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Chasing Backlash

Discussion in 'CNC IN THE HOME SHOP' started by TomS, Jan 30, 2017.

  1. TomS

    TomS Active User H-M Supporter-Premium

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    Been chasing .003" of X axis backlash on my PM-932 CNC for four days now and haven't made any headway. Haven't checked the Y axis yet. It's time to tap into the Hobby-Machinist brain trust.

    I've measured the backlash two ways. One method I used is to mount a .0005" DTI in the spindle and touch off on the vise. My other method is mounting a mag base with a 1" travel DI on the saddle, again touching off on the vise. I preload the dial indicator and set them to zero and also zero the Mach3 X axis DRO. I then jog the table in the opposite direction in .001" increments until the DI needle just begins to move and note the X axis DRO reading which is .003"-.004". For a comparison and to eliminate stick slip I tried the second method using a 1" travel indicator and jogged the table 1/2" and got the same backlash readings.

    I've adjusted the gib several times and run the tests and no change in backlash except when I adjusted it too tight. I also adjusted the preload on the ball screw AC bearings. Having .003" to .004" of backlash I added .006" to the shim pack. Backlash is still .003" to .004". If I take shims out backlash increases.

    Checked the ball screw for lateral movement with a DI and it's zero.

    I installed Linear Motion Bearing double ball nuts a few months ago and was able to cut a pocket that was within .0005" of being round. Haven't tried this again so maybe it's time to retest.

    What is considered acceptable backlash? Zero would be nice but I don't think it's a realistic expectation.

    Am I checking backlash correctly? From what I've read on the internet I am.

    Could it be bad AC bearings? These are the original Chinese bearings that came with the bearing blocks.

    Could it be the coupling? I'm using split clamp style double diaphragm couplings with two set screws on each end. Although I don't like to run with rigid couplings, as a test I'm go to either make or buy one to eliminate that possibility.

    Thanks in advance for your advice.

    Tom S.
     
  2. JimDawson

    JimDawson Global Moderator Staff Member Director

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    0 is acceptable backlash and with double ball nuts should be achievable. It sounds like you are setting up correctly. My preference is to use the spindle at the indicator base point

    I don't particularly trust the Mach3 DRO for a test like this. With the motor removed, I would put a ''degree wheel'' on the lead screw, preload in one direction then rotate 1 turn in the preload direction, then 1 turn back . The difference on the indicator is the backlash. The degree wheel could be a stiff piece of paper with a single mark on it, just taped to the end of the lead screw or coupling. The larger the diameter the better to get maximum resolution.

    Are the AC bearings installed in the correct orientation?
     
  3. TomS

    TomS Active User H-M Supporter-Premium

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    I'll give your recommendation a shot in the morning. I've had the drive end of the lead screw apart so many times in the last four days I can do it with one hand and blindfolded.

    For info I cut a pocket and the bore matched my measured backlash. I enabled backlash compensation, or so I thought, and recut the pocket. The bore was .005" out of round! Double checked my inputs ; .003" in X and .0018" in Y. Even tried negative numbers and it locked up the motors.

    Double and triple checked the AC bearing orientation and confirmed they are installed back to back. Whats interesting is I measured bearing bore depth, cover spigot height and bearing widths to calculate the preload shim pack. Put everything together with the calculated shim pack (.005" bearing preload) and had .0125" of backlash. Hmmm! I think the inner races were contacting each other. Added a .015" shim and ended up with .003" backlash. Added another .005" shim to the shim pack and still had .003" backlash.

    I adjusted the gib until it stalled the motor then backed it off until the motor ran the full length of travel without stalling. Didn't want the table too loose and giving me false readings.

    Tom S.
     
  4. JimDawson

    JimDawson Global Moderator Staff Member Director

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    Do you have enough clearance between the outer races? They should have a spacer between them.
     
  5. TomS

    TomS Active User H-M Supporter-Premium

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    The bearings drop into the housing. Haven't measured the clearance but my guess is about .002" loose. No spacers but I'm adjusting preload with shims between the outer races. Is that what you are referring to?

    Tom S.
     
  6. JimDawson

    JimDawson Global Moderator Staff Member Director

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    OK, yes. I guess I would have to see how the assembly goes together to really understand. Normally I would expect the outer races to have a fixed spacer between them, then set the preload with a nut on the lead screw to tighten the inner races against each other. I would think the outer races would be tight axially in the housing, maybe set with shims, but tight. 0.002 radial play seems a bit loose, but shouldn't really affect the axial play of the lead screw. Normally the support bearing on the other end of the lead screw is pretty loose in the bore, but it is supposed to float axially.

    upload_2017-1-30_22-23-1.png

    And a bit better picture of the bearings

    upload_2017-1-30_22-25-57.png
     
    Last edited: Jan 31, 2017
  7. TomS

    TomS Active User H-M Supporter-Premium

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    Your second picture closely resembles my setup. The only difference is the bearings are recessed in the bore about 1/16" and there is a spigot on the cap that fits the bore and locates the cap. The shim pack serves the same purpose as the spacer. Yes, the support bearing floats in the housing..

