Andrew, I didn't know Cliff meant THAT small. If I had to measure accurately a
1mm diameter hole to a .0025mm tolerance, and had lots of money I would use an air gauge. If I didn't have lots of money I would make my own expanding ball gauge. The hardest part would be splitting it. My thinnest slitting saw blade is .2mm thick and I know thinner ones can be had so even a 1mm expanding ball gauge could be split in a home workshop. I wold think that a horologist would have the drills to put a hole through the center of the gauge and maybe even the slitting saw. Just ruminating about this makes me want to make one. Eric
-------------------------------------------------------------------------------- Be my guest Eric . Small holes I can drill, but turning then halving a 1 mm steel ball is beyond my skills.
Just out of curiosity, as I don't have lots of money, what is an air gauge in this context?
Cliff.