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Redrawing and Ironing Cylindrical Shells, Part 2

Wednesday, January 29, 2020
 

Last month, we examined the differences between cylindrical redrawing and ironing, and guidelines for choosing diameter and thickness reduction percentages. This month we turn our attention to punch-nose geometry and draw-sleeve design.

Punch-Nose Geometry


Fig. 1—Relationship between shell radii (blue) and die radii (black). Adapted from Die Design Handbook, 3rd Ed.
One fundamental rule exists when it comes to the size of the punch nose radius for successive redrawing of cylindrical shells: Each should be proportionally smaller than the preceding operation.

In general, the punch-nose radius for drawing will be four to 10 times the material thickness in order to prevent thinning in the bottom of the shell. Very thin materials require much larger radii, as much as 20 times material thickness.

When employed in the first draw operation, sharp radii may cause thinning in the wall of successive redrawing operations, showing as a line or depression. This impression moves higher up the wall with each additional reduction.

A specific relationship between the redraw shell radius and the die-entry radius exists to minimize metal thinning in each redraw operation, as shown in Fig. 1.

  • The center of each redraw radius, approximately 0.125 in. outside of the previous cup wall (point A), depends on material thickness and the diameter of the preceding shell. The objective: Have the outside radius of the shell contact the die entry radius at approximately 45 deg. of tangency.
  • The center on the punch-nose radius should be slightly inside of the following shell diameter (point B).
  • The center of the punch-nose radius for the final two operations, on about the same line (point C), maintains the flat on the bottom of the shell.


Fig. 2—Angular-corner relationships.
Adapted from Die Design Handbook, 3rd Ed.
Shells also may be drawn with angular corners. These beveled edges, often employed to minimize the severity of plastic flow, help align the shell in the next redraw station. However, because wrinkling is more likely to occur with angular corners, they generally require smaller reduction ratios, especially in the absence of an internal blankholder.

The angled corner profiles for the redrawing operations are developed from the finished shell, as shown in Fig. 2.

  • The angle in the bottom of the preceding shell should start at a point equal to approximately one-fourth of the bottom radius in the finished shell (A).
  • Angles for addition redraws should start midway between the draw reduction diameter (Y) of the previous shell (Y/2),
  • Angle (Z) will vary, depending on material thickness: 30 deg. for materials less than 0.030 in. thick; 40 deg. for materials 0.030 to 0.060 in. thick; and 45 deg. for materials 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 secondary 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 significant bending and unbending over the die radius. The transition 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.


Fig. 3—Redraw-sleeve profiles.
Adapted from Advanced Diemaking (NTMA).
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 additional 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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