3 Fundamentals of press design

Blanking processes
to complete breakthrough of the metal as the movement of the punch
progresses. A force-time diagram of a typical blanking process is provided
in Fig. 4.6.7. A rough fracture zone then forms on the cut surface
underneath the shearing zone (cf. Fig. 4.7.8, left): The sheet metal material
springs back after the blanking process, causing it to clamp onto the
lateral surface of the punch. The sheet metal has to be separated from
the punch by means of a stripper during the return stroke.
Due to the wide variety of applications in stamping plants, only
processes used in the production of single parts by closed dies will be
described here.
Positioning of blanks in the strip and material savings
Guidelines on the economical layout of blanked, punched and drawn
parts:
Close cooperation between component developers and die designers
can help to substantially reduce material waste.
Workpieces should be designed with the smallest possible surface
area; their shape should be such that they can be lined up or nested in
each other in the sheet metal strip so as to ensure minimum waste. The
arrangement of parts illustrated in Fig. 4.5.2 b, for instance, is more
favorable than that in 4.5.2 a. The best possible material utilization is
achieved where the surface shapes are completely interlocking, i.e. can
be blanked of the strip without leaving any scrap whatsoever (Fig. 4.5.3).
The drawback of scrap-free blanking is that burr occur on both sides of
the workpiece and the design of the blanking tools needed in this case is
more complicated.
In cases where it is impossible to design workpiece shapes so that optimum
in-line arrangement is possible in the strip, it may be possible to
a b
Fig. 4.5.2 Arrangement of parts in the strip: b offers greater material savings than a
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
269