Fanuc Backlash adjustment

[stang623]

I am working with a fanuc 15M kitamura, how does the backlash work? Machine was bought second hand and tapping is not great. I believe it has a 15 or 16 value in the backlash for the Z axis. With an indicator and the handle mode set on 0.0001" increment I see it jump like 0.0005" when switching directions.

I was thinking about changing the value to 11 instead of the 15, does this make sense? Is there a unit attached to backlash?

Thanks,
Anthony

[Al_The_Man]

Normally the backlash direction can be selected, and when a reverse in direction occurs, there is normally a rapid move of the servo to take up the set backlash only, IOW the motor shaft only should move, the axis itself should not jump, if it does, it is usually a sign that the backlash amount is too large, try it back to zero and see how much movement of the shaft occurs before the axis starts to move, this can be done by attaching a dial gauge and watching the axis display.
Al.

[sinha_nsit]

Is pitch error compensation same as backlash compensation?

[Dan Fritz]

No. Pitch error compensation inserts or deletes a detect unit or two as the machine moves along it's axis. This compensates for a ballscrew that has been ground inaccurately, or one that does not hold to exact tollerances over it's entire length.

With pitch error compensation, you set the number of detect units (usually .001 mm on a metric machine) at each of 128 or 256 points along the ballscrew. If the screw has pitch error, the compensation adjusts for it, but it has nothing to do with backlash. Don't mess with pitch error compensation unless you have a way to accurately calibrate the machine over large distances (i.e. a laser calibration device), and your machine is in a somewhat temperature-controlled environment.

Backlash compensation inserts a few extra detect units of motion whenever the the axis reverses direction. Use backlash comp sparingly, because it's worse if you over-comp for backlash. Also, before adjusting for backlash, be sure the axis is well lubricated to prevent ballscrew "wind-up" and be sure the thrust bearings on the ballscrew are properly adjusted. Backlash comp does not eliminate backlash. It just compensates for it. Imagine if you had a LOT of backlash in a ballscrew assembly. The table would be "loose", so you could move it back & forth without turning the ballscrew at all!. That's not possible to compensate for electronically because the machine's axis is simply not rigidly controlled.