Hello ! I have some trouble when I use G76 on my lathe,If I want metric 1.50 pitch I must program 1.47 to get right pitch.
The feed is 100% and I can`t find any options in spindle setup or in general config to pitch down or up ?
I have check the spindle speed with a external rpm meter and compare with the computer and it`s correct.
Does anyone know something about this problem?
Sorry my bad English!

Is the "Steps Per Unit" for the Z axis set accurately in motor Tuning ?
Have you verified this with an indicator or other means ?
RC

Yes the motor tuning is correct.

Is the thread out of pitch all the way down its length or is it just at the start for a few turns?

If the pitch error is at the start only :-

What can happen is the cutting load of the tool can stall the spindle for a split second and throw out the pitch.

Also you have to allow for acceleration of the tool, in most cases 6mm (1/4") depending on the spindle speed and reaction time of your machine, the heavier the saddle the more time allowed.

If its all the way down :-

Your machine calibration is out.

Graham.

1 slot in index disc or 4 ?
If 1, you might try Grahams suggestion in the neighboring G95 topic.
RC

Good timing Graham.

Hi Graham !
Yes the thread is out of pitch all the way down the length
I have try to use the timing option on the spindle with 4 pules but I don't get`it to work.
When i try the timing option I made one of the pulse a little bigger then the other three.
I only use one puls though
I have rebuild a old support lathe and I use the gearbox on the lathe to the spindlespeed
When I use G76 I must program the speed. For example G76 S335 Z-10 X-1.88 P1.47 C0.5
This way I got the right threding.