Q:  We are using a CNC lathe machine with GSK 980 TDc system , sometimes the sizes of a finish part is too big, so we adjusted the tool compensation on the GSK980TDc, then made the next one and found that was too small.

We checked the backlash of the ball screw and it seemed there was 0.1mm range error of the lathe machine carriage, so we adjusted the carriage, then the backlash become 0.25mm, we doubt that it was not backlash, because it should not be so big. Maybe some where is loos.

We use below G codes to check the tool post repeat accuracy, the result seemed ok.

G0 U80

G04 D2

G0 U-80

Now we are lost, what problem should it be?

A: According to above description, this is not the CNC setting issue, we think the problem should come from the mechanical parts, please check again carefully.