The work with stone I've been doing has been marvelous fun, but there's a significant problem: like math, stone is hard. The carving has to go very slowly, taking very shallow cuts, and it is absolute hell on the bits I've been using. Carbide is great material for wood, plastic, and even some metals, but rock? I can easily kill a bit even on a small piece like the "noncompliant" carving.
So on a tip from the assembled wisdom of the internet, I decided to try a diamond bit. I happened to have a narrow diamond bit intended for engraving for a Dremel tool, which just happens to use the same 1/8" collet as most of the other bits I'm using.
Turns out that it works wonderfully. Working stone still requires slow speeds and shallow cuts, but I could turn up the speed a little bit and double the nearly-negligible depth per pass. Now, twice almost-nothing is still not very much, but it's still effectively halving carving time, which saves tremendous wear and tear on my router.
I decided to make some appropriately themed coasters for my lovely and talented spouse. I laid out the designs to carve a bunch of different symbols within a grid, getting all the carving done in one long job so I could move on to concentrate on post-processing later. I started with cheap slate tiles from the hardware store, but slate's limitations quickly became apparent. The natural variation in the surface of the slate, which is one of the things that makes it attractive, turned out to be greater than the depth to which I was cutting (about 0.8 mm). There were spots where I got no carving at all because the bit never got as far down as the stone, and at least one other where the stone was sufficiently thick that I ended up carving into much greater depth than intended and ended up snapping the bit. Is OK; I had a spare. Next up was white marble tile. The flat surface proved much friendlier to carving.
For post-processing, I experimented with a few more things. I had some metallic ink in dropper bottles, so I tried filling the carved recesses with it. Turns out that works pretty well, too.
I probably should have cut the coasters up first before using the ink, but it turned out OK in the end. Instead of taking the extra time and bit wear to have the CNC machine carve the coasters out all the way, I just pulled out the trusty old tile saw (though I did have the CNC lay down some guide lines). The only real problem I had is that one of my few successful slate pieces flaked a whole layer off the bottom. That would have been a problem had I been worried about thickness. The ink, while not water-proof, was sufficiently water resistant that I could clean them up with a quick rinse and wipe. A little adhesive felt on the bottom, and...
Some of those are unfinished. I realized too late that I don't have red, so the Superman and Flash ones will need a bit of work, and I've got a Huntress likewise awaiting purple. However, I think the carving went well here.