Fixing the bearing preload on my mill spindle (PM-25MV)
Problem: excessive lateral and vertical spindle play
Solution: take it all apart, put it back together, and hope for the best!
A few days ago I had ~.006" of runout with my drill chuck. Not great, but not totally unreasonable either. Yesterday, I had anywhere from .010" to .025" runout. Even with a collet it wasn't much better, and I noticed I could grab the spindle and move it a noticeable amount to each side. Definitely not good!
After taking it apart, it looks like the clamping bolts that tighten the threads on the preload nut were loose. Not sure if they were just never tightened properly or eventually worked loose with some heavy milling (I did quite a bit of work with a roughing endmill Saturday), but either way the preload nut backed off a turn or two.
With tapered roller bearings, even a couple thousandths of vertical play leads to significant runout. With the nut backed off, the inner race of the top bearing was free to lift up a pretty noticeable amount.
This leads to all sorts of problems, so I had to fix it right away. For now, the spindle seems to be more rigid than it's ever been, so hopefully it stays that way for another 2 years, at least!