Page 59 - MetalForming March 2013

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Tooling by Design
approximately 22 percent, 17 percent and 12 percent in subsequent redraw operations, providing the ratio of mate- rial thickness to blank diameter (t/D x 100) is 0.25 or greater.
Keeping friction to a minimum, while controlling mate- rial flow, is the key to successful deep drawing. Excessive blankholder pressure forces the aluminum to stretch. Use just enough pressure to keep the aluminum sheet from wrinkling, and use standoffs to control the gap between the die face and the blankholder surface.
Use a lubricant designed for drawing and forming of alu- minum. Lubricants that work well for steel sheet most like- ly will not work well for aluminum. Don’t guess—contact a lubricant specialist.
Product Design Guidelines
Specify part radii carefully. Small radii will localize strains and promote stretching, and the part may fail quickly, while large radii will reduce the contact area between the blankholder and the part flange. This increases the tenden- cy to form wrinkles in the area near the die radius.
Generally, specify punch radii for aluminum stampings between eight and 10 times material thickness, and die- entry radius between five and 10 times material thickness. Whenever possible, keep punch radius larger than die radius to reduce the tendency for the punch to penetrate through the material. MF
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