Boxford 260 VMC rebuild

[mike-f]

Well, the spindle was a right faff, but the ball screws? No surprise there. These VMC 260s have a bit of a reputation for knackered ball screws. They’re 14mm with a 2.5mm pitch, and to top it off, they’re overhung with no end support. The single nut uses tiny 1.6mm balls, so the load rating’s a bit on the skimpy side. Finding replacements? Good luck. The nut’s a screw-mounted type, not a flange-mounted one, which are ten a penny.
Once I’d got the machine back together and wired up for a test, I braced myself to see how bad things were. The X-axis was the worst offender at nearly 0.18mm backlash. Y-axis wasn’t far behind at 0.09mm, and Z was the least awful at 0.05mm. I did track down a Chinese supplier offering rolled screws for £90-100 each, plus shipping. But knowing my luck, the replacements would probably be dodgier than the originals. So, I thought, why not have a crack at re-balling the ones I’ve got?
I started with the X-axis screw, dismantled it, measured the balls, and ordered replacements. When they arrived, the real fun kicked off. Those 1.6mm balls are so small they vanish the moment you drop one—like they’ve got a personal vendetta against you. For the first go, I meticulously counted out the exact number of balls from the old screw, then counted out the same number of new ones. Sounds simple? It’s not. After thoroughly cleaning the nut, I packed the grooves with ball screw grease and, with the patience of a saint, placed each ball into the grease. This took forever. Once all the balls were in, I slipped in a turned nylon filler rod to hold them in place temporarily. Fitting the screw back was a doddle… after a few practice runs, mind you. The result? Backlash down to less than 0.01mm—good as new, if not better.
The Y and Z axes were a breeze by comparison. I ditched the ball-counting nonsense. Instead, I slapped as many balls as I could into the grease, popped in the filler rod, held it just past the return path tube, and kept feeding in balls until the track was full. Worked like a charm first time and took about 20 minutes per screw. Piece of cake, really. Anyone else tackled ball screw woes on one of these? Or am I the only one daft enough to try?