3 Fundamentals of press design

276 Sheet metal forming and blanking
When using a bevel ground punch or female die, the blanking force
is reduced by at least 30% compared to punches or female dies with a
flat surface, as only a certain part of the die is engaged at any one time
(Fig. 4.5.10). However, overall the blanking work remains the same, as
the reduced force acts over a longer stroke.
If there are several punches mounted in a die, it is also possible to
reduce the blanking force by causing the punches to act in sequence
(Fig. 4.5.9 g). However, in order to ensure a smooth press operation and
reduce the stress on the dies, when using this method, the height difference
between each punch force should not be greater than the penetration
depth of the die until the fracture of the sheet metal (around
0.3 to 0.4 · s). Bevel grinding of punches or female dies and offsetting in
the case of several punches also help to reduce noise during the blanking
process.
Example:
Assuming a 2 mm thick, hard brass sheet Ms 63 (kS = 380 N/mm2 ) is to be
perforated using a flat ground punch of 113 mm in diameter. The sheared surface
is:
2
A = d⋅π⋅ s = 113 mm⋅π ⋅ 2 mm =
710. 0 mm
S
Accordingly, the blanking force will be:
F = A ⋅ k = 710⋅ 380 = 270, 000 N =
270
kN
S S S
The blanking work is determined with the following equation:
W x F s Nm resp. kNm
= ⋅ ⋅ [ ]
S S
with the blanking force F S and material thickness s. The factor x [–] takes
into consideration the actual progression of force when blanking and
depends on the material. It lies within the approximate range of 0.4 to
0.7, whereby the lower value applies for brittle materials, a large blanking
clearance and thick sheet metals, and the upper value is used primarily
for tougher materials, a small blanking clearance and thin sheet metals.
For approximate calculation, the following equation applies:
2
W S FS s Nm resp. kNm
3
= ⋅ ⋅ [ ]
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998