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).
Figure 21. Design alternative to following
recommended slot-to-form spacing is punching out the entire area.
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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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