3 Fundamentals of press design

22 Basic principles of metal forming
2.1.4 Combinations of processes in manufacturing
Various combinations of different forming processes or combinations
of forming, cutting and joining processes have been found to be successful
over many years.
Stretch drawing and deep drawing, for example, assume an important
role in the sheet metal processing industry (cf. Sect. 4.2.1). During
stretch drawing, the blank is prevented from sliding into the die under
the blank holder by means of a locking bead and beading rods or by
applying a sufficiently high blank holder force (Fig. 2.1.32). As a result,
the blank is subjected to tensile stress during penetration of the punch.
So the sheet metal thickness is reduced.
Deep drawing, in contrast, is a process of forming under combined
tensile and compression conditions in which the sheet is formed under
tangential compressive stress and radial tensile stress without any
intention to alter the thickness of the sheet metal (cf. Fig. 4.2.1).
For example when drawing complex body panels for a passenger car,
stretch drawing and deep drawing may be conducted simultaneously.
The tool comprises a punch, die and blank holder (Fig. 2.1.32). The
blank holder is used during stretch drawing to act as a brake on the metal,
and during deep drawing to prevent the formation of wrinkles.
Modern pressing techniques today permit the desired modification
of the blank holder force during the drawing stroke. The blank holder
forces can be changed independently at various locations of the blank
holder during the drawing stroke. The blank is inserted in the die and
clamped by the blank holder. The forming process begins with penetra-
punch
blank holder
blank
die
drawingwithblankholder stretchforming deepdrawing
=
F St
F Bl F Bl F Bl F Bl
s 0 s 0 s 0
Fig. 2.1.32 Overlapping deep drawing and stretch forming in the sheet metal forming process
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
s1
F St
+
s 0