[bbone]

I'm using a bridgeport tormax 13.5 manual lathe to cut a m20-1.5 metric thread. when I engage the half nut on the same # every time, it doesn't follow the same tool path. It's like it shifts about 1/2 of the pitch. I am not moving anything except the cross slide in & out. Any suggestions.

[handlewanker]

Yeah, you're cutting Metric threads, so you have to keep the half nuts engaged all the time.....reversing the chuck with the half nuts still engaged back to the start each time.

When you cut Imperial threads, that is threads per inch, you are actually cutting one pitch for every revolution of the chuck, but when you cut Metric threads you are actually cutting a pitch distance for each thread.....so a 1mm thread pitch is the same as cutting 25.4 tpI......and 1.5mm pitch is about 15.5 tpi approximately, which in the Imperial system would mean you can't unclasp the half nuts at end of cut as it's considered an odd pitch, and that goes for any other Metric pitch too.

With a threading indicator and an Imperial leadscrew, you can disengage and re-engage the half nuts at specific marks on the threading dial, provided the thread pitch is a multiple of the leadscrew pitch.

I have an old 10" Colchester Bantam lathe with flat belt drive, dating from 1920/30 era and it doesn't have a leadscrew indicator dial, but I go into reverse at each end of thread cut....problem solved...or is it......I don't have a reverse mechanism on my lathe due to the fact that the chuck is screwed on and to go inot reverse under power would make the chuck unscrew.

So I fitted a handle that plugs into the end of the spindle bore, and with an electric clutch to stop the drive at the end of cut, I just wind the carriage back to the beginning, with the half nuts still engaged, by hand.....I even cut a few fine threads with the handle drive too when I want to.

I usualy run the lathe at 100 rpm for screw cutting, so the handle doesn't cause a vibration problem and for normal turning It's removed anyway.

When I rebuilt the lathe back in the 80's, it was immediately apparent that stopping and starting the motor too often (1 hp single phase) was not an option, so I adapted and fitted the electric clutch from a cars air conditioner to the drive train, and so the motor runs contiouously but you can stop and start the lathe a hundred time a minute if needed without harming the motor, also I get total control over the stopping point of the lathe carriage when screw cutting.

I hope this helps.
Ian