Page 57 - MetalForming March 2016

cushion may offer a better solution, by allowing the user to selectively apply pressure at specific locations on the flange. This will suppress wrinkles with- out restricting material flow at other locations.
A flat side on an otherwise round cup represents another type of asym- metrical cup. The blank material at the flat portion of the cup may flow too
rapidly and leave loose material in that portion of the cup wall. Material flow in the area of the flat may require a high- er localized pressure or restriction, via a draw bead.
In summary, the deeper the depth draw and more complex the part shape is, the more sensitive the deep-drawing process will be to variations and inac- curacies in blank holding forces. MF
RESISTANCE WELDING ONE-SPOT-SHOP
Tooling by Design
Elliptical Cup
Wrinkling most severe at small radius
Circular flange
Fig. 3
greater the need to precisely control blankholder forces. Forces must be high enough to promote stretching of the dome but not too high as to restrict material flow when drawing the cup wall. Ideally, the blank edge will begin to draw in immediately after stretch- forming completes. Otherwise, the material will thin too much and cause the dome to tear or fracture.
Modern blankholder technology, such as programmable servo-cush- ions, allows stampers to accurately control blankholder pressure and cre- ate pressure profiles that vary through- out the press stroke. This technology permits the application of high blankholding forces to induce stretch- ing and then instantaneously (within milliseconds) reduce those forces to allow proper material flow to produce the cup wall.
Axisymmetric cup drawing is rather straightforward and easily understood. However, not all cup drawing is sym- metrical about the cup axis. Consider deep drawing elliptical shapes, for example, where the uniform compres- sive loads found in axisymmetric cup drawing are disrupted. This causes wrinkles to form in the flange due to unbalanced peripheral flow of materi- al toward the punch (Fig. 3). Metal- formers can reduce these compressive loads by notching the blank in the smaller radius area, provided that the notch will not affect the final product trim. Again, a programmable servo-
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MetalForming/March 2016 55
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