I am building a CNC milling machine , the long axis has a 20mm x 5mm ; 1m80 ball screw drive.
It is generally fine but at random points there is a stiff spot which causes a stall at higher linear speeds. This is at the higher end of the stepper motor speed where it does not have a lot of torque. After stall I can feel a tight spot which is repeatable and I hear a ball flick past and obstruction. My impression by touch was that this was a plastic rather than hard obstruction.
I suspected either ingress of some foreign material or that it was catching on the edge of the plastic inserts which guide the balls to skip back inside the ballnut. Strip down and inspection revealed the latter.
These are "cheap" chinese ballscrews which have a plastic rather than metal insert and three independent internal tracks of one turn each, rather than the multi-turn single track and external return path common on western manufactured items.
It seems that the sticking point comes when there is slight off axis load. The thread is pretty well set at both ends but it seems that the stick happens at places where then is a little resistance to turning the bar. I doubt this is really the root cause but rather the condition where the resistance to movement causes balls to want to push sideways. This would normally be constrained without problem if items with metal inserts or single track paths.
I could slow down, gain a little torque and push through, though I suspect this may just deteriorate and end up locking more seriously once in use. Sweeping under the rug does not look like a good solution.
Replacing the entire ball-thread assy. is excessively expensive due to the length however all these chinese things seem to be production pick and mix to match arbitrary production dimensions which are poorly controlled. If I buy a new nut, I probably will not match my screw and/or ball diameters.
Here is a photo of the defective piece.
The arrow indicates the fine edge where the plastic guide has been deformed. Compare to the other side where it tapers to a point. There may be resistance further down the track, the balls back up and this is the point with least resistance to sideways movement. The other tracks seem fine. This one sits a little more proud of the body, which is probably a manufacturing defect.
My advice at this stage would be , if you decide to buy chinese crap, chose items with a steel insert not these plastic ones.
I'm unsure what to do about this. Is this a known issue, anyone have any suggestions of how best to fix the problem?
Ballscrew platic inserts
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