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.
In this case I'm using a ceramic 'stone' with a medium grit;
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 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.