X/Y axis rigidity

Discussion in 'Calibration, Help, and Troubleshooting' started by pSimon, Oct 6, 2016.

  1. pSimon

    pSimon Member

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    Just built myself a fitting for a travel gauge I had to level the bed.

    The levelling part is easy enough now...

    But, I'm noticing that the bed seems to rise away from the corners, by about 0.1mm in Y, with the highest point in the centre of the bed.

    "Seems" is the operative word, I think - I'm pretty sure the glass isn't warped (this is cold). I can get a further contribution of about 0.04mm by moving the carriage about, so I'm pretty sure this is the X/Y axes flexing.

    Is this a normal amount? Has anyone else measured (or noticed) this?

    If it's not normal, what do I need to look at?
     
  2. Jasons_BigBox

    Jasons_BigBox Well-Known Member

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    I've thought about these types of issues for a while. I think you can expect that there will be some variation across the bed. Any combination of distortion in the glass, the heated plate pushing on the glass, the z-axis bearings not being level plus any sagging of the x and y-axis rods will inevitably lead to changes in the nozzle position relative to the glass face.

    If you can use the Mesh Bed Levelling (MBL) and do it with the bed at temperature then any variations should be effectively removed. It's worth trying a few times to get the feel for the how much load is on the strip of paper when you do it. It's worth writing down the measurements at each point too, you should get reasonable consistency but I wouldn't expect the exact same measure each time.

    Give it a go!

    Jason
     
  3. pSimon

    pSimon Member

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    I hadn't expected total flatness, but I have to admit that the magnitude I'm measuring is a bit larger than I expected.

    I'm on stock firmware - to get mesh levelling, I need different firmware, don't I? (My machine is a dual hybrid) Which is the suggested firmware, at the moment?

    Also makes me wonder if a smaller stepper on the direct extruder might help. I seem to recall seeing a smaller "pancake" motor being used for the direct extruder somewhere. Do these have adequate torque for the extruder?
     
  4. pSimon

    pSimon Member

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  5. Miasmictruth

    Miasmictruth Well-Known Member

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    Yeah I have the same issue, I put a really nice straight edge on the glass and can verify that it is indeed flat, which means the sag is definitely in the xy assembly.

    I think it could be partly due to the assembly as I and I am sure probably you tightened everything down without any support to the xy assembly, I am somewhat doubting the rods themselves are sagging that much but I could be wrong.

    I bought some 123 blocks and if I get time I plan to loosen up the the assemblies rest them on the blocks then tighten everything back up and see if there is still sag, just haven't had the time.
     
  6. Alex9779

    Alex9779 Moderator
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  7. pSimon

    pSimon Member

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  8. pSimon

    pSimon Member

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    New firmware in.

    Mesh levelling numbers seem to match the ones I get by measuring, and having finally got the PLA to stick, printing in progress.

    One minor problem though; using Alex's Simplify 3D profile, [right extruder only], the head doesn't home properly in the ooze dump bucket - it whacks into the side.

    Is there a patched version somewhere (or did I just get the wrong profile?) The one I got was about 3 months old.

    This one: https://github.com/Alex9779/BigBox-Settings/tree/master/Hybrid-Dual/Simplify3D

    I imagine I can figure out the gcode changes, but re-inventing the wheel is pointless.
     
  9. Alex9779

    Alex9779 Moderator
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  10. pSimon

    pSimon Member

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    Thanks muchly.

    Sterling work with the firmware!
     
  11. Digi2life

    Digi2life Member

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    I'm seeing the same issue. I put a dial indicator on the end effector and noted a rise of about .25mm/.010" toward the center of the bed, which I also think is sag on the XY carriage.

    Is this normal and deemed acceptable?
    Auto level only senses the corner heights, so the correction makes the nozzles hit near the center of the bed and too far away at the corners. Is that correct?
    How does mesh leveling help?
     
  12. Digi2life

    Digi2life Member

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    @pSimon, I measured this much sag with the pancake motor on the direct extruder. But, the dial indicator did add some weight. I have a test indicator coming in the mail which is much lighter. I'll post back when I have some firm numbers.
     
  13. Jasons_BigBox

    Jasons_BigBox Well-Known Member

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    Mesh levelling measures nine points, the corners, mid points of each side and the centre of the bed.
     
  14. pSimon

    pSimon Member

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    I can report that mesh levelling does help a lot.

    Not ideal, to use software to compensate for a hardware shortcoming, IMO, but short of a major re-design, it will do.

    Thanks for the confirmation, though.
     
  15. Alex9779

    Alex9779 Moderator
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    A rod will always bend if you fix it on the ends and apply a force in the middle.
    The matter is how much and the question is if that has an influence on the result or the job you want to do with it...
    I must admit that the 8mm rods may be "just enough" and a little stronger ones would limit the deformation to a range it won't matter for us.
    Maybe if the deformation is under 0.02mm it would be enough for most of the layer heights we use (0.2 is my favourite, sometimes I printed in 0.1 especially with my UM2).
    But if you go lower than 0.1 like 0.06 or someone here tried 0.04 then 0.02 will be again 50%...
    The thing I learned in engineering is that you have to deal with tolerances and deformation, you cannot get rid of it or your dimensions and costs will explode and you will still have them. Also other problems may arise if you just make everything stronger.
    So I think some kind of measuring the error and compensating that error with other techniques is inevitable...
     
    Jasons_BigBox and Miasmictruth like this.
  16. pSimon

    pSimon Member

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    As you say, 8mm rods are probably "just enough" ~0.15mm is a fine(ish) layer height all on its own, though, so without compensation, prints would be a whole lot worse. Certainly MBL made all the difference for my machine - I was having a lot of trouble with the first layer - far more than I ever had with my old RepRapPro.

    (If you have a travel gauge fitted, it's quite alarming to see how little pressure it takes to get a deflection though)

    I'd be happier with better stiffness, but I'm not wildly upset (not enough to go to the trouble of trying to fix it, for instance :) ). Engineering is about acceptable compromises, after all. (I am an engineer, as it happens - just not a mechanical type.)

    Knowing my machine is typical in this regards, and having a means of compensation is fine - I was just a little concerned initially that I'd got something wrong in the assembly.
     
  17. mike01hu

    mike01hu Well-Known Member

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    I agree with @Alex9779 that the rods are just good enough but the weakness in this system really comes from the width of the bushes. A distributed load on the rods rather than a almost point load by one bush will cause less deflection. Using two short bushes per rod separated to fit across the width of the carriage will decrease the deflection substantially. Maybe a better mechanical Engineer than me could do the math or even model it in SolidWorks to show the deflection.
     
  18. Miasmictruth

    Miasmictruth Well-Known Member

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    I was thinking about maybe someday converting the XY system to makerslide or something though weight wise. Not sure how that would work out.

    Unless I am crazy I was thinking aluminum extrusion would be stiffer.
     
  19. pSimon

    pSimon Member

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    Admit I had the same thought :)

    I have a few lengths of V-Slot kicking about that I bought to experiment with. Not sure how durable the track is long-term though...
     
  20. Miasmictruth

    Miasmictruth Well-Known Member

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    They use it on the xcarve right? I would think it should be pretty good, but that of coarse is simply assumption. One argument against it is that as a cnc it doesn't move as fast or as much as a 3d printer I would think.
     

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