3 Fundamentals of press design

Methods of forming and cutting technology
tion of the punch to perform a stretch drawing process in which the
wall thickness of the stretched blank is reduced. The bottom of the
drawn part is subsequently formed.
The deep drawing process begins once the required blank holding
force has been reduced to the extent that the blank material is able to
flow without generating wrinkles over the rounded sections of the die.
At the end of the drawing process, the blank holder force is frequently
increased again in order to obtain a reproducible final geometry by
respecting the stretching portion of the drawing stroke.
In addition to deep drawing, body panels are additionally processed
in the stamping plant by forming under bending, compressive and
shearing conditions. A characteristic of the bending process is that a
camber is forced on the workpiece involving angular changes and swivel
motions but without any change in the sheet thickness. The springback
of the material resulting from its elastic properties is compensated
for by overbending (cf. Sect. 4.8.1). Another possibility for obtaining
dimensionally precise workpieces is to combine compressive stresses
with integrated restriking of the workpiece in the area of the bottom
dead center of the slide movement.
Formingis almost always combined with cutting. The blank for a sheet
metal part is cut out of coil stock prior to forming. The forming process
is followed by trimming, piercing or cut-out of parts (cf. Sect. 4.1.1).
If neither the cutting nor the forming process dominates the processing
of a sheet metal part, this combination of methods is known as
blanking. Where greater piece numbers are produced, for most small
and medium-sized punched parts a progressive tool is used, for example
in the case of fine-edge blanking (cf. Sect. 4.7.3). However, solid forming
processes often also combine a number of different techniques in a
single set of dies (cf. Sect. 6.1).
The call for greater cost reductions during part manufacture has
brought about the integration of additional production techniques in
the forming process. Stacking and assembly of punched parts, for example,
combines not only the classical blanking and forming processes
but also joining for the manufacture of finished stator and rotor assemblies
for the electric motor industry (Fig. 2.1.33, cf. Fig. 4.6.22 and
4.6.23). Sheet metal parts can also be joined by means of forming, by
the so-called hemming or flanging (Fig. 2.1.34).
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
23