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I have a Chinese CNC mill (CNC3020T, though several different devices go under this name), and its Z axis was very imprecise, often being randomly off position by as much as 0.5mm. I've disassembled the linear actuator and discovered several problems with it.

First problem is that they apparently forgot to lubricate the linear ball bearings. I make this conclusion because the rails have a set of grooves ground into them, and after wiping the rails with a tissue the only thing that is left is the finely powdered metal, with no traces of oil or other lubricant.

rails bearings

Second problem is the nut. I expected to see a ballnut, but in reality it is just a piece of threaded PTFE! The leadscrew rotates smoothly in it, but there is quite some lateral movement, i.e. I can tilt it slightly without any opposing force.

PTFE nut

Third problem is the overall mounting. In the picture below, the top left screw has been sheared in the factory and then they hid their mistake by tapping a larger thread and putting in a shorter screw that doesn't actually hold anything in the top plate. So the whole assembly was fixed in three, rather than four, points. However, the remaining screw was quite tight.

assembly

So my closely related questions are:

  • Is the assembly even salvageable? How do I verify that linear ball bearings, the PTFE nut are relatively undamaged?
  • Can I just rotate the rails by 45° to get smooth surface again?
  • What do I lubricate the linear bearings with? Do I clean them before lubrication? I have an ultrasonic cleaner.
  • Any other advice on maintenance of the whole assembly? There may be something that I missed.
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  • $\begingroup$ The RepRap page reprap.org/wiki/Lubrication says that PTFE-based lubricant would be the best for both leadscrew and linear bearings. Leadscrew seems to be lubricated using it. I'll try it. $\endgroup$ – whitequark Jul 10 '14 at 12:56
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I've cleaned the bearings of residue using dimethoxymethane aerosol and lubricated them with PTFE-based oil lubricant. It works pretty well now.

More details here.

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