fanuc OT threading problem

[prash]

FANUC OT :While threading;first two cuts are OK but the third cut will spoil the work done by previous 2 cuts (in other words the start position of tool will be changed third time).
Turning of the machine is OK.
All the pulleys, encoder, ball-screw and other suspected mechanical parts have been replaced already without luck.
The same program works fine with other machine.
Parameters are original (not changed).Checked several times.

In summery:Program,Parameters,Mechanical parts and Encoder are OK.
What spoils threading after second cut?
THANK YOU ALL.

[gridley51]

Are you running a canned cycle?
If so which one?
Does it thread ok if you programme it longhand?
What machine is it?

[wmpy]

I had this problem before on a machine where the belt between the spindle and the tach was slipping.

Also, could your part be slipping in the collet or chuck?

[prash]

@gridley51: I am not a programmer. I am man of electronics.The programmer says he checked all G codes and the program which he uses for this machine works OK with other machine.I will get the program from him and post it. (currently I am on another project ). @wmpy:Slippage !! Well mentioned.will have to check this.
Thank you both for reply.