3 Fundamentals of press design

4 Sheet metal forming and blanking
4.5 Blanking processes
In blanking operations, a difference is made between open and closed
contours (cf. Fig. 2.1.29). Open contour blanking techniques are used
mainly for blanking strips made of sheet plates. This technique is similar
to the action of a pair of scissors (cf. Fig. 2.1.31). The blanking elements
used for open blanking can be either longitudinal or circular
knives. In closed blanking processes, the sheared contour is closed, for
example when piercing. Here, the process is applied using blanking
dies: The relative movement of the blanking punch to the female
blanking die seperates the metal (Fig. 4.5.1). The punch makes contact
with the sheet metal, initially causing elastic deformation. The plastic
deformation stage then follows, leaving the sheet metal with a permanent
camber. The upper edge of the sheet metal then bends and draws
in, followed by a shearing action which leaves a visible, smooth area on
the cut surface. If the shearing strength is exceeded, cracks are formed.
These generally run from the edges of the female blanking die and lead
contact of the
punch
elastic and plastic
deformation
Fig. 4.5.1 Phases of the blanking process
shearing and
crack formation
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
breakthrough stripping