Ask The Stamping Expert: How can we prevent misfeeds?

A: With more than 35 years of experience working as a setup man, toolmaker, and toolroom manager and now as director of engineering, I am happy to tell you that, given the right circumstances and working with extreme precision in your setup, you can avoid misfeeds.

Three Detection Methods

First, you should have three detection methods as standard in all your dies and presses:

1. Pilot missed – A spring-loaded pilot is designed into the tool so that if the feed does not pitch the material precisely while the die is running, the pilot can retract without breaking when the tool closes and there is no corresponding pilot hole in the raw material strip because of a short-feed. When the pilot retracts, we sense the head of the pilot in the top die shoe and stop the press. If there is 0.001-in. clearance between the pilot and the pilot hole in the strip, any feed error greater than 0.0005 in. (half the diameter of the difference between the pilot and pilot hole) will activate the stop.
2. Micron sensor - This measures a distance when the die is closed. It is very accurate, and if a slug pulls up into the die or the strip buckles in the die, the sensor will detect it immediately. The drawback with these two methods is that if a misfeed or slug problem occurs, by the time you detect it, it’s too late to stop the press, and inertia will close and crash the die.
3. Tonnage monitor – This unit measures the force required in the stamping. If a misfeed or a slug pull occurs, a tonnage monitor should detect it—but again, it’s too late. However, the tonnage monitor can be extremely valuable when you have a condition that is slowly causing tonnage to rise. Then you can stop the press before breakage. If your die section slugs are gradually getting tighter, you can stop the press before they split the die section and break punches.

More Ways to Prevent Misfeeds

The challenge with sensing is that you have to detect a problem as early as possible so you can stop the ram before the tooling crashes. One way is to install two feed cycle sensors in the die: one to sense the pilot hole in the base material on pitch and another to sense the pilot hole in a location at 50 to 70 percent of the feed cycle (if the feed cycle is 180 degrees, it starts at 270 degrees and finishes at 90 degrees). You can set the sensor electronics for the first sensor to stop the press if the pilot hole has not moved when your press is at 300 degrees. That’s well before the ram hits top center.

Stopping the press on the way up can give you three times more stopping time to avoid the crash. This is great if the die strip becomes totally obstructed, such as when a broken punch remains stuck through the die strip in the mating die cavity. If the die strip starts to move but gets tight, if the second sensor is not made by 360 degrees or halfway through the feed cycle (top dead center of the press cycle), the sensor will stop the press. You also can use the first sensor to check not only at the start of the feed cycle, but also at the finish. Anything is better than waiting to detect the misfeed when the tool is closed.

You can also install a buckle sensor. Build two support plates, covering the top and bottom of the raw material from the feed rollers to the die entrance, as close as possible. If the die strip is obstructed when you run the tool, the feed rollers will slip on the raw material – there’s no place for it to go. Do some trial-and-error testing on the bench to determine the minimum gap (or hole) you need to cut in the top liner so the raw material will buckle when the die strip is obstructed. Mount a sensor in the center of the buckle. That way, at the start of the feed cycle, it will quickly detect if the strip is obstructed. This unit’s sensitivity depends somewhat on the feed length and subsequent height of the buckle.

Good luck, and happy stamping!