DIY lead screw repair

I acquired a Bridgeport series I milling machine two years ago. It seems that the previous owner took good care of it. Little by little I have taken it apart, cleaned/oiled/greased it, and checked/replaced every component.
The machine came with lead screws in the X and Y axes. A couple of days ago I decided to take them apart again to adjust the backlash (0.0085” and 0.0030”). Once I had taken the Y axis screw apart I measured the bearing balls and found that there was a difference in size of 0.003”. Since this variation is too big for a lead screw I decided to buy new bearings from McMaster (P/N 9528K11); 0.0001” tolerance and RC60 hardness.

Here is the question, after I cleaned everything and put the screws together, there is some grinding noise in some parts of the screw. The interesting thing is that the grinding noise does not happen in both directions. Can anyone shine some light on what may be happening? The screws did not grind before and the nuts and threads seem quite clean and in good shape. Thanks.

Do you have ball screws instead of Acme?

OK, so these are BALL screws, as opposed to the standard acme-thread lead screws, then. There is an ongoing discussion in a couple of places regarding ballscrew refurb for CNC application that might be worth reading. Do a search for ballscrew and have a look (there are actually dozens of discussions that have occurred on this forum). There are an awful lot of parameters related to the interaction of a ballnut with its screw.

I can't particularly address your question, other than to point at the likelihood that new balls of different size (albeit uniform size) may be riding in different areas of the ball trough, which may be worn in some way or have some other finish problem that is showing up as the grinding noise you observe. It might be useful to experiment with different size balls to see if you get different results without a deterioration of backlash result. Additionally, there could be a cleaning issue with some section of the ball loading that is showing up periodically as the balls recirculate.

Other thing is, that the old balls oftern were sized large then a smaller one then large etc, its so they don't rub and grind, the smaller ones act as spacers bettwen the big ones doing the work! Other thing you could have done is get a ball stuck in a gap bettwen one of the return - ball circuits!