 Peter Ulintz
Technical Director

# Redrawing Square and Rectangular Box Shells

December 1, 2019

At a recent seminar, as in past columns, I addressed deep drawing of square and rectangular box shells. Based on the resulting questions and discussions, it has become apparent that a technical column on redrawing square and rectangular shells might be beneficial. Fig. 1—Draw reduction graph for box shells

Determining the Required Number of Draws

In determining the maximum depth of a drawn (in one operation) square or rectangular box shell, the size of the corner radius is the main factor, with radius of the drawn part always specified to the centerline of stock (the midpoint of material thickness).

In general, square box shells can be drawn from a flat blank to a depth equal to about 80 percent of their width with a relatively small corner radius. Rectangular box shells can be drawn to greater depths than square boxes. The depth of draw increases with the increasing width of the longest side. The formability diagram in Fig. 1 illustrates the relationship between the maximum depth of draw (h-max), the width of the longest side (w) and the size of the corner radius (Rc) on the number of draws required for a rectangular product.

More specifically, a box shell with a corner radius between 3⁄8 and 1⁄2 in. may have a depth of draw of six times its corner radius, while a box shell with a corner radius exceeding 1⁄2 in. generally has a depth of draw of four to five times the corner radii. In both cases, the distance between the centers of the corner radii must be greater than, or equal to, six times the corner radii. If less, the shell’s depth of draw decreases proportionately.

Determining Redraw Die Parameters When the allowable depth of draw is shallower than finished-product requirements, the process requires a redraw operation. The accompanying table from J.W. Lengbridge, Theory and Practice of Pressing Aluminum serves as a guide for assessing the necessary number of draw steps to produce rectangular shells, based on the ratio of the wall height to the corner radius (h/r). Though developed for aluminum, I find that the table also works well for steel.

The amount of reduction between redraws depends on the corner radius, which shrinks incrementally with each reduction. When two or more reductions are required, take these steps to determine the length and width of each shell:

• Starting with the finished part geometry, multiply the corner radius by 3.
• Multiply using a constant of 0.75 on corner radii greater than 1/2 in., regardless of the corner size.
• Add the product for the calculation above to the current length and width, which will be the length and width of the preceding draw shell.

The corner radius for the previous draw will be approximately four to five times larger than the final part. Do not lay out the new radius from the centerline of the final part. Instead, it should provide a small ledge from which to draw. Redrawn to its final shape, a large portion of the part material in the corner will flow into the sidewalls with little compression. Fig. 2 depicts this method, where: Fig. 2—Relationship between the first draw and redraw (from Die Techniques)
R = Corner radii of the finished shell

A = 5R

X = approx. 1⁄8-in. drawing edge

D = 3R; or 0.75 in. when R > ½ 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 gradually 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:

• The inside dimensions on the final part drawing will equal the corner radii and punch-nose radii for the final redraw punch.
• 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 thickness 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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