Monday, June 8, 2015

Sharpening a Case Trimmer Cutter

If you hand-load ammunition, one thing you occasionally have to do is trim the cartridge cases; they stretch a bit over time, and there's a maximum length. Fortunately, they make case trimmers for this situation.

The case trimmer, like this one by Lyman Universal, has a 4-flute cutter. And like any cutting tool, with use it eventually gets a bit dull. It might even pick up a nick on an edge.
You can buy replacement cutters. You can even buy a carbide replacement for some. Or you can be cheap, like me, and sharpen it. Here's the cutter, which screws out of the trimmer shaft:
You can see the four cutting flutes(the hole in the center is for a pilot that keeps the case centered).  While there are several ways to do this, I'm doing the 'all by hand' way*, which starts with some type of sharpening stone.
Depending on just how dull the cutter is, you might be able to get by with just a fine stone; if it's really dull, or has nicks, start with a medium-grit; it'll cut faster.

In this case I'm using a ceramic 'stone' with a medium grit;
If you're using a standard sharpening stone, use plenty of oil just as you would with a knife. Start with working the head in an oval, making sure it stays flat. After 6-10 loops, turn it 1/4 turn and repeat. Wipe clean and examine. The freshly-cut areas will be much brighter than the untouched areas, and you'll be able to see how much you'll need to do. If yours was only slightly dull you might already see the whole of each cutting face bright; if you've got areas still dull, or nicks to remove, go back to work with the stone.

At this point, start working the cutter in a figure-8 pattern. Every few cycles, turn the cutter a bit. And occasionally reverse the pattern, too. Do everything you can to make sure it keeps cutting evenly all around. Check regularly. When the whole of all four faces is bright, you're done with this stone. If you started with a fine, you're done with the main work.

If you've been using a medium, go to a fine or very fine and do it again. At this point you'll want to examine it under a strong light so you can see when you've removed all the marks from the coarser stone. When they're all gone, this part is done.
After medium grit.
After fine grit.
You can take it to a finer stone if you wish, though for this it probably won't make a noticable difference. You have one final step: Just like with a knife, sharpening will leave a bit of a burr or 'wire edge' on the cutting edge, where traces of steel rolled over and the stone wasn't fine enough to remove it. They've got to be removed, and that's simple: get a fine stone small enough to fit, hold it square against the face of the flute, and draw it down, making sure it stays flat. Just a few strokes on each face will do. Good chance you'll be able to feel the difference between the first stroke and the last as the burr is removed.
Don't have a variety of sharpening stones? You can still do all of these steps: just use wet/dry abrasive paper, found anywhere from Wally World to auto parts shops. This stuff can be had in grades anywhere from very coarse up to X000, a grit so fine it doesn't really feel abrasive (seriously, some of this stuff is so fine it feels like copy paper). For this, the finest you'll need is 400 to 600 grit, with 180 or 220 as the first step to clean up the faces.
  • You need a flat, hard surface to use it with. 
  • Cut a sheet in two so you have a 8.5x5.5" piece to work with. 
  • Soak it in water for ten minutes or so before using. 
  • Put the paper on the hard (and waterproof) surface, and start working the cutter on it with light pressure so it doesn't cut or tear the paper. 
  • For cleaning up the burr, cut a strip and back it with something like a small, fine file.
That's all there is to it. Clean it off well (I like to take stuff like this outside and blast it with brake cleaner), dry it, put a dab of oil on to prevent rust (don't forget to put a bit on the threaded shank!) and give it a try.

1 comment:

  1. I tried this method on a standard RCBS trim pro cutter, on a hard arkansas stone with Corbin "diamond edge" oil. Painted face with Dykem, applied carefully to stone at 90 degree angle. It looks beautiful but will not cut worth a tinker's dam. Was there a relief angle on the faces?


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