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Fanuc 21it Backlash


Dec 13, 2002
Benicia California USA
I have a small flat bed CNC Lathe at home.
Its a Harrison Alpha 1330s, Running a Fanuc 21it control.
Doing some maintenance and notice something with the "Z" axis movement.
Using the MPG hand wheels with the movement step set to .0001" i see the carriage jump .002" when i change direction of the slide...
It is not lost motion but rather excess motion.
So if i am moving the "Z" one step at a time (.0001") in the minus (-) direction and change to going in the plus (+) direction, the actual move jumps .002" in the plus (+) then continues moving normally (.0001" per step)
Guessing this is backlash comp. Is this normal or has someone set the correction too high.
If so, is there a way to correct this.(procedure)

Thanks for any help or advice here.
Cheers Ross
It is over compensated in the area of travel you are checking. Not uncommon for an older or heavily used lathe to see comp correct near the chuck and over comped further away.
It could have had new thrust bearings installed at some point in its life. They may have comped it to 25 while the bearings had lots of wear, then never re-set it after changing them. Or, simply not set correctly from the getgo.
Check carefully that the excess motion is actually linear, and not the saddle rotating on the ways. The design of these machines makes them extremely prone to that. The ways can be immaculate, and the turcite for the front V way quite worn.