3

I'm using leather punches to cut polypropylene film, but the round one isn't sharp enough to do a nice job. The film cuts nicely with a scalpel or scissors but I need 5mm discs (with a 3x2mm hole in them, cut using the rectangular punch, or a more complex cutout using a scalpel).

leather punches, edge-on

The rectangular one also needed sharpening, but this was easy -- a small diamond file and an approach like sharpening a chisel.

leather punches, side-on

But I just can't get a good edge on the round one. I don't know if I'm having trouble holding it at a consistent angle, or what the problem is. I I've got sharpening stones, a diamond file, and various abrasive sheets.

Is there a trick to sharpening small, hollow, round, steel things like this?

  • I do not know how to sharpen, but I do know that if you use them on wood, the ends of the grain, so mostly the end of a bar, you get a much better result, less damage so less need of sharpening and it works easier as well. That is, you use the wood under whatever you want to cut the hole in. – Willeke Jan 12 '18 at 17:35
  • @Willeke that's interesting. I've been using a plastic cutting mat (and before that a pad of paper; I may have to cut on a paper template to align). I'm sure I can arrange a decent sized block of endgrain sanded smooth enough for working at this scale. – Chris H Jan 12 '18 at 17:40
5

The approach is the same. You remove metal from the outside of the periphery of the punch end, at an angle roughly the same as it was originally.

The contour of the round punch adjacent to the cutting edge looks rounded in your picture. This makes the edge stronger, but makes it trickier to get the sharpening angle right when using a flat stone. The punch won't get sharper unless you remove material right up to the opening, producing a keen edge.

You'll have to pick your sharpening angle (I'd start with 45-degrees) and stick with it until your grinding marks meet the opening. With something like this it's sensible to mount the punch in a drill or drill press and use the stone against the edge while it's rotating. Best results come from holding a consistent angle, which is easier when the drill or other rotating device is fixed to a bench or whatever, not hand-held. I suppose you could try holding the stone fixed and run a hand drill guided by hand, but avoid holding both in hand.

If you're brave and have a steady hand, you could try using the hand-held drill approach against a a bench grinder or belt sander instead of a stone. Only problem there is when things go wrong the go wrong quickly. Might be something to work up to once you've gotten a feel of how it works against a stone.

| improve this answer | |
  • I've got a drill press so that sounds good. I don't have a bench grinder so I won't be tempted to try that. And I can make a jig to control the angle of the stone and make it easier to hold. – Chris H Jan 12 '18 at 17:52
  • The drill press seemed to do the trick (a test on several layers of thinner plastic went through them into the workbench). I've only got quite fine stones, and ended up doing a bit of reshaping with 240 grit wet/dry paper before I could get a decent edge. One addition: keep the stone moving. If you hold it still you'll wear a noticeable dent in it almost instantly. – Chris H Jan 21 '18 at 19:52
1

the drill press is a good idea for being able to get the same angular cut or grind on the tool in question I would also suggest getting some needle files. you can usually pick them up at Harbor Freight for a dollar or two if not you might be able to roll up some fine grit sandpaper small enough to where you can get it to fit the inside diameter of the tool. and depending on the thickness of the material that you're cutting you might find that a different or slightly different angle cut might last longer and give you a better finished product.

| improve this answer | |
  • After the coarse reshaping I did use a needle file to deburr the inside, but the steel of the punch seemed rather hard. Too hard for a file (much harder than my steel lawnmower blade, for example). – Chris H Jan 26 '18 at 7:58

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.