I have started having trouble holding size on my machining center, especially when using circular interpolation. The ball screws seem to be running quietly and there are no strange noises when the slides move. Do I need to have the ball screws repaired or is it a problem with the control?
It could be a problem with the control but it is more likely to be mechanical.
First, measure the part to try and determine which axis is mis-positioning. If it is both, proceed one axis at a time using the following steps. Determine if the encoder (pulse generator) is connected directly to the ballscrew or attached to the servo motor. If the encoder mounted motor is connected directly to the ball screw with a coupling or the encoder is mounted to the end of the screw, check for movement in the ball screw thrust bearings with an indicator on the end of the screw. If an encoder mounted servo motor has a belt or gear drive to the screw, the adjustment could be incorrect. If these checks fail to account for the tolerance error it is likely there is backlash in the ballscrew.
It is possible, with the servo disconnected, and if the backlash is more than 0.001 inch, for movement to be felt using a light touch.
There could also be a problem with the spindle bearings, if the metal being removed is not uniform, as with a casting.