Chapter 7
- Designing for Stamping Production
Design Guidelines for Piercing
For both economy and performance, the following general recommendations should be followed for piercing.
Holes. Minimum hole diameter should be equal to or greater than 1.2 times the material thickness. With stainless steels and other high tensile strength alloys, two times the material thickness is recommended. (For hole size prior to tapping consult the chapter on Designing For Machining).
If hole sizes are less than the recommended minimum diameters special hole punches must be used at increased initial costs and expensive upkeep, or drilling (a more expensive operation) may be required.
Punched features are measured from the punch side of the material, because the hole dimension on the burr side is typically larger and less easily controlled. If straight holes are necessary, reaming or shaving is required at extra cost (see Figure 15).
Figure 15. Reaming or shaving is required at extra cost where straight holes are necessary.
A web--the dimension between hole and edge--should be a minimum of two stock thicknesses (Figure 16). For long slots, this dimension should be three to four times the stock thickness (see Figure 17). With holes, webs equal to one stock thickness produce a slight edge bulge, which increases as the web decreases.
Figure 16. Pierced holes should have a minimum diameter equal to 1.2 times T (material thickness), except for hard materials. Recommended minimum hole spacing (edge-to-edge) is 2 times T.
If webs are not two times the thickness, distortion may occur, or harder materials may crack. However, there are alternatives: an ear around the hole, or a notch (Figure 18). The former increases blank size and cost, while the latter does not.
Figure 17. Minimum distance of a slot edge from another feature like a hole should be at least 2 times the material thickness (T).
Edge-to-hole spacing should be maintained at a mimimum of two times stock thickness (see Figure 17).
Hole-to-form spacing is normally 2.5 times the material thickness plus the bend radius. Otherwise, the hole distorts (Figure 19). If the hole is to be tapped, slightly less spacing is needed, but the hole must be tapped after forming to avoid thread distortion.
Figure 18. Webs should be at least 2 times the stock thickness (T) to avoid distortion or bulging. Alternate designs include a notch or, a pierced hole within an ear.
Figure 19. To avoid distortion, holes should be located a minimum distance (D) from forms. D = 2.5T + R, where T = material thickness and R = bend radius.
Figure 20. Long slots should be positioned at a minimum distance (D) from forms. D = 4T + R.
Slot-to-form spacing for long slots (Figure 20) should be at least four times the material thickness plus the bend radius. If the slot is the only opening in the part, additional cost is minimal. But if other holes are required, the holes must be punched first, the part is formed, and then the slot is punched. This approach involves added operations. Another alternative is to punch out the entire area (Figure 21).
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.
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Excerpt taken from Design Guidelines for Metal Stampings and Fabrications -- 2nd Edition copyright © 1995 Precision Metalforming Association
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