On my old (built in 1965) Millrite MVI vertical mill, one annoyance is the difficulty setting the table to a specified X (side-to-side) location. One would overshoot, in both directions. My first theory was that this was because the X-axis feeedscrew and nut were worn, but I wondered if there was an adjustment needing attention, so I got a bright light and looked at the screw as I cranked the table back and forth. I could see that things were moving a bit at the left end, where the leadscrew is fixed to the table by two opposing thrust ball bearings, so I decided to take this assembly apart and de-spooge it.
This assembly has never before been disassembled, I think. The three hex socket cap screws that hold the left end "bell" to the table were glued in place by well-hardened spooge, and took some force to disassemble, and there were lumps of spooge everywhere. Oddly, the deep blind threaded holes into which these cap screws go were filled with oily swarf. Cleaned everything out.
It turned out that one of the thrust ball bearings was caked with hardened grease, being unable to move, and so the whole assembly rotated, forcing the preload nut to turn and loosen, so everything could move. This was a major cause of lost motion.
Soaking the two thrust bearings in acetone for four hours dissolved the caked grease and allowed free rotation and full cleanout. Bearings looked OK, so packed them with Mobil-1 synthetic grease and reassembled the end assembly. The preload nut is round and engages threads machined into the feedscrew, rotation being prevented by a setscrew in the nut. I didn't like the idea of mashing a cup-point setscrew down on and mangling those threads, so I put a little piece of annealed copper in under the setscrew, so the copper would mash and conform to the steel threads. I tightened the nut first by hand, and then by an added
1/8 turn with a pin spanner wrench, only then driving the setscrew home, so there is a slight drag when cranking the screw.The lost motion is now about 0.025", which isn't much better than what it was before, but it does seem easier to set the table to a desired location. I'll use the mill for a while and see if this is really true. With a DRO, the real issue is ability to set to a desired location, not the accuracy of the screw. Both the leadscrew and matching nut (5 tpi) are made of steel, but the nut (being far shorter) should have suffered more wear than the screw.
Joe Gwinn