Chapter 7
- Designing for Stamping Production
Design Guidelines for Formed Features
Forming, too, has its own set of guidelines, all of which are formulated
to avoid potentially costly design pitfalls.
Formed features that do not meet the minimum
recommended length, generate additional expense, since the part must be formed long and
then trimmed to size. The short portion beyond the form should have a minimum length equal
to 2.5 times material thickness plus the radius, as measured from top of the form to end
of the blank (Figure 22).
Figure 22. Form height (H) should be at least equal to 2.5T
+ R.
Form-to-hole spacing should be great enough so that
hole distortion does not occur (see hole-to-form section under piercing design).
Restrictive tolerances on the outside of a V-shaped
form may require bend relief notches (Figure 23). Without bend relief, edge pucker
(bulging) will result.
Figure 23. When bulging is not acceptable,
bend relief on a V-shaped form is required.
Figure 24. Bend-relief notches on formed L-shaped parts. R
= Radius. Example A is not reccommended. Examples B and C are recommended designs. Notch
width = 2 times material thickness, min. 0.060 in. (1.5 mm). Notch depth = radius plus one
material thickness (T).
Formed, L-shaped Parts. Designers should include a bend
relief notch to avoid cracking or fracture (Figure 24).
Grain-direction constraints are particularly important
in harder stock. The line of a sharp V-bend should not be parallel with the grain
direction, otherwise cracking or fracture is likely to occur. Many parts function just as
well with the grain in the longitudinal direction. When forming harder materials with the
grain direction, an appropriately large inside radius is required to avoid cracking (see
Material Selection Chapter).
Burrs on the outside of a bend can create
"checks" or small fractures. This is particularly evident in parts with two or
more bends made in opposite directions (Figure 25). The effect is more predominant with
harder-temper alloys and can be eliminated either by deburring prior to forming or
punching additional holes to relieve stress.
Figure 25. Under tension from forming, the burr side of a
blank may develop fractures, which may affect cosmetic appearance.
Go to the Design Guidelines Overview
Go to the Glossary
Excerpt taken from Design Guidelines for Metal Stampings and Fabrications -- 2nd Edition copyright © 1995 Precision Metalforming Association
Purchase the new Third Edition of Design Guidelines for Metal Stampings and Fabrications copyright © 2004 Precision Metalforming Association at Marketplace today!
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