Chapter 7 - Designing for Stamping Production
Pre-Design Basics
The effective design of parts is determined by the inherent
characteristics of the metal forming process. Functional features, tolerances, and
dimensional relationships all have well-established guidelines. Before getting into design
specifics, however, discussion of some basics may help to explain certain constraints in
metalforming.
Figure 10. Standard terminology that refers to cutting or
shearing metal with a punch and die. In practice, dimensional measurements are made at the
shear area.
When a punch and a die are used to cut or shear metal, approximately the
upper third of the material's thickness is essentially cleanly cut, and the remaining
portion fractures and breaks through. In the process, a slightly depressed area on the
upper surface is formed. This characteristic is called "roll-over" or "pull
down."
Figure 11. Specially shaped tool is used to
punch a tab in either a one or two-step operation, depending on design requirements.
In practice, pull down is affected by two factors: temper of the
material and structure. Essentially, softer materials increase pull down.
Dimensional checks are made at the shear area or "cut band"
(Figure 10). Punched holes and slots are measured from the punch side because on the
opposite side, the break out of the material--influenced by necessary die clearance--may
affect the accuracy of the opening. Material type, ductility and die clearance can have an
effect on size and reproducibility of openings.
Punched tabs on a part are normally fabricated in one of two ways. Most
commonly, a special punch is used to make a horseshoe-shaped cut, which is then bent out
in a secondary operation. The other method is to shear form, where the tab is lanced and
bent at the same time. The latter approach creates more burr and is slightly less accurate
(Figure 11).
 Figure 12. Generally, radii for corners should be 1/2 of
the material thickness or greater, with a 0.015 in (0.4 mm) minimum radius preferred.
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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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