screwcutting in a ML7

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.

Check the tooth count on your stud gear.

Something is amiss there. You should be able to cut an 8tpi thread without even using the thread chaser dial. Just engage anywhere and it should work. Ditto for any thread with tpi a multiple of 8, eg 16, 24, 32, 40 etc.

As Pete says above check the tooth count on your gears. Should be a 20 tooth on both the output stud of the reversing mechanism and the leadscrew, with idlers of your choice in between.

Myford did make a 21 tooth gear that can easily be mistaken for a 20 without careful counting.

Otherwise, you must have something loose somewhere that is moving. Either in the gear train or the toolpost etc.

PS You can check your carriage movement with a dial indicator while rotating the chuck exactly one turn. Carriage should move .125" for an 8 tpi thread.

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