3 Fundamentals of press design

370 Sheet metal forming and blanking
Determining the blank length for bent workpieces
The blank length of the part to be bent is not equal to the fiber length
located at the center of the cross section after bending. The extended
length of bent components 1 [mm] is calculated as
whereby a [mm] and b [mm] stand for the lengths of the two bent legs,
and v [mm] is a compensation factor which can be either positive or
negative (Fig. 4.8.3).
It should be noted that � is the bending and � the opening angle of
the bent part. Values for v are contained in DIN 6935 or can be calculated
for every required angle:
– for � = 0° to 90°
– for � > 90° to 165°
with correction factor k [–]:
– k = 1 forr/s> 5and
1 r
– k = 065 . + ⋅log
f or r/sto5 2 s
The calculated extended length dimensions should be rounded up to
the next higher full millimeter.
b
α
l = a + b+ v [ mm]
f or opening angles 0° to 165° and
l = a+ b [ mm]
f or opening angles > 165° t
o 180°
⎛ ° ⎞ s
v= π ⋅⎜
⎟⋅ r+
⋅ kR⋅ r+ s mm
⎝ ° ⎠
β
180 – ⎛ ⎞
⎜ ⎟ – 2
180 ⎝ 2 ⎠
a
b
( ) [ ]
⎛180°
– β⎞⎛s⎞180° – β
v= π ⋅⎜
⎟ ⋅ ⎜r+
⋅ kR⎟ – 2 ⋅ ( r+ s )⋅ tan mm
⎝ 180 ° ⎠ ⎝ 2 ⎠
2
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
β
a
[ ]
Fig. 4.8.3
Geometry of bent legs