Chasing Backlash

[TomS]

Maybe I missed something but I do not see how the ball nut was eliminated as a possible source?

Jay - other than the dial indicator check I've already done what other checks can I do to either eliminate the ball nut or identify it as the culprit?

Tom S.

 

[jbolt]

That's what I suspected. What the heck backstops the inner race of the inner AC bearing?

There is a spacer between the inner race and a shoulder on the screw.

This is a close representation.

 

[jbolt]

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?

The ball nut is attached to the saddle. The screw is attached to the table via the AC bearings and block. They both move together so if the AC bearings are good I would not expect to see movement there. You need to measure with the TDI mounted off the table. I would lock the Y axis, mount a TDI in the spindle and bolt a 123 bock to the table or reference off the side of a vise and then apply force to the table in both directions to see if you are getting any deflection.

 

[JimDawson]

That is kind of a basakwards system. I would want the outer races anchored solid in the housing with a spacer between them, and set the preload with a double locknut on the lead screw. If it was mine, I would be changing that in a heartbeat at the first sign of trouble.

 

[jbolt]

That is kind of a basakwards system. I would want the outer races anchored solid in the housing with a spacer between them, and set the preload with a double locknut on the lead screw. If it was mine, I would be changing that in a heartbeat at the first sign of trouble.

I'm not speaking for Tom but that is essentially how it is as you described except instead of guessing what the preload is with a thick shim and just tightening a nut, thin shims are stacked between the outer races to set a known distance. Adding or removing shims changes the preload amount.

The Chinese AC bearings and blocks have a large tolerance variation so to get them to work properly it is important to measure the fit of the bearings in the block as well as the play in the AC bearings. Once I know what the minimum shim thickness to get the bearings in full contact (no play) is after measuring all the components I then add .001" shims at a time to add preload.

 

[JimDawson]

I understand what you are saying. I have always set preload by feel rather than shimming, but an absolute dimension on the preload makes some sense.

 

[TomS]

The ball nut is attached to the saddle. The screw is attached to the table via the AC bearings and block. They both move together so if the AC bearings are good I would not expect to see movement there. You need to measure with the TDI mounted off the table. I would lock the Y axis, mount a TDI in the spindle and bolt a 123 bock to the table or reference off the side of a vise and then apply force to the table in both directions to see if you are getting any deflection.

I'm learning something new every day. I'm out of town until Tuesday. Will check it when I get back.

Tom S.

 

[jbolt]

I understand what you are saying. I have always set preload by feel rather than shimming, but an absolute dimension on the preload makes some sense.

Having had a Chinese AC bearing fall apart in my hands I don't trust doing anything by "feel". I was talking to a bearing tech at one of the distributors I use about bearing preload and he said they order matched sets ($$$) with the internal races ground to a specific width so that when installed and the nut is tight the inner races are in full contact with each other and a preset preload is established.

That's how I came up with using shims to set the preload. I may not know the value of the preload but at least I have a reference value if it turns out to be too loose or the bearings wear prematurely.

 

[jbolt]

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.

cs900,

Are these the the typical Chinese double ball nuts? If so could you elaborate on how to adjust them?

 

[cs900]

well this was before they started offering the double ball nuts they have now, but yes it's 2 chinese single nuts preloaded with belville washers.