Why doesn't an 8mm threaded rod fit into a bearing meant for 8mm smooth rods?

Discussion in 'Guides, Mods, and Upgrades' started by W1EBR.Gene, Mar 11, 2017.

  1. W1EBR.Gene

    W1EBR.Gene Well-Known Member

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    I have bearings from the kit that fit onto the 8mm smooth rods (like the back y-axis drive rod). The same bearing does not fit on the threaded 8mm z-axis rods. Does anyone know why?
    Thank in advance!
     
  2. mike01hu

    mike01hu Well-Known Member

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    It is because the outside diameter of a M8 screw is between 7.84 and 7.98mm, so 8mm bearing will be too loose. The answer is to get a friend with a lathe to turn down the threaded rod end to say 6mm to fit a 6mm bearing.
     
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  3. W1EBR.Gene

    W1EBR.Gene Well-Known Member

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    Hi Mike, I am finding just the opposite; the bearing does not fit over the end of the threaded rod because the rod is too thick.
     
    #3 W1EBR.Gene, Mar 13, 2017
    Last edited: Mar 13, 2017
  4. Stian Indal Haugseth

    Stian Indal Haugseth Well-Known Member

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    Well then one or both of the parts are out of spec. These tings comes in various tolerances and if the shaft is close to the max and the bearing is close to the minimum there is a chance they will not fit. Try all the bearings on all the shafts and sort them by who fits best (reserve the best for Z shafts if any fit). I did this and found surprising differences. I would contact E3D if these are BigBox parts and they do not fit or snag/move badly.
     
  5. mike01hu

    mike01hu Well-Known Member

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    I am not at home this week so can't measure my lead-screws but I would assume that the E3D ones are to the correct tolerances but @Greg Holloway may be able to comment. As far as I am aware, it was important for the nut and lead-screw to be matched so the two may be outside the M8 spec., as I would not expect the bearing ID to be wrong. Have you measured your lead-screws with a digital calliper or micrometer?

    As yours appears to be larger, then I would suggest that judicious use of a file while the lead-screw is rotated at very low speed with a power drill will make a fit that should be adequate. You would need to support the lead-screw at each end in 8mm holes cut in blocks of wood screwed to a length of wood or held in a "Work-Mate" and have someone hold and operate the drill whilst you do the filing; a bit crude but cheaper than a lathe for a minor fix.
     
  6. Greg Holloway

    Greg Holloway Administrator
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    No part of the BigBox design requires the leadscrew to be supported with 8mm ID bearings, ergo we did not do any testing to see if they do.

    The important part is that they are matched to the brass nut. I expect the newer 8mm pitch leadscrews are slightly larger than 8mm as we had to slightly widen the couplers to take them.
     
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  7. mike01hu

    mike01hu Well-Known Member

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    You will have seen the comment from @Greg Holloway and it is right that provided the corner rods are secure and true and the flexible coupler is correctly fitted any run-out should be managed by the flexible coupler. Of course, there is another camp that says if the lead-screws are supported at both ends the coupler can be solid and any run-out is restricted to lead-screw straightness. For ease of design and cost considerations the original method works but the beauty of the BigBox is that it is hackable and people can make their choice in to which direction to go.
     
  8. W1EBR.Gene

    W1EBR.Gene Well-Known Member

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    This is something that I was tweaking, since even with my best attempt at tuning, I still saw some eccentricity at the top of the lead screws. I was surprised that the 8mm ID bearings (the ones supplied by Greg for the Y-axis drive shaft) did not fit over the lead screws and wondered whether there was something about lead screw sizes.

    I am interested in the integrated stepper motor - lead screw combination. The lack of flexibility between the motor and screw may force me to adjust the rods or bearings, which is actually what I think I should be doing if everything is straight and I am still having problems... at least, it makes sense to me. :)
     
  9. Stian Indal Haugseth

    Stian Indal Haugseth Well-Known Member

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    Hmm.. I missed the threaded part :p
     
  10. mike01hu

    mike01hu Well-Known Member

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    What is important is the actual printed results and my machine is stock and produces acceptable results.
     
  11. W1EBR.Gene

    W1EBR.Gene Well-Known Member

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    The engineer in me always wonders about what can be done, at a reasonable cost of time and money to make a worthwhile gain in performance.:)
     
  12. mike01hu

    mike01hu Well-Known Member

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    That's where I come from and my well-tuned machine defies the need to change things just for the sake of it. It is not a toy anymore but is a tool in the box! ;) . . . OK, I lied a bit about it not being a toy.:rolleyes:
     
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  13. Stian Indal Haugseth

    Stian Indal Haugseth Well-Known Member

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    Where would we be without toys?
     
  14. CHP

    CHP Member

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    Which it is supposed to. The bed is constrained in X and Y direction by the 4 smooth rods + linear bearings. The leadscrews need to be free at one end to compensate their misalignment.
     
    #14 CHP, Mar 14, 2017
    Last edited: Mar 17, 2017
  15. W1EBR.Gene

    W1EBR.Gene Well-Known Member

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    Thanks, guys, for all of your responses! The support on this forum is great!
     
  16. R Design

    R Design Well-Known Member

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    Hmm. That may be the thing.

    When you have just 2 rods you can perhaps more tightly constrain the platform strung between them than when you have 4 rods. Just as it's never possible to set two things up perfectly so 3 things must be set up less perfectly, 4 things even worse and 10 rods would necessitate a tremendous amount of slack to assure smooth linear motion over 300mm !

    Remember the first batch of kickstarter rods had to be sent back to be ground down to allow for a bit more play?

    So I'm thinking that maybe leadscrew wobble has been an issue in print-sidewall quality:

    a) because we did have wobbly leadscrews;

    b) because in a system with 4 rods there must be more play than in a system with 2 rods;
     
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