Thanks for the quick response.
The turbo carver is an option, but at the moment, it’s probably a little out of my price range. Mounting it wouldn’t be an issue, I have a mill and lathe, so custom parts aren’t really a problem.
The type of engraving tool I was thinking of was (I’m not sure about this forum’s ebay link posting policy) the type of thing you find if you search ebay for ‘engraving pen’. I’d probably replace the diamond tip with a sharper carbide one.
Another option that may or may not be a smart one would be to coat the entire ring in something like polystyrene and have a pen that writes in (precisely controlled quantities of) acetone, thus dissolving the polystyrene at that point. The ring could then just be etched. Or the reverse: the pen could write with some contaminant and the ring could be anodized afterwards, leaving that region clear. I’m not sure about the accuracy of something like that though.
Extra fine sharpie type result is probably ok (ish).
The other option would be to buy the engraving kit for my Proxxon MF70 mill and turn the horizontal motion into rotation. I’m not sure how much accuracy that would give…
