Note: To prevent hunting in servo (a few thousands of an inch of back and forth motion while out of trace at zero velocity) the BACKLASH COMP may have to be set .0001 or .0002 inch less than the actual lost motion. BACKLASH COMP helps the machine position accurately in CNC mode only. A typical machine has less than .003 inch of lost motion, as the lost motion increases machine oscillations in trace increase, surface finish is worsened and accuracy is reduced. Typical reasons for a sudden increase in backlash is that the ball screw thrust bearing has loosened or the screw nut has loosened from the slide. Ball screw nut or gearing backlash adjustment may be required.
Check for excessive lost motion in axis. (Machine with scales or inductosyns).
Note: If in pencil trace while reversing direction the tracing head feels like it "clicks" and/or a "clunk" is heard, this may indicate lost motion.
The lost motion is difficult to measure when you have scales and inductosyns since there is no direct measure of the motor position. To check the lost motion you must be able to visually see how far the axis motor has turned. Places to look for motor motion are under the motor end cap, watching the motor shaft or servo feedback device shaft, a motor timing belt gear or the ball screw. The largest diameter surface e.g. timing belt gear is usually best to see the motor motion. To determine the distance moved, either watch the distance moved by the shaft when you jog the axis .010 inch or calculate what one rev of the motor is in distance of table movement (i.e. screw pitch times the motor gearing.)
CAT I: Set BACKLASH COMP touch field to 0 on the SYSTEM SETUP/SYSTEM MACHINE X, Y or Z menus. (This field should be at 0 for scales or inductosyns).
CAT II: Set BACKLASH COMP touch field to 0 on the SYSTEM SETUP/X, Y or Z AXIS PARAMETERS menus. (This field should be at 0 for scales or inductosyns).
Set jog move for .020 inch at 5 IPM.
Jog in one direction twice to take up lost motion and mark motor shaft, gear or ball screw to non moving reference member of machine.
Jog forward and jog back .020 inch.