I have an ML7 with an 8tpi leadscrew driven by changewheels. From time to time I have cut threads about 20tpi with no trouble. Recently I tried to cut an 8tpi thread, which should have been easy: engage the clasp nut at any mark on the indicator. But when I made the second cut, having engaged the clasp nut at the same indicator mark as for the first, I got a second helix about 1/32nd inch from the first. And then a third; all three regular, but not on top of each other as they should have been. I have done the obvious things like cleaning the leadscrew and clasp nut. Any advice about what's wrong will be very welcome.

You are both right. I thought I had 20teeth at both ends of the train, but following your advice I looked again and found 21teeth on the tumbler end. It's marked "21", so I have even less excuse. The thread that I was cutting was too short for the pitch error to be seen without more accurate measuring than I was using.

Many thanks Pete and Hopper

Alan