The machinist should have remove the nut first before machining. It could have been slid off onto some threaded rod or even a wooden dowel.
It may be that there is some swarf in the returns. Since you have had it apart now, you could investigate. Take the balls aout and hit it with some compressed air. Wear eye protection and don't point it at anything. A towel wadded up may be your best target. You may get some pipe cleaners to feed in the returns to clean them up.
I have heard of stuffing these with oversize bearings for a greatly reduced backlash. If you think balls are awol from the assembly, this may be an option for you.
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Lee |