    Thanks,


    Tom S.
     
  8. TomS

    TomS Active User H-M Supporter-Premium

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    Well your advice about not trusting the Mach3 DRO's was right on. I set up a disk on the ball screw, installed and aligned a pointer, preloaded the ball screw in one direction, zeroed the DI, then reversed screw rotation exactly one turn. I have .007" of backlash, not .003" to .004"! For reference the DRO read .0043".

    Before running the test above I adjusted the X axis gib using the Bridgeport method. I ended up with .0015" of indicator movement and called it good. Before adjusting the gib my backlash was .005". I'll back the gib off a bit.

    I tried measuring ball screw rotational torque using my in/lb torque wrench. My lowest setting is 50 in/lb and the screw turned easily. I even backed off the torque wrench setting one full turn and still didn't work. Suffice to say rotational force of the ball screw in less than 25 in/lb.

    Although they are Chinese origin I don't think the AC bearings are the problem. Also rechecked ball screw longitudinal movement. Still zero. Don't remember off hand what the preload is but it's probably around .010". I know it's way too much but I am chasing an unknown and with preload set at about .005" I had backlash. I wanted to see if increasing preload would change anything and it didn't. If I go below .005" preload backlash increases to about .012". Maybe more using the degree wheel test.

    I'm kind of at a loss as to how to approach this. I've measured it a dozen different ways using three different dial indicators and adjusted the AC bearing shims pack thickness more times than I can remember. I thought it might be the coupling but setting the jog at .0001" and carefully watching rotation in both directions I can see the coupling and ball screw rotating in unison. If I was chasing a couple of tenths then I might dig deeper but .007" of rotational lag would be quite noticeable.

    Here are a couple pictures of the driven side bearing housing components. The two cylindrical items at the top are seal/compression sleeves. The grease seals run on the OD and the length is such that when the bearing lock nut is tightened the sleeves and AC bearing inner races are compressed against the shoulder on the ball screw.
    20170131_120026_resized.jpg

    This shows the register on the cap better than the picture above.
    20170131_120040_resized.jpg
     
  9. JimDawson

    JimDawson Global Moderator Staff Member Director

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    Given that the backlash changes with the shim packs, and the leadscrew/table/ballnut seems to have 0 backlash, then it would seem that the problem is in the AC bearing area. Now the question is how to fix it.

    I would be threading the lead screw for a preload nut even if it requires a redesign, but I have very little patience for this kind of adjusting system. If that doesn't fix the problem then I would be tweaking the ball nuts or mounting system.
     
  10. jbolt

    jbolt United States Active User H-M Supporter-Premium

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

    JimDawson Global Moderator Staff Member Director

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    I was going by Tom's quote above. But you are correct, the ball nuts have not really been eliminated.
     
  12. TomS

    TomS Active User H-M Supporter-Premium

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    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?
     
  13. TomS

    TomS Active User H-M Supporter-Premium

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    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.
     
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  14. JimDawson

    JimDawson Global Moderator Staff Member Director

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    Tom, I think you're right, time for a break. :faint:

    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.
     
  15. jbolt

    jbolt United States Active User H-M Supporter-Premium

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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.
     
  16. TomS

    TomS Active User H-M Supporter-Premium

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    I'll try and take one in the morning before I head out of town.

    Tom S.
     
  17. cs900

    cs900 United States maker of chips Active Member

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    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.
     
  18. TomS

    TomS Active User H-M Supporter-Premium

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    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.
    20170201_082522_resized.jpg
     
  19. TomS

    TomS Active User H-M Supporter-Premium

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    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.
     
  20. JimDawson

    JimDawson Global Moderator Staff Member Director

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

    TomS Active User H-M Supporter-Premium

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    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.
     
  22. jbolt

    jbolt United States Active User H-M Supporter-Premium

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

    This is a close representation.
    ACBearingBlock.jpg
     
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  23. jbolt

    jbolt United States Active User H-M Supporter-Premium

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    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.
     
  24. JimDawson

    JimDawson Global Moderator Staff Member Director

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    That is kind of a basakwards system. :confused: 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.
     
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  25. jbolt

    jbolt United States Active User H-M Supporter-Premium

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    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.
     
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  26. JimDawson

    JimDawson Global Moderator Staff Member Director

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    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.
     
  27. TomS

    TomS Active User H-M Supporter-Premium

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

    Tom S.
     
  28. jbolt

    jbolt United States Active User H-M Supporter-Premium

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    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.
     
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  29. jbolt

    jbolt United States Active User H-M Supporter-Premium

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    cs900,

    Are these the the typical Chinese double ball nuts? If so could you elaborate on how to adjust them?
     
  30. cs900

    cs900 United States maker of chips Active Member

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    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.
     

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