3 Fundamentals of press design

Bending
Table 4.8.1 gives the smallest admissible bending radii for bending
angle �up to a maximum of 120°. For bending angles �> 120°, the next
highest value applies: When bending Q St 42-2 sheet steel materials with
a thickness s = 6 mm at right angles to the rolling direction, for example,
the smallest admissible bending radius r = 10 mm for � �120° and
r = 12 mm for � > 120°.
Springback
When designing a bending die, it is necessary to consider springback that
occurs after unloading. Springback characteristics differ depending on
the material type. Springback occurs with all types of forming by bending,
when bending in presses, folding, roll forming and roll bending.
As a result of springback, the bending die angle � does not correspond
precisely to the angle desired at the workpiece � 2 (Fig. 4.8.1). The
angle ratio is the so-called springback factor k R, which depends on the
material characteristics and the ratio between the bending radius and
sheet metal thickness (r/s):
With � 1: angle at the die (required bending angle) [°],
� 2 : desired angle at the workpiece (after springback) [°],
s: sheet metal thickness [mm],
r i1 : inside radius at the die [mm],
r i2 : inside radius at the workpiece [mm].
The springback factor k R for various materials is given in Table 4.8.2.
� 1
� 2
s
r i2
r i1
k
R
α2
ri1+ 05 . ⋅s
= =
α r + 05 . ⋅s
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
1
i2
[ ]
–,
Fig. 4.8.1
Elastic recovery after bending:
s sheet metal thickness, � 1 required
bending angle, � 2 desired angle,
r i1 inside radius of die, r i2 inside
radius of workpiece
367