Mud
Diamond
- Joined
- May 20, 2002
- Location
- South Central PA
Cblair suggested I post this here, and see how the thread progresses.
I'm redoing the X and Y ballscrews in a smallish VMC, 18 X 30, 25 HP, square ways, 600 IPM max. 2" diameter screws, .400" pitch. They seem to be in good shape but I have the machine apart to inspect and clean the Turcite and wanted to go over the ballscrews at the same time before I put it back together and into service. They are a lot of work to get to, I don't want to do this again anytime soon.
So I have lots of questions, this being my first time.
1. - The big one - Which is preferrable, all the balls the same size, or using spacer balls?
I spoke to Express Ballscrew Repair a bit and he strongly preferred using spacer balls, because he felt he could use more preload to get a tighter more accurate screw. The concensus of previous PM threads is that using all the same size balls increases the load capacity and that the screw will last longer with less wear. So are both of those opinions true, and which is preferrable?
2. - This machine has glass scales on all 3 axes. Does having scales change how much preload you would like to have in the screw? Since the scales determine position instead of the screw, I can see that you could maybe run less preload, possibly have less wear and not lose accuracy. If less preload is necessary, perhaps there's no downside to using all the same size balls.
3.- If you have the preload set with spacer balls, then reload the screw with all the same size balls, the same size as the larger balls previously, will the drag increase?
4.- With alternating ball size, does the size of the spacer balls matter? When replacing the balls with new, do you replace the spacer balls also?
5.- How do YOU measure ball size? I've read about elastic deformation of balls measured with a micrometer. I'm using a comparator micrometer checked against gage blocks. I have no good way to measure the force of the anvils on the ball, but my guess is it's slightly less than a starrett or B&S mic cranked down with the thimble. If the gage blocks that make the comparator mic read the same as the balls add up to .2584, what size are the balls? I can discern closer than .0001 with this mic, easily .000025, and it repeats, can't speak for it's true accuracy except to compare to gage blocks.
6.- How much ball size change is needed to make a significant preload change? If I have a screw with what seems like zero clearance, it has no measurable movement between the screw and nut and turns easily, how much larger do you go with the balls in one step to make a change?
Guess that's enough questions to start a thread. I'll have more.
I'm redoing the X and Y ballscrews in a smallish VMC, 18 X 30, 25 HP, square ways, 600 IPM max. 2" diameter screws, .400" pitch. They seem to be in good shape but I have the machine apart to inspect and clean the Turcite and wanted to go over the ballscrews at the same time before I put it back together and into service. They are a lot of work to get to, I don't want to do this again anytime soon.
So I have lots of questions, this being my first time.
1. - The big one - Which is preferrable, all the balls the same size, or using spacer balls?
I spoke to Express Ballscrew Repair a bit and he strongly preferred using spacer balls, because he felt he could use more preload to get a tighter more accurate screw. The concensus of previous PM threads is that using all the same size balls increases the load capacity and that the screw will last longer with less wear. So are both of those opinions true, and which is preferrable?
2. - This machine has glass scales on all 3 axes. Does having scales change how much preload you would like to have in the screw? Since the scales determine position instead of the screw, I can see that you could maybe run less preload, possibly have less wear and not lose accuracy. If less preload is necessary, perhaps there's no downside to using all the same size balls.
3.- If you have the preload set with spacer balls, then reload the screw with all the same size balls, the same size as the larger balls previously, will the drag increase?
4.- With alternating ball size, does the size of the spacer balls matter? When replacing the balls with new, do you replace the spacer balls also?
5.- How do YOU measure ball size? I've read about elastic deformation of balls measured with a micrometer. I'm using a comparator micrometer checked against gage blocks. I have no good way to measure the force of the anvils on the ball, but my guess is it's slightly less than a starrett or B&S mic cranked down with the thimble. If the gage blocks that make the comparator mic read the same as the balls add up to .2584, what size are the balls? I can discern closer than .0001 with this mic, easily .000025, and it repeats, can't speak for it's true accuracy except to compare to gage blocks.
6.- How much ball size change is needed to make a significant preload change? If I have a screw with what seems like zero clearance, it has no measurable movement between the screw and nut and turns easily, how much larger do you go with the balls in one step to make a change?
Guess that's enough questions to start a thread. I'll have more.