Ask the Stamping Expert: Fixing die shortcomings

Q: We have a large stamping tool running in a 300-ton press. A changeover tool makes a left part and a right part, each of which is about 0.1 in. thick and shaped like a peanut. One-half of the part (lengthwise) is coined offset one material thickness in depth in addition to a number of other strengthening embosses and assembly holes.

A: Our design team talked to the reader further to understand the current state of the tool design. Following are problems we identified and the actions we recommended. They fall into two categories: robustness and design changes.

1. Robustness: Part variation is caused by the tool flexing and poor guidance. The press has no bed holes for scrap. The die is on parallels, which are small and spaced out over the 6-ft. tool length. The die shoe, at 3 in. thick, is flexing. The parallels need to cover the entire surface of the bottom of the tool, except the very minimal areas where scrap needs to fall through. Four 3-in.-dia. down stops are placed at the corners. The three-plate design uses four stripper guide pins at the outer corners of the strippers.

We recommended the following changes:

• Add several parallels, some up to 6 in. wide, under the forming stations in the tool.
• Increase the die shoe thickness from 3 to 5 in.
• Add four more evenly spaced down stops and increase their size to 3- by 4-in. rectangles where possible. Make sure the ram is stopped at dead bottom of the stroke in exactly the same spot every cycle.
• Increase the number of stripper guide pins to eight, evenly spaced, and change the design from ball cages to roller cages, which will offer 10 times more rigidity in guidance. Apply this advice to the die shoe as well. Increase the diameter of all guide pins by 1.0 in., to 4 in. When proper guidance is achieved, you could almost remove the press guides and still run the tool.
• Add a hardened steel backup plate sufficiently thick so that the die inserts can sit on it, rather than directly on the die shoe.

2. Design Changes: Variation inherent in the stripper clamping is causing inconsistent forming of the blank. The biggest design issue is in the offset station in the tool. The part must be bent up into the stripper plate. The form insert is 0.1 in. higher than the die chase. As the tool closes and the stripper clamps the material, the bend goes into the stripper plate forming punch cavity, essentially bending the part with the stripper force. As the tool continues to close and the stripper continues to collapse, at some point the forming punch contacts the material to be formed and rebends the part offset. This bending and rebending is a fundamental flaw that leads to variation.

Following are the actions we recommended:

• Add slug darts to hold down scrap.
• Add lifters and some additional pilots to keep the strip level.
• Make sure the forming punch is spring-loaded to a point flush with the stripper face when the tool is open. Add a nitrogen spring pack loaded behind the punch with enough force to bend the part and prevent the punch from backing up. This way, when the stripper contacts the material, the forming punch will prevent the material from bending into the stripper form punch cavity. As the stripper compresses, the part is formed and hits home on the backup plate at dead bottom.

After all these changes were made, the new tool is now up and running flawlessly. Changeovers require no tool tweaking. Service intervals are repeatable and predictable. Scrap has been eliminated. The toolroom maintains to the CAD prints, with no tweaking or shimming.