Ask the Stamping Expert: Why does my tooling yield such big variation?

A: So many tooling laws have been broken here:

1. Be consistent.
2. Nothing changes unless something is changing.
3. Find it—fix it.
4. If it needs to be done, it needs to be on the print.

I could go on.

I advised this reader first to eliminate “tribal knowledge.” Documentation must be such that anyone can understand what needs to be done. If you cannot document, measure, and do it consistently, then don’t do it.

He needs to engineer in the three R’s:

1. Replicate. To ensure tooling components can be replicated on the machine regardless of who makes them, put plenty of detail in the print: tolerancing, surface finish, breaking corners, even the manufacturing process (jig grind or wire EDM). Your goal should be that if the part is made to print, it will be exactly the same every time.
2. Reliable. Is the part design such that the tooling component will not flex, chip, break, deform, or compress? If not, fix it.
3. Repeatable. Can all tooling components be serviced, changed over, and replaced every time by any toolmaker for the same manufacturing results? If not, you need to identify why and engineer out the variation.

Second, building a new die of the same design will yield the same results. If nothing changes, nothing will change.

The current die, a 6-ft.-long tool, runs in a 300-ton press. The press bolster has no scrap opening, so it is run on parallels. The die shoe is 3 in. thick. I advised the reader to put a block of steel under the die between the parallels and machine it with a 0.050-in. gap between it and the shoe. The reader set a piece of lead on it, stamped a part, and measured the lead at 0.045 in. A flex of 0.005 in. is a major problem.

The die has four 2-in.-dia. guide pins on roller bearings. The press is 50 years old, with no record of a rebuild. Both of these factors are issues. The tool makes three different parts, and the changeover components nest in pockets clustered on each other. If one component is off location, it will move all the others. If two are off, the accumulation is a bigger problem.

The most critical dimension on the part is the rivet hole-to-hole dimension. The holes are stamped several pitches from each other. The reader adjusts the offset bend in the part by adjusting the press shut height, overhitting the down stops. So what in the tooling is compressing when this is done?

The solution here is multifold:

• Re-engineer the tool with 5-in.-thick shoes to prevent flexing.
• Use six roller guide pins (not ball guides) of 3 in. dia. This offers three to five times the guidance accuracy.
• Pocket no more than two inserts next to each other, since the press offers enough bed space to grow the tool a few pitches.
• Engineer the tooling to yield part consistency on critical-to-function dimensions.
• Stamp the rivet holes in the same station and at the same time.
• Install 12 die shut height stops, 2 to 3 in. square, all equally spaced.

The reader cannot depend on a 50-year-old, 300-ton press hitting home at exactly the same time every time. Inspect and tighten up and maintain the press ram and gibs to manufacturer specifications.

When in doubt, go back to the basics.