Z wobble

Discussion in 'Guides, Mods, and Upgrades' started by Dr Jeep, Apr 15, 2016.

  1. Dr Jeep

    Dr Jeep Well-Known Member

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    I had very slight Z wobble on my first print, you can just see it on the Benchy hull in the foreground. Nailed it on the 2nd one with just one change.

    I changed the Helix couplers on the Z motors for Plum couplers, they are much firmer than the Helix ones so you do need to make sure everything is aligned, but they don't try and unwind and cause Z errors and you don't have to do that thing where you have to stretch them up the Z axis lead screw..you have to add some washers to the lead screw end caps though as they sit about 5mm higher than in the helix ones.

    It's all feeling pretty dialled in, I need to try a full height print but waiting for my filament order to be dispatched (cough) and someone to reply to my ticket about swapping a reel over from ABS to Everyday PLA.
    IMG_0734.JPG
     
  2. Trevor

    Trevor Well-Known Member

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    I hear good things about plum couplers. They're exponential more costly than the standard flexible couplers. You do any further tests for a more conclusive comparison on taller prints?
     
  3. R Design

    R Design Well-Known Member

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    The Z axis wobble is a very slight banding? (I probably do have a little of that.) That can come from the couplers? Not being on straight?

    In fact, can anyone explain what couplers are for?

    And what happens if the existing couplers slip down a bit? Are there any symptoms in the print? (I can imagine that one might be pushing harder than the other, very easily....)
     
  4. Trevor

    Trevor Well-Known Member

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    The CNC guys refer to these as jaw couplers or spider couplers. Lovejoy couplings are the favorite. They help in backlash as well. The elastomer inside can be customized for less or more rigidity depending on purpose. Definitely more practical for more heavy duty machines such as CNC mills. I never see them on 3d printers which is why I'm a bit curious of their application.
     
  5. Kick2box

    Kick2box Well-Known Member

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    This is not Z wobble. Traces from retraction at layher change and small calibration issues. And do PID tuning. My opinion...
     
  6. Dr Jeep

    Dr Jeep Well-Known Member

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    Yeh still other minor issues but the banding every 4 layers or so vanished when I changed the couplers.

    That's for the tip on pid tuning Kick2box

    Richard asked on G+ for me to post a link here. These are the ones I used, the machining isn't great and they don't have a grub screw for the flat on the motor shaft so personally I'd look about and try and find better ones.

    http://www.ebay.co.uk/itm/281054481556?
     
  7. Dr Jeep

    Dr Jeep Well-Known Member

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    Trevor, I'll try a taller print but away for the weekend now, and I don't have quite enough trust to run it remote from octoprint yet when I am not around.
     
  8. PsyVision

    PsyVision Moderator
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    Jeep - They're the same that I have. Took ages to arrive from China.

    I have done PID tuning and still had the same issue as you that I fought for ages. Adjusting the couplings (making sure they're as flat as possible) did the trick.
     
  9. R Design

    R Design Well-Known Member

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    What do you mean "making sure they're as flat as possible"?

    I still don't understand how couplings are related to print anomalies....
     
  10. PsyVision

    PsyVision Moderator
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    As in tram and straight.

    I recorded a timelapse of a print and could see the top of the coupling was wobbling because the coupler wasn't coupling them properly:



    So every turn (or X layers) it would cause a band like Jeep sees.
     
  11. Dr Jeep

    Dr Jeep Well-Known Member

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    Yeh it's kinda illogical but basically if you run with a bent lead screw or one that is running out of true due to a coupler, then what happens is the lift becomes very slightly uneven with a repeating pattern.

    Essentially if there was 1mm lift per turn, you'd be getting 0.6mm for the first half turn and 0.4mm for the second half...because your layers are say a 8th of a turn of the screw it generates this banding up the walls of the print, because half your layers are slightly flatter.

    k8200 owners know all about this as that printer shipped with a hopeless coupler and a bent lead screw, it's one of the first mods anyone does to that machine.

    What you want to aim for really is with the bed at the bottom, you want the tops of the lead screws to be staying as central as possible in the holes at the top when they turn. I am not quite there yet, mine aren't quite there yet when the bed is at the bottom and I am not sure you will ever get them to run 100% true, but the less wobble there the less chance of layering patterns in the print.
     
  12. R Design

    R Design Well-Known Member

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    Interesting.

    Thanks for the explanation guys.

    I must say, I thought the way the BB setup works is that the 4 rods hold the bed steady in the horizontal plane and the Lead Screws are allowed to wobble freely at the top so they can't disturb it.

    If anything, I thought the idea of a coupler was that it could bend and flex to compensate for any motion of the lead screw whilst transmitting torque all the while.

    But now you've worried me about Lift from the Couplers so I'm going to go and stare at them.

    Did the whole "loosen motor" thing in the calibration (a couple of times) so fingers crossed.

    ;)
     
  13. Old_Tafr

    Old_Tafr Well-Known Member

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    Although the loosen motors and dropping the bed probably helps, it is only possible to tighten the two outside screws before moving the bed up again. I suspect that just tightening the two outside screws (on each motor) first then moving the bed up, tilts the motor slightly.

    This would then give the wobble. A dial gauge on the top surface of the coupler may help to see how flat it is. The wobble on the coupler would then cause the lead screw to wobble... most obviously at the top where it is free to "wobble" :)
     

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