Is my lead screw bent?

Discussion in 'Build Help' started by Henry feldman, Mar 20, 2016.

  1. Henry feldman

    Henry feldman Well-Known Member

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    Well apparently the embed movie tag here doesn't support vimeo or youtube, so here is a link. Is my lead screw bent? The left side one wobbles a LOT while the right has a trace amount of wobbling.
     
  2. Kick2box

    Kick2box Well-Known Member

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    Maybe you have to refix the coupler, and don't tighten the grub-screw. I had some wobble too before carefully centering the motor.
     
  3. Dr Jeep

    Dr Jeep Well-Known Member

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    If in doubt whilst you have the coupler undone anyway..pop the screw out and roll it on a known flat surface ( probably not the bed of the printer you'd best not risk scratching it )
     
  4. mike01hu

    mike01hu Well-Known Member

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    It looks like your coupler connection as @Kick2box says.
     
  5. Dr Jeep

    Dr Jeep Well-Known Member

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    I've bought some plum couplers as I am pretty convinced those cut flexible ones shouldn't be used with the end on loads they get supporting the weight of the bed.

    They have a slight coiled spring effect (you can compress them between your fingers if you try) so that must mean they drift a bit when raising and lowering the bed. The plum couplers I bought are much stiffer..maybe it doesn't make much difference overall or maybe they will be too stiff l but I am going to assemble with the supplied couplers and try a before and after comparison once I have the rest of the printer dialled in.

    My current printer has the Z screws supported by a bearing and a nut from the top which carries the weight and so the spiral cut on the coupler doesn't get compressed by the weight of the z motion system.
     
  6. Dr Jeep

    Dr Jeep Well-Known Member

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    Right so just looked at some data sheets and those helical shaft couplers are designed to flex on axial load as well and I can't help thinking that is undesirable when they are supporting the weight of the bed as it effectively means the z axis alignment is essentially sprung..interesting, Mike you are the engineer here am I barking up a wrong tree ?
     
  7. Alex9779

    Alex9779 Moderator
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    They are designed for that and they are designed to work horizontally.

    But in this case this does not matter because if you just did what the build manual said then you inserted the lead screw into the coupler. Nothing more. If the screw of the upper part was completely loose then gravity helped you to do it right.

    The lead screw goes the full way through the coupler until it touches the bottom of the hole in the coupler.

    As you might have noticed the motor axis is smaller than the lead screw. If you have a look at the coupler you will see that the bigger hole goes down in the coupler to the point where the flexible part ends.
    So if you insert the lead screw the full way down until it hits the ground of the hole all vertical forces you apply are not transmitted by the flexible part but go directly to the motor axis.
    You can say that this way for vertical forces you disabled the flexible part. Only if you pull out the lead screw a bit so it does not touch the bottom of the hole then the flexible part is active. But in this case this is not wanted as you mentioned because the vertical forces can push the bed down like when the nozzle scraping over something or anything else.

    The only thing that might happen is that a radial force applied through the motor and the lead screw running not smooth and reacting instantly with turning to that force might widen the flexible part radially which will result in a small length gain in axial direction. This can happen too if you accelerate the motor at high rates because of the moment of inertia of the lead screw.
    But your lead screw should run smooth in the nut and you do not use very high accelerations. So the radial moment the motor applies to the coupler should be instantly transformed into a radial movement of the lead screw.
    By the way this can happen only in one direction in this case how the couplers are designed, when the motors turn clockwise so the bed moves upwards. Upward moves do not really have any effect on print quality as they are only done when you do a z hop on retraction. That's the only case in a print job when the bed moves upwards.
    If the motor turns anti-clockwise the coupler would compress but because of the form lock of the lead screw in the coupler this cannot happen.
     
  8. mike01hu

    mike01hu Well-Known Member

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    You are right Dr Jeep but I suspect that when designed it was considered that the loading on the coupler changes only a minor amount during the printing process so that any change in height is minimal; note that is preloaded by the weight of the bed anyway. Being sprung is not an issue, as the spring rate in a coupler will be too high to have an effect; backlash in the mechanical chain would have a much greater effect on print quality and that is not a problem with this design provided the builder observes good mechanical practices. Now that there are lots of BB's working you can see from the published results that print quality is very good after tuning.

    I am backer 499 so still have to get to grips with the beast so I am grateful to all you guys for ironing out the bugs before I get there . . . OK, I lied as I would love to be in the thick of things now!!!
     
  9. Trevor

    Trevor Well-Known Member

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    I see this happen a lot and it seems quite normal given the length and diameter of the lead screw. I know many people use those flanged pillow block bearings up top to keep it from "wobbling" on their repraps. Some people like it, some don't. What's the consensus on applying it on the BigBox?
     
    #9 Trevor, Mar 23, 2016
    Last edited: Mar 23, 2016
  10. mike01hu

    mike01hu Well-Known Member

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    I think that, because the 4 corner rods effectively secure the position of the bed assembly, the need for a top bearing on the lead screws is reduced.
     
  11. Alex9779

    Alex9779 Moderator
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    I don't really care at the moment. If you watch my time laps of the rocket you see the wobbling. But I don't see any effect on the print. The alignment seems to be worse when the bed is low.
    Just have a look at the play of the lead screw in the nuts. Mine can move about 0.5mm in the screws. Not axial but vertical to the screw axis. So there is enough to play to compensate the wobbling if the four corners are stiff enough...
     
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  12. Henry feldman

    Henry feldman Well-Known Member

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    I'm printing a tall rocket, so we shall see...
     
  13. Mike Kelly

    Mike Kelly Volunteer

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    You can't really tell if it's bent inside the printer. You'd need to take it out and roll it along a flat surface and see if it's distorted.

    Probably just slightly misaligned. Loosen up the mounts a bit, raise the Z up and down and adjust slightly until it stops wobbling.
     

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