December 13, 2001
The author discusses what not to do to manage slug ejection. He mentions several tooling maintenance errors that he has made over the years, as well as what machine operators can do if they want to experience slug ejection difficulties. He concludes by stating that the easiest solution to slug management is to keep the tooling in good condition and to use the correct die clearance for the material.
I used to listen to an eight-track tape of Buffalo Springfield while I ran a kick punch. If you've never seen a kick punch, it basically is a lever-operated C-frame press. You cause the punch tip to lower by pushing a pedal forward with your foot.
I remember running a job on some 16-gauge mild steel that called for a 9/16-inch-diameter hole. The punch would make an authoritative pop as it went through the metal. I'd have to give the pedal a pretty decent shove for each hole. To help the stripper, I was generous with the oil. About 20 holes were done, and I was in metaphysical harmony with Stephen Stills. I reared back to kick another hole when, surprisingly, the pedal moved only about an inch before it stopped. I was too slow to realize what was going on before my chair and I were launched backward.
The oil had stuck the slugs together into a neat little column. The slugs weren't falling down the chute and had completely plugged the die. As a result, I learned to listen for the frequent clunking noise of the slugs hitting the bucket.
Some years later, I was setting up the turret press for a 24-gauge steel job. I needed a die for a 18-inch-diameter punch and, naturally, the die needed sharpening. By the time I was done with it, it was sharp, but pretty thin, way under the minimum die height. I did my impersonation of an expert and judged that it was good enough to punch the thin sheet. With enough shims, I had the die set at nearly the right height.
Murphy supervised the programming of that job. The station with the 1/8-inch hole was toward the end of the tape. You all remember paper tape, right? I got bored and wandered off. The turret wadded up that steel sheet like it was paper. The excessively ground (worn-out) die didn't grab the slug, and the slug popped back into the sheet and locked the blank. The clamps did the rest. The hole mess (pun intended) took less than a second to occur. I spent most of the rest of the shift getting the wad out of the machine and fixing its broken cover doors. Boy, was Dad (the owner) proud of me.
Believe me, I have every reason to be humble. I tell you these stories not as a scourge, but to point out a few things that are easy to overlook. Maybe someday you'll have your own horror stories about tooling maintenance errors that led to slug problems.
You might try using really dull dies. The stringy burrs hold the slugs together, almost in the same way as an excessive amount of oil does. It is hard with today's turret presses to get slugs to stack like they did on the kick punch, but it'd be fun to try.
Or, if you are punching brittle material, use a really big die clearance. For fun, grab some 0.080-inch 2024-T3 and load a 1-inch punch with a 0.030-inch clearance die. It is likely that the slug will stick to the face of the punch and lift out of the die. The good news is that the blank probably will rip out of the clamps instead of wadding up like that thin steel did for me.
Of course, if this is too much fun, the tooling manufacturers offer punch tips with plastic slug ejectors. You even can buy dies with a negative taper to help to grab the slugs.
If you are very careful, you can sputter metal in the slug chute of the die to make the slug stick. The danger is that the sputtered bumps reduce the die clearance. If the die is sharpened so the bump rises to the cutting edge, you've changed the shape of the die. This guarantees burr problems.
If you have the budget, you can buy a vacuum to suck the slugs through the chute. I haven't bought one yet, but it seems like kind of a good idea.
Some of the newfangled (what is a fangle, anyway?) machines feature air-blow tooling to push the slug through the chute. This is a great idea, particularly when running an unattended process.
Probably the easiest solution to slug management is to keep the tooling in good condition and to use the correct die clearance for the material. If you have any doubt, contact your tooling supplier because it solves problems like this for a living.
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