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11-03-2013, 12:33 AM #1
Ballscrew backlash - Tree 325 CNC
How much backlash is acceptable in ballscrews? I'm looking at a used Tree Journeyman 325 CNC, the X axis has essentially 0 backlash, the Z axis has 0.0005", but Y has about 0.002-0.003" depending on position. I think the thrust bearing is not pretensioned correctly and is contributing about 0.001-0.0015" of the total, based on indicating the end of the screw while jogging the axis. I didn't have time to figure out what backlash comp was set in the Dynapath 20 control, but judging based on how the screws turned when reversing, it's minimal, < 0.001"
The machine appears to be in good condition, ex-Boeing equipment. Asking price is $4300. Does this seem reasonable? Could the backlash be reduced by reballing it, or is it a more major repair to get the backlash out?
11-07-2013, 12:33 AM #2
If the Y axis backlash varies .002-.003 all along the axis that would be a different problem than the backlash being "zero" everywhere except a particular section of the screw. That much backlash in just one section would indicate wear in that section of the screw. That would leave you with the option of having the screw re-ground (a number of shops advertise this service) or getting a new ballscrew.
If it's even along the axis, then, yes, probably the thrust bearing needs to be tightened down - or replaced.
11-07-2013, 06:23 PM #3
Troubleshot a variable backlash issue (Y axis on a Mori MV-40).
The screws holding the ball nut into it's bracket were stretched slightly and the screws holding the bracket to saddle were also loose.