I've got some good news... ok, so it is a bit of both, good and a far bit less bad news. If you are like me, its good news, because I found what I believe to be the reason my mill has more backlash than it should and it is fixable. The bad news is, it probably will cost 6 angular contact bearings and milling some new bearing / stepper motor mounts to do this the "right" way however there may be a way to do it inexpensively too ( see below ). In explaining this I am going to focus just on the X axis but this should apply to all three axis. On my PM 940 CNC the X axis stepper motor is on the left side of the "table". A nut is used to tighten a set of bearings that fix the end of the ball screw directly to the "table". This nut, when tightened should apply enough force to remove backlash but the bearings should allow the ball screw to rotate without any friction unless the nut is overly tight. I had tightened mine up sufficiently to ensure I *should* have a minimum amount of backlash defined by the balls and the screw itself. The ball screws in the PM940 are supposed to be be of a good grade, so I was hoping for the best. Sadly, I'm seeing a minimum of .002 of backlash. I kind of put it out of my mind as other things needed attention but it always bothered me that my mill had a significant deficiency. HOWEVER, I remembered something I read about ball screw ends from china sometimes are designed poorly. Basically they use two bearings but stupidly the inner races are allowed to touch each other. Basically the two bearings are back to back, acting as one bearing. So even if they were two sets of angular contact bearings, if the inner races are allowed to tighten against each other they have no value other than allowing low friction when turning. The bearings side play is then what defines the amount of backlash. Check out my little video that shows the two bearings spinning together with just a small amount of force squeezing them together by hand.
Also, they don't appear to be angular contact...
One additional detail is that the right side of the table also has a nut assembly that holds the hand cranks. When I first got the mill I removed all the hand cranks and nuts to ensure that the ball nut assembly would not be bound up with opposing force.
There is one possible way to do this on the cheap and use the existing castings and possibly the existing bearings. Just find a sufficient washer and place it between the large outer races. I haven't tried to remove the existing bearings because they appear to have been pressed in with no way to press them out without damaging ( pressing via the inner races is not good, but maybe necessary ), I haven't tried. I'm going to order replacement bearings before I try to remove them. Then I'l at least have a way to machine something that willl replace the castings, etc, if needed.
sleepy... gnite
Jake
One additional detail is that the right side of the table also has a nut assembly that holds the hand cranks. When I first got the mill I removed all the hand cranks and nuts to ensure that the ball nut assembly would not be bound up with opposing force.
There is one possible way to do this on the cheap and use the existing castings and possibly the existing bearings. Just find a sufficient washer and place it between the large outer races. I haven't tried to remove the existing bearings because they appear to have been pressed in with no way to press them out without damaging ( pressing via the inner races is not good, but maybe necessary ), I haven't tried. I'm going to order replacement bearings before I try to remove them. Then I'l at least have a way to machine something that willl replace the castings, etc, if needed.
sleepy... gnite
Jake
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