Thanks!
I can't find the cut 8 Vallorbe, but I did find cut 8 Grobet. These have a kind of diamond cross section, and will cut a tapered slot. Smooth on the width, so I could use it on a rest, but I like your idea of freehanding it under my scope. Practice, practice, practice
I think you've given me enough info to move forward, I'm really glad I asked before buying the cut 4!
Thanks again
(a little bit later)
Heh. Found an interesting suggestion in Fried's Bench Practices for Watch Repairers.
His idea is to take a single edge razor blade, set the edge on a cut 4 needle file, and then give it a good whack with a hammer. He says this will form teeth in the edge of the razor, good enough to file a screw of "moderate temper".
I check a blade, it's 0.2mm wide. Perfect. I also have some cut 4 needle files.
I've got to try this!