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I've built a a PrintrBot inspired printer with a moving printbed for the X-Axis movement. It's controlled by a RAMPS board running Marlin firmware. And there is a problem with the X-Axis positioning. Four S-Shaped blocks

As can be seen in the image the X-Axis is not really stable. The blocks should be 10x10x10 mm, and the top and bottom are perfect. The waving does not appear in the Y-direction, it's only in the direction of the bed movement.
Each block was printed at different speed, from medium to dead-slow. I limited the acceleration and in the last block I also limited the non-printing speed so the bed would never experience a higher acceleration than with the printing itself. But the wave effect remains. And it's also remarkably repeatable so it looks more like a synchronisation error somewhere. Does anyone has a clue, or experienced a similar effect ?

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    $\begingroup$ Welcome, @CeesMeijer! If you try to move the bed manually along the x-axis, does it have any slack or "deadzone"? (Deadzone would be the distance the motor has to move before the bed starts moving too.) $\endgroup$
    – Tormod Haugene
    May 18 '16 at 10:37
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    $\begingroup$ @TormodHaugene slack you've mentioned would rather cause irregularity or pseudo-regular changes but layer by layer. He mentioned that entities on the picture comes from different printouts, this suggests that these are not irregularities and also not pseudo-regularities, these are regular repeatable changes so they have to be caused by repeatable cause. What do you think? $\endgroup$
    – darth pixel
    May 18 '16 at 10:46
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    $\begingroup$ flag @TormodHaugene : There is absolutely no 'deadzone' in the bed movement. I've checked this thoroughly, because that was my first idea as well. $\endgroup$
    – Cees Meijer
    May 18 '16 at 12:46
  • $\begingroup$ @CeesMeijer - it is not the bed that is moving to erroneous positions, it is the printhead being angularly offset to the left or right. It looks like you probably have one of those 2mm spacing x 4 start screws that advances 8mm per rotation, which matches the vertical period of the error it introduces as an apparent eccentricity or bend in it puts a torque on the Z assembly that introduces this cyclic left-right error in the location of the printhead relative to the rest of the machine. If you print something very long in Y you may find the error greater at one end than the other. $\endgroup$
    – Chris Stratton
    May 18 '16 at 18:57
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This is the issue of your z-axis rather than x- or y-axis because printing speed doesn't have any influence on the results.

I bet:

  • your threded rods are bent or
  • nuts on these rods have too high clearance or
  • nuts on these rods have eccentricity or
  • couplings (between motor and threded rods) are bent or
  • these couplings are badly fitted

All above suggestions concern z-axis.

[edit] because it's PrintBot its z-axis has the following disadvantage

enter image description here

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  • $\begingroup$ None of these apply.. The way the printer was built makes it almost impossible for any eccentricity in the threaded rod (it's only one) to pass through to the printhead (you can see what I mean in if you look at the 'Printrbot simple). Also the movement is only on the X-Axis. I know this is not really clear from the picture but actually the front and back of the cube are quite smooth. $\endgroup$
    – Cees Meijer
    May 18 '16 at 12:53
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    $\begingroup$ @CeesMeijer I've looked at PB simple. Now I'm almost absolutely sure that one of my suggestions plays a role. PB simple is stiff enough in y-axis as it has 2 rods (so printer cannot swing in front-back direction) but left-right it really can. You can check it without actually printing - If I were you I'd check with the right angle (angle gauge) while moving in z-axis layer by layer. According to your picture the deviation is about +-1mm so you will be able to check it by eye. $\endgroup$
    – darth pixel
    May 18 '16 at 17:13
  • $\begingroup$ @CeesMeijer see edit $\endgroup$
    – darth pixel
    May 18 '16 at 17:32
  • $\begingroup$ You're absolutely right, and I was horribly wrong... After multiple attempts to solve the problem in by tuning the motor drive, this is exactly what I found. The motor of the Z-Axis is not correctly aligned and therefore the threaded rod makes the the movement as you illustrated. thank you very much for your help. $\endgroup$
    – Cees Meijer
    May 21 '16 at 10:07
  • $\begingroup$ @CeesMeijer you are welcome! Good to hear you've solved your problem $\endgroup$
    – darth pixel
    May 21 '16 at 12:17

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