3 Fundamentals of press design

Fine blanking
it is also possible to determine the mean pressure pm [N/mm2 ] applied
on the punch:
FS + FG
N
pm
=
⎡ ⎤
2
A ⎣⎢ mm ⎦⎥
or when punching a hole, whereby the counterforce is generally taken
as 10% of the punch force:
p
The mean pressure p m must not exceed the 0.2% compression limit R p0.2
[N/mm 2 ] of the perforating punch (p m �R p0.2 ). Accordingly, on the basis
of the previous equation for the maximum ratio of sheet metal thickness
s to perforating punch diameter d, the following equation results:
If we assume that the perforating punch is made of high-speed steel
S6-5-2 with R p0.2 = 3,000 N/mm 2 and HRC 63-64 and that the tensile
strength of the fine blanking material is 500 N/mm 2 , the following s/d
ratio results when blanking with counterpressure:
and when blanking without counterpressure:
m
s
d ≤
=
St
44 .
1
s
d
⋅s⋅ Rm⋅f ⎡ N
d ⎣⎢ mm
Rp02
.
≤
44 . ⋅R ⋅f
This maximum s/d ratio is generally assumed to be 1 in normal shearing
practice.
Part configuration: flat and formed
The geometric shape of a part, the thickness of the sheet metal and the
characteristics of the material determine the production possibilities
available by fine blanking. In order to ascertain whether a part can be
manufactured using fine blanking, its degree of difficulty is determined:
S1 (easy), S2 (medium), and S3 (difficult). Here, the various formed elements
such as slot widths, section widths, hole diameters, tooth forms,
corner angles and radius must be evaluated with the aid of Fig. 4.7.12.
m
1
[ – ]
2
⎤
⎦⎥
3000 , 3000 ,
= = 150 .
44 . ⋅500⋅09 . 1980 ,
s
d ≤ 3000 , 3000 ,
= = 167 .
4⋅500⋅09 . 1800 ,
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
343