Colchester/Clausing Z Axis Inaccuracies

Hi Folks
I wonder if anyone out there can help with a problem that's gotten me beaten:

Lathe: CNC2000L, Fanuc 0TA

I have a big z axis inaccuracy that is proportional to the required movement; an MDI programmed W-600 will be 12mm short of travel,
W-300 is 6mm short, W-150 is 3mm short etc.

The same goes for MPG moves.

I cannot see any looseness/slippage in the pulleys and the parameters all seem to check out with my earlier backups. Backlash was considered
before these tests and is quite small anyway.

Any suggestions would be greatly appreciated.

Regards
Rob

.... made more confusing by Z -ve moves being inaccurate and Z +ve moves being ok....

It was the drive belt having wear on one side only of some of the teeth making the belt 'jump' occasionally.