3 Fundamentals of press design

274 Sheet metal forming and blanking
ble dies or, where double dies do not represent a viable investment due
to small batch sizes, the use of a feed system for offset blanking.
Blanking force and blanking work
The necessary blanking force F S [N] for punches with flat ground working
surfaces (closed blanking contours) is calculated with the following
equation:
FS = AS⋅ kS = lS⋅s⋅ kS [ N],
whereby AS is the sheared surface in mm2 and kS the resistance to shear,
shearing strength or relative blanking force of the sheet metal expressed
in N/mm2 . The sheared surface is determined from the length of the
sheared contour lS [mm] multiplied by the thickness of the sheet metal
s [mm], whereby the length of the sheared contour is taken to mean the
sum of all sheared edge lengths in mm. If the ratio of the punch diameter
d to the sheet metal thickness s is greater than 2, the following
equation is sufficient for an approximate calculation of the shearing
strength kS: N
kS = 08 . ⋅R
⎡ ⎤
m 2 ,
⎣⎢ mm ⎦⎥
whereby R m [N/mm 2 ] is the tensile strength of the sheet metal.
The blanking force must not exceed the nominal press force within
the nominal force-stroke curve, given in press specifications, as otherwise
the machine will be overloaded. If the sheet metal thickness is
larger than the nominal press force stroke position (i.e. distance before
BDC, where nominal press force is given), then the permissible press
forces are smaller (cf. Sect. 3.2.1). These can be ascertained from the
load versus stroke diagram included in the operating instructions.
The force of the return stroke for stripping the workpiece off the
punch is around 3 to 5% of the blanking force when the ratio of the
punch diameter d to the sheet metal thickness s is around 10 (d/s = 10).
In the case of smaller d/s ratios, the return stroke forces increase substantially,
amounting with d/s = 2 to around 10 to 20% of the blanking force,
while this drops further with greater d/s ratios. A greater return stroke
force is required for stripping tough materials than for brittle ones. The
return stroke force must be taken into account when designing the
punch and dies, and in extreme cases also when dimensioning the press.
Lateral forces can also occur during blanking operations. Particular
attention must be paid to these, in particular when the strip layout has
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998