Page 15 - MetalForming December 2019

Fig. 2—Relationship between the first draw and redraw (from Die Techniques)
D=3R;or0.75in.when R > 1⁄2 in.
Ensure that the punch- nose radius, around the top of the draw punch, is equal to or larger than the corner radius along the straight sides, and gradu- ally increase to the same size as the corner radius in the corners.
To determine the punch-nose radius size for first draw and final redraw operations, follow these steps:
rial in the corner will flow into the side- walls with little compression. Fig. 2 depicts this method, where:
• The inside dimen- sions on the final part drawing will equal the corner radii and punch-nose
radii for the final redraw punch.
RESIS
WELDIN
A = 5R
X = approx. ing edge
1
⁄8-in. draw-
Tooling by Design
x
A R
Shape of first draw thru
D
center of shell
B C
Shape of finished shell thru center of stock
R = Corner radii of the finished shell
• The corner radii for the first-draw punch equals radius A in Fig. 2, minus half of the material thickness.
• The punch-nose radius C is the sum of the wall reduction distance D plus one-fourth of the final punch- nose radius (D+1/4B in Fig. 2) minus half of the material thickness.
To illustrate, assume that the punch- nose radius along the flat sides for the final redraw in Fig. 2 is 0.125 in. and D is 0.375 in. The punch-nose radius along the flat sides for the draw die would be 0.375 + (0.125/4) = 0.406 in. Now subtract half of the material thick- ness to establish the final radius size for the punch.
• Blend the punch nose radius around the corners of the punch face to match the vertical corner radii. This provides a corner that looks somewhat spherical, or “balled off.”
Next month: Redrawing cylindrical parts without a blank holder and in combination with ironing. MF
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