Chapter 7
- Designing for Stamping Production
Design Guidelines for Drawing
The following recommendations apply to the design of drawn
parts:
Shapes are virtually unlimited because of
the ability to perform multiple draws. Round is the easiest to draw, followed by square
with adequate corner radii. Irregular shapes and those that combine two basic shapes into
one are much more difficult and costly to produce.
Radii should be as generous as possible to facilitate
drawing. Normally, the punch radius and die radius (R1 and R2 in Figure 26) should be a
minimum of four times the material thickness.
The part radius (R3 in Figure 26) should be a minimum of six times the
material thickness with suitable drawing-quality material. Small parts may require larger
radii. Generally, the larger the radius, the easier and less costly it is to draw the
shape.
Figure 26. Generous radii are recommended for drawn
shapes. For minimum internal radii: R3 = 6T (material thickness); R1
= 4T.
Depending on the height of the drawn parts, corners can be
designed to a certain degree of sharpness by successive drawing operations that
progressively reduce the corner radii (Figure 26).
Additional Considerations
Burr direction should be indicated on the part drawing.
Otherwise, for flat parts, burrs are assumed to be on the "near side" of the
drawing. On formed parts burrs are assumed to be on the inside of the form. Some slide
forming equipment allows the placement of burrs on either side of the form.
Squareness (angularity). Normal variation on formed
90° bends is usually ±1°.
Feature distortion is more likely to occur when various
design feature like holes or slots are too close to an edge, form, or each other.
Die marks are most apparent in drawn parts, which
undergo much more deformation than formed ones (Figure 27). Likewise, grippers, if
required to hold down a part, may result in significant marking on the part surface. Other
types of tooling may also cause marking.
Figure 27. Die marks characteristic of drawing.
Flatness should not be over-specified. Requiring
flatness of less than 0.003 in./in. (0.031 mm/cm) may require a secondary operation at
added expense.
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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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