- Joined
- Feb 8, 2014
- Messages
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I was going by Tom's quote above. But you are correct, the ball nuts have not really been eliminated.
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Chasing Backlash
- Thread starter TomS
- Start date
I was going by Tom's quote above. But you are correct, the ball nuts have not really been eliminated.
I attached a mag base to the table and set the indicator tip on the end of the ball screw. Traversing the table in both directions the DI needle did not move. If the backlash was in the ball nuts wouldn't I see that on the DI?
I decided to go back to square one and remeasure the bearing housing and other related parts. Hopefully I can lay it out so it's understandable. Here goes.
The bearing housing bore is .701" deep. The bearing cap spigot is .078" tall. Each bearing is .315" wide. Doing the math, with the cap assembled, the effective bore length is .623". Stack two bearings together (.315" wide) and my calculations result in .007" of preload. I assemble all these parts onto the ball screw, including the front and rear spacer sleeves, and tighten the retainer nut. Set my mag base on the table and positioned the DI on the end of the ball screw. Moving the screw by hand I get .016" of backlash. What am I missing? After five days of no progress I had to walk away.
Tom S.
The bearing housing bore is .701" deep. The bearing cap spigot is .078" tall. Each bearing is .315" wide. Doing the math, with the cap assembled, the effective bore length is .623". Stack two bearings together (.315" wide) and my calculations result in .007" of preload. I assemble all these parts onto the ball screw, including the front and rear spacer sleeves, and tighten the retainer nut. Set my mag base on the table and positioned the DI on the end of the ball screw. Moving the screw by hand I get .016" of backlash. What am I missing? After five days of no progress I had to walk away.
Tom S.
- Joined
- Feb 8, 2014
- Messages
- 11,176
Tom, I think you're right, time for a break. 
Not sure where the problem is, but I'm afraid you are going to find something simple.
Post a picture of the bearing area on the ball screw when you get a chance.
Not sure where the problem is, but I'm afraid you are going to find something simple.
Post a picture of the bearing area on the ball screw when you get a chance.
- Joined
- Dec 3, 2013
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Two bearings at .315 ea (assuming this is the measurement of the outer race) is .630 so stacked they are .007 taller that the pocket and cap. In this case the outer races are being compressed by the cap. This does not show the play in the AC bearings. To get the pre-load you need to measure the offset of the inner and outer race with the inner race suspended by the outer race and bearings. Once you know the offset and take into account if the inner and outer races have different widths then you can determine what the minimum shim thickness is to have the balls in full contact with the inner and outer races when stacked. Any shimming above this sets the pre-load.
Tom, I think you're right, time for a break.
Not sure where the problem is, but I'm afraid you are going to find something simple.
Post a picture of the bearing area on the ball screw when you get a chance.
I'll try and take one in the morning before I head out of town.
Tom S.
- Joined
- Mar 21, 2016
- Messages
- 251
so I had a fairly similar issue when I did my PM45 conversion. Had .003-.005 of backlash i just couldn't get out using a double ballnut set-up just like you. Well turns out I had too much preload on the double nut set-up. Went back and took all preload off the second ballnut and had .007 backlash. I then progressively increased the preload until I got to around .0005. I tired getting it closer, but seems to be that if I increase preload any more my backlash increases again. So I left it at that and I currently attribute it to something other than the leadscrew. Using the backlash comp and a good leadscrew mapping I hold +/-.001 fairly well.
Hopefully it's something simple like that, casing backlash can drive you nuts! I can understand why people just go buy tormachs now, lol.
Hopefully it's something simple like that, casing backlash can drive you nuts! I can understand why people just go buy tormachs now, lol.
Jim - here's the picture. From the shoulder to the end of the bearing fit area is 15/16". Length from the shoulder to the end of the threads is 1-1/2".
Tom S.
Thanks for the suggestion. I can do that. It's just a matter of pulling the table off. Not sure how a Linear Motion double ball nut is put together. I know there is a key that keeps them from rotating.
Tom S.