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.
Design Guidelines for Blanked Features
Regarding blanking, specific recommendations should be followed to obtain the most economical and functional part design.
-Corners may be designed sharp (less than 0.015 in. (0.4 mm) radius), but often this involves additional cost to address increased die wear and burr formation. Generally, radii for corners should be 1Ž2 of the material thickness or greater, with a 0.015 in. (0.4 mm) minimum radius preferred (Figure 12).
Figure 13. Notches in blanks should not be narrower than 1.5 times the material thickness. Notches can be extended and narrowed slightly if they are tapered about 0.004 in. (0.1 mm) or greater--on one end.
--Notches and tabs on blanks should generally not be narrower than 1.5 times the material thickness. With a 0.060 in. (1.5 mm) stock thickness, a 0.090 in. (2.3 mm) wide notch or tab is readily cut.
Typically, notch and tab lengths can be up to five times the width. In marginal cases, the metalformer can use a heavier die, punch or stripper at a slight cost increase. Alternately, notches and tabs can be extended and narrowed slightly if they are tapered, i.e., if they are a little narrower--about 0.004 in. (0.1 mm) or greater--on one end (see Figure 13).
Depending on the material, hardness and shear characteristics, notches and tabs can be de-signed with widths as small as one material thick-ness and with lengths up to six or seven times the width--usually at additional tooling cost.
Notches and tabs should not be too close to another feature or too long and narrow because of die weakening. If close proximity to notches is necessary, secondary operations may be required.
Cutoffs. Three basic types of cutoffs apply to blanking: straight or square cut-off; half-round or partial radius; and full radius (Figure 14), with square being the most economical.
The full radius option is not recommended because the tangential blending of the radius into the outside material edge creates a condition where there is no material to be cut. This is called "feather edge" and in this condition the material will extrude along the die edge for an unavoidable, large burr. (See Figure 14 for design solutions)
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!
|
