3 Fundamentals of press design

384 Sheet metal forming and blanking
shaft
frame
Fig. 4.8.24 Stand for a complete stand exchange
forming roll
base plate
In accordance with DIN 8586, roller straightening is classified as a
bending process using rotating tool motion (cf. Fig. 2.1.3). Bending
back unwanted curvature by means of a precisely defined bending
process which must be exactly coordinated with the starting radius has
proven to be impossible in practice. Instead, the technique used
straightens the metal by imparting alternating bending operations
using gradually increased bending radii. This technique is known as
roller straightening (Fig. 4.8.25).
Bending and counter bending must each take place within the plastic
range of the material in order to ensure that the bending direction
is retained following elastic recovery, i.e. the yield strength of the material
must be exceeded. On the other hand, care must be taken to ensure
that the straightened material does not sustain damage. If bending
is too pronounced, brittle materials can develop slight cracks at the
surface. The last roller must be set in such a way that it causes the
bending process which generates a flat sheet following elastic recovery
(Fig. 4.8.26).
As a result of alternate bending, the residual stress in the sheet material
is reduced as the number of bending processes and the bending
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998