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  1. #1
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    Ball screw question

    Hello,
    I am looking to see if anyone knows the size of the balls used in the c7 ball screws used in the shopmaster over the past couple years? I am seeing 3 thou backlash and would like to try an oversized ball to help with this. I can see the screw move in small movements of the handle but no movement is registered on the dro or a test dial indicator. Would prefer not to have to disassemble an axis to find out.
    Thanks

  2. #2
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    Re: Ball screw question

    Here is a pretty good video on re-packing those screws.
    Not sure what the sizes are, my older unit used the NOOK screws with .125" balls- these look similar in the video, but are most likely metric.

    https://www.youtube.com/watch?v=GXdt5M8ZeoE

  3. #3
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    Re: Ball screw question

    Metric screws do not necessarily use metric balls. I repacked some Misumi 8mm screws that use .0625" balls (1.5875mm) per manufacturer's specs. In any case I think I'd want to measure the existing balls before ordering new ones to improve accuracy. Even if you know what they are supposed to measure, you don't know what's actually in there. They could be undersized due to any number of reasons including wear or poor quality control. When I took those Misumi screws apart I found someone had put 1.5mm balls in them, I can only guess why. So as much as it sucks, I think you should take one apart and measure ahead of time, unless you're going to order an assortment to have on hand when you do take it apart. If the nuts have removable returns you can open them up and pop a few balls out without unthreading the whole screw.

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  5. #5
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    Re: Ball screw question

    Thank you for the info so far.

  6. #6
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    Re: Ball screw question

    The Mfr for my 2010 were Thomson and there is a etch PN# on mine which I was able to look up. Spec noted .125 nominal and most were .1235 some .123 and many .124 I did not see .125 show up for any of the dozen or so I tried! They are really easy to repack once you get the hang or it! 10mins for 69 balls .125
    If you pull out several and they come out .123/4 then you can go up to the precision .125's to help I would think? I wonder if you could go even higher? PS- I had 69 per screw in mine.
    I am a newb too.. But over about 10months and a total rebuild, I've earned a few striped and loved getting the hang of this great hobby and industry. So - YMMV on this but it took me to a much better consistency place w/ my screws. let me know what you get on the ball sizes if/when you pull it. I'm curious.

    Happy New year CG

  7. #7
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    Re: Ball screw question

    Hmmmm.......you may have another problem nobody seems to address or talk about when it comes to backlash in used ballscrews.

    The problem with a different screw, the Acme thread one is that the screw is soft and wears in the middle where the most action happens leaving the ends in almost new condition.

    To take up the slack in an Acme thread you use a jam nut that backs against the main nut and so eliminates the slackness caused by the main nut wearing......it's a sliding fit and sparsely lubricated at the best of times,........adjusting for wear in the middle makes it tight at anything past the middle, so the adjustment is as close as the drive will tolerate and still move.

    With a ballscrew, if you get backlash the first thing that comes to mind is the balls have worn......never the screw????

    It is quite feasible to think that if you get any backlash the screw needs replacing period.

    Fitting oversize balls means you will get a tightness at the ends or progressively past the centre point that puts a load on the stepper motor also progressively which means you don't notice the tightness if the centre part is now OK.

    Having to eventually come to terms with this predicament made me think that the answer is to add another ball nut to the screw similar to the jam nut method used in Acme threads.

    The design is simple, just screw another ball nut onto the screw, (if you have the room lengthwise for travel), with the flanges of the two nuts facing each other......one nut will be in a housing attached to the table, whatever.

    Then fit threaded studs through the bolt holes in place of the original cap screws and pull both nuts together with NYLOCK nuts to prevent them from vibrating loose.

    Once they're adjusted they will eliminate the backlash without having to think about odd balls and worn screws.

    A lot depends on the overall amount of travel you have with the screw as you might lose some....the thickness of the second ball nut, or 42mm, but it's a simple fix.
    Ian.

  8. #8
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    Re: Ball screw question

    I don't think this is a case of wear at all, I think it's pretty common for these inexpensive screws to have that amount of backlash when new, isn't it? I think the problem is exactly what CG found, a mix of different (mostly undersize) balls that were within the manufacturer's range of tolerance for that particular low-cost screw. The fix is not oversize balls, but simply installing the correct size balls and maintaining a tighter tolerance in ball size. Instead of the factory +0/-.002" just throw out any ball that's not within +0/-.0005". I would find it hard to believe that any part of a ball screw assembly is worn out after only a few years of hobby use, unless it was seriously abused.

    I would also take a good look at the thrust bearings that mount the screw to make sure the screw isn't moving laterally, that will have the same effect. I seem to recall reading about these machines using cheap radial bearings to support the screws.

  9. #9
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    Re: Ball screw question

    That's a solution, but a very complex one to a simple problem. I installed my own ball screw on the quadra lift portion of my machine, and I can tell you they are HARD on the surface. Once you cut past the surface hardening, the material turns quite easily. For one of these things to wear through that hardness would take years and years of production work. I would look for the ball size as the issue and also double check all brackets, thrust bearings, belts and pulleys for any other source of backlash. The pulleys especially, because they use a lock screw on a flat on the shaft, and constant forward-reverse motions can loosen them up.

  10. #10
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    Re: Ball screw question

    Yes.....there are many factors apart from the obvious ones like loose thrust bearings, but with all those checked off and you still have play, reloading with balls is still a hit and miss affair.

    This is one reason I'm going to add the second ball nut to the bridge mill/router Y axis or table drive I am currently in the course of building, mainly to off set the eventual wear that can occur and the backlash that can't be over looked.

