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Author Topic: G76 pitch  (Read 3724 times)

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G76 pitch
« on: March 04, 2008, 11:21:21 AM »
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!
Re: G76 pitch
« Reply #1 on: March 04, 2008, 11:44:48 AM »
Is the "Steps Per Unit" for the Z axis set accurately in motor Tuning ?
Have you verified this with an indicator or other means ?
Re: G76 pitch
« Reply #2 on: March 04, 2008, 12:02:57 PM »
Yes the motor tuning is correct.

Offline Graham Waterworth

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Re: G76 pitch
« Reply #3 on: March 04, 2008, 12:20:37 PM »
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.

Without engineers the world stops
Re: G76 pitch
« Reply #4 on: March 04, 2008, 12:22:02 PM »
1 slot in index disc or 4 ?
If 1, you might try Grahams suggestion in the neighboring G95 topic.

Good timing Graham. :)
Re: G76 pitch
« Reply #5 on: March 04, 2008, 01:34:15 PM »
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.