Threading cycle in 0i-td not working

hi friends...

In our machine threading cycle is not running properly.
The cycle was running okay few days back but now whenever we run threading cycle G92, G76 the machine starts the cycle normaly.
After one thread cut when it goes for the second cut to increase the depth it changes the thread start point and when it goes for third one it again changes the thread start and this goes on with each cut and the result is rough turning instead of threading.
Whereas the pitch remains the same in every thread but somehow it changes the start point of the cut.
Sometimes in the center of first thread pitch and every time it varies in multiple thread cutting and we already have tried G92 but the condition remains the same.
We have checked the repeat-ability of machine.
we have checked the backlash of machine.
The spindle RPM remains constant with +/- 1.
We have checked threading parameters of the machine.

Every thing seams to be okay but still machine is doing rough turning instead of thread cycle:confused:
Please help and thanks in advance...

Even though the rpm shows to be consistent, your problem is likely a spindle encoder, or encoder belt. Lots of Fanucs do not use the encoder for speed feedback, only for Ipr and threading. Not sure of your machine type, if it's a belt drive spindle or integral spindle. Integral spindles use a built in type sensor, so adjustment/ cleaning/ replacement may be required.

Thanks for reply underthetire

Its a Lathe with belt driven spindle. The encoder is also through belt.
This machine is new and the belt condition is just like new and spindle encoder pulley is tightened properly.
Can you explain how to check 1pr of position encoder. I think its through parameter 3117 and can then check the 1pr in diagnosis.
In diagnosis it will show position degree of 1pr.
Whats your opinion.
Tomarrow I am going to check for it.

Sorry for late reply..
The problem we found was of position coder pulley key.
After dismentling position coder we found that pulley key was missing.
So now threading cycle is working fine.

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