Page 13 - MetalForming February 2020

Profile A
Profile B
Fig. 3—Redraw-sleeve profiles.
Adapted from Advanced Diemaking (NTMA).
greater than 0.060 in. thick. For all thicknesses of stainless steel, angle Z should be 45 deg.
• Radius (r’) at the intersection of the angle and the wall is approximately 0.6Y.
Redraw-Sleeve Design
To redraw successfully, the sheet metal must flow freely without developing wrinkles. This can be achieved without a blankholder, using a reduced draw reduction percentage and a suitable draw edge profile (e.g., angular corners), but a redraw sleeve often is used to achieve maximum draw reductions.
A redraw sleeve serves multiple purposes in the die. Its main function: control material flow during the deformation process through a combination of applied pressure and restraining forces inherent to the sleeve geometry. Its sec- ondary functions: serve as a locator for the incoming shell and strip the redrawn shell from the punch after forming.
The basic shape of the redraw sleeve conforms to the draw edge profile inside the shell. Fig. 3 depicts two redraw sleeves with different profiles. Profile A provides the greater flow restraint due to its relatively abrupt profile and signifi- cant bending and unbending over the die radius. The tran- sition of the more gradual profile B maximizes material flow and imparts less bending-related strains. However, both profiles can be modified to provide freer flow or greater restraint, though profile B generally is more desirable for sleeve-type redraw operations.
Regardless of the profile chosen, the draw sleeve should
not fit too snugly around the punch post. One option: Relate the clearance (C) between the sleeve and the post (P) within the following limits: C = P + 0.001 in. per inch of post diameter, with C = P + 0.0004 in. minimum for small dies and C = P + 0.010 in. maximum for larger dies.
The outside diameter (OD) of the draw sleeve must be sized to provide a slip fit for the incoming shell. Measure the inner diameter of the previously drawn shell above the tangency of the bottom radius and subtract 0.002 in. (for very small diameter shells, subtract 0.001 in.). Use this as the starting size for the draw sleeve OD. Before making addi- tional adjustments to the sleeve OD during die tryout, check for proper venting of the punch and die cavity. Trapped air and drawing fluids will impede loading the incoming shell over the draw sleeve if not properly vented.
Finally, the working surfaces of the redraw sleeve should be highly polished, including the top surface and around the radius or bevel edge, and down the sidewalls to a depth slightly greater than the depth of the redrawn workpiece. Polishing must be performed in the direction of material flow and to a mirror-like finish. Although it may be easier and faster to mount the redraw ring in a lathe and rotate it to polish, this will not provide long production life due to the polishing direction being 90 deg. to metal flow. Any small scratches, even very fine ones, or asperities in this direction increase friction, impede material flow and accelerate galling and wear. MF
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