
punch edge. This would lower the maximum
force by spreading the work over a larger depth,
as seen in Fig. 1.5 and 1.6.
Fig. 1.4
Load-stroke curve of a blanking/piercing operation
Fig. 1.5
Punch force profiles for different tip geometries. PVD, physical vapor deposition. Adapted from Ref 1.7
4 / Sheet Metal Forming—Processes and Applications
1.3 Part edge Characteristics
Different Zones of a Part Edge
The shear edge is made of different zones
based on the method of
material deformation
that has occurred. The ratio of the different
zones is influenced by different parameters,
such as the punch-die clearance,
material prop-
erties, and punch corner radius, to name a few.
In general, it is preferred to have a large shear
zone and smaller burr.
The zones and deformation modes of a
blanked part edge are given as follows and in
Fig. 1.7:
•
Rollover
zone
(
Z
r
)
:
Caused
by plastic mate-
rial deformation
•
Shear zone
(
Z
s
)
:
Smooth and shiny area, cre-
ated during material shearing
•
Fracture/rupture zone
(
Z
f
)
:
Rough surface,
results after the material cracks
•
Burr zone
(
Z
b
)
:
Caused by plastic deforma-
tion
•
Depth of crack penetration
(
D
cp
)
:
Angle of
fracture zone, depends mainly on clear-
ance
•
Secondary shear:
Created if cracks do not
run toward each other and material is sheared
again
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