I bought a previously retrofitted BP clone mill and I'm just getting it up an running with a new PC based control.
I'm seeing more backlash on the X and Y lead screws than there should be- .001" on the X and 0.004" on the Y.
The backlash is uniform on the full length of both screws, so it looks like the nuts are worn but not the screw themselves. So far I can't find any manufacturers names on the screws, and I can't see the nuts very well without taking the table off.
Is it possible to replace just the nut on a ball lead screw or will I have to replace the whole set to get rid of this backlash?
Thanks for the suggestions on the thrust/preload issue fellas, I'm looking at that now and a quick test (tapping the end of the screw with a rubber hammer moves the table through the backlash zone) is showing it may be a preload issue or worn out thrust bearings. I'm going to yank it apart and I'll let you know what I find.
After disassembling it, it became clear that the "semi-pro" that had installed the ballscrews had hacked things a little. The Y screw preload wasn't set correctly and the way the motor mount was designed you couldn't adjust it. I reworked it so there was some preload and got the 0.004" Y backlash down to 0.001", same as the X axis. Increasing the preload any more didn't get rid of the last 0.001", but the machine has 6204 plain ball bearings in place for these thrust bearings, and when you put in ballscrews you're supposed to upgrade those bearings to 7204 angular contact type, which handle thrust loads more accurately.
I'll leave it as is for now, 0.001" is good enough for the time being, and I'll upgrade it to 7204 bearings at some point down the road.
There's nothing fancy in the bearing setup, a couple of 6204 bearings back to back with a thin (0.064") spacer between the inner races to give some preload room. There is a ring with 3 bolts that compresses the outer race of the outside bearing towards the inside bearing creating the preload. I believe this is the standard setup on all BP type mills.
I replaced the 6204's with SKF 7204 angular contact bearings as suggested on the http://www.rockfordballscrew.com/ on the X axis hoping to get the last 0.001" of backlash to go away, but even with the preload cranked up pretty high I've still go the 0.001" backlash. Backing down on the bearing preload adds in more backlash, but I can't get it less than 0.001".
At this point I figure its either got to be wear in the nut (backlash is constant over the whole screw, so I don't think its the screw) or some slop in the center yoke, possibly from a so-so installation job, which would be consistent with some of the other work that was done by (or for) the previous owner. I'll probably have to let it slide for now, but I guess some where down the road I'll take the table off and try to figure out where the remaining backlash is coming from.