    I would say that with a machine already made it would probably be difficult if not at all possible to fit a second nut to the screw(s) due to the screws being fitted to give the travel and not over length, whereas the machine I'm building can have the screw lengths made to suit the needs.

    There's no machining involved and the back up nut is infinitely adjustable for years if not millennia of wear that could/will occur.

    I'm considering this design at the outset as with cheap ball screws you can't expect to have zero backlash no matter how you look at it, and a ball screw with even a smidgeon of backlash that you can't ignore is a pain in the rump.

    I would not imagine that having radial bearings instead of angular contacts to do the thrust side of the drive would give any long lasting results and a penny saved is a pound foolish on that score.

    The same applies to the mounting brackets and the framework they are attached to, specifically at the drive end.....they need to be rock solid.
    Ian.

  11. #11
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    Re: Ball screw question

    My machine new has a backlash problem that can be seen at the ball nut. Jt indicated no adjustments possible so I am going to take apart all ball screws to install either the correct or oversized balls. Yes there are a couple of brackets that I will be upgrading but the ball screw issue is very visable right at the nut. Thank you for all the info everyone.

  12. #12
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    Re: Ball screw question

    Hi, that's interesting that you have the wear problem at the nut, as in between the screw and the nut etc........one of those items has worn somewhat.

    Repacking with new balls on a worn screw is a waste of time in that case....the screw only has a case hardening skin to resist the wear from ball contact, same as the balls, but they're probably completely through hardened.

    I have read that there are alternate balls in the nut that are undersized (only a couple of thou probably) to act as spacers for the main balls in the nut.......the purpose of which is to stop wear from the balls driving against one another as they will rub when in contact, whereas if you have alternate balls undersized, they will rotate in the opposite direction to the main balls and roll without rubbing.

    You can see this phenomena if you place two rollers between two flat surfaces and push one of them against the other....the sides in contact with the top and bottom will roll but the sides touching each other will rub against each other as the rollers rotate,

    Putting an undersized roller between the two rollers that is smaller will enable the contact to be rolling as the undersized rollers cannot drive against the top or bottom and cause friction and so will rotate in the opposite direction.

    This is logical to me, but I don't know if this is common practice in ball screws or just a hypothesis.

    On that basis, I would not hesitate to suggest that if you load the nut with balls of the same diam you will get accelerated wear from the balls rubbing against one another........this does not happen in a ball race as the balls are separated in a cage specifically for that reason.

    I've never investigate this at all, but I would suspect that it "could" be the same in a needle bearing with crowded needles.
    Ian.

  13. #13
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    Re: Ball screw question

    I just picked up a used Tri-Power with CNC and ball screws. It's got .011" backlash on the X axis, .010" between the screw/nut and .001" at the thrust bearings. I tore right into it, found some interesting stuff. The screws are supported by plain bushings with open frame thrust bearings. The screws have welded shafts on both ends rather than being machined, the welds were covered with heat shrink tubing as if to hide the fact that they were welded. because of the welds the nut cannot be removed. The X screw is a Thomson, the Y is a Nook. Neither nut has wipers. Both nuts and all four thrust bearings were full of chips. I'm not the original owner of this machine so I can't say for sure that it came this way. I was planning on rebuilding these screws, but I may just replace them with new ones and machine the ends to fit proper bearings.

    Attachment 263458
    Attachment 263460
    Attachment 263462

  14. #14
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    Re: Ball screw question

    Hi....wow, real precision engineering........I "like" the welding on the screws as in hilarious.....LOL.

    BTW, what's a tri-power?
    Ian.

  15. #15
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    Re: Ball screw question

    ShopMaster Tri-Power Eldorado Bridgemill. It's the model just before the Patriot.

  16. #16
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    Re: Ball screw question

    Your mob on the other side of the pond will be more familiar with that model.

    Will it be worth rebuilding?

    A bridge mill.....I take it that means a fixed gantry and moving table........Linear ways?....you could be in for more surprises in that department.
    Ian.

  17. #17
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    Re: Ball screw question

    This is some really good info! Thanks!

  18. #18
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    Re: Ball screw question

    you can buy the wipers.. It's still on my list. Call Thomson directly. If you do this soon, pop back with the notes on the PNs.. I'll promise to do the same. Its in need of some love it would appear but I guess once you inspect the screw, you can elect to go from there. Keep us posted.

  19. #19
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    Re: Ball screw question

    That was a common way of converting the old machines to ball screws. I did the same on my original 1995 machine. The NOOK ball screws are only 5/8" diameter, and the original acme screws had 19MM shafts, so it was easier to cut the ends off the acme shafts and weld them to the ball screw. If done right in a lathe, they come out fine. On my conversion I drilled and reamed each piece for a pin to keep them aligned . In order to remove the nut you need to take the balls out. I found the same issue with my NOOK ball nuts with excess backlash. When I measured the balls, they were 0.1235" diameter, but the NOOK spec sheet showed 0.125". I called NOOK and asked about the undersized balls and was told that if I ordered the " Custom Fit" nuts at a higher price they would be fitted with the 0.125" balls. Seemed to me like some sort of sales gimmick to get more money out of you, since they could just fit the proper balls in the first place. I just bought a box of 0.125" balls from my bearing supply house for a few dollars and did the upgrade myself. This brought the lash down to near zero- if I remember correctly, there are 67 balls in each nut.

  20. #20
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    Re: Ball screw question

    My x axis screw wobbles a bit, it either got bent or was not welded straight. The nut was making crunching sounds from all the chips that got into it. Due to these factors I believe I'll be replacing the screws and nuts rather than rebuilding them. I did pull a few balls and measure them, .1240 to .1245" on the y axis that had about .006" play.

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