3 Fundamentals of press design

282 Sheet metal forming and blanking
u
a 1
a
Fig. 4.5.12
Dimensions for piercing and blanking on the punch and the
female die
clearance generally reduces the necessary force and work requirement,
and thus also tool wear; with a smaller clearance, in contrast, a qualitatively
improved cut surface and greater part accuracy are frequently
achieved. In conventional blanking with blanking speeds in the region
of 0.1 to 0.2 m/s, the optimum blanking clearance is between 2 and 10 %
of the sheet metal thickness (cf. Fig. 4.7.6), whereby the lower value
applies to thinner or softer sheet metals.
When piercing sheet metal, the dimension of the pierced hole is
determined by the blanking die; the opening in the blanking die must
accordingly be selected to be twice the blanking clearance greater than
the blanking punch. When blanking external contours, by contrast, the
female die determines the size of the blanks. The punch must accordingly
be configured smaller by twice the blanking clearance.
Example:
A 3mm thick medium-hard steel sheet is to be processed in a progressive tool
(cf. Sect. 4.1.1). Initially a hole with a diameter of 10 mm is to be pierced, after
which the circular blank with a diameter of 28mm (external contour) is to be
blanked out of the metal strip (Fig. 4.5.13):
1st stroke – piercing:
The piercing hole punch is given the nominal dimension 10mm. The female
blanking die has the diameter
3mm ⋅6%
d = 10 mm+
2 ⋅ = 10 mm + 036 . mm =
10. 36mm
100%
2nd stroke – blanking:
The female blanking die has the workpiece dimension 28 mm diameter. The
blanking punch has the smaller diameter:
d = 28 mm– 036 . mm =
27. 64
mm
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998