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Mendel wobble in z-axis of prints

Posted by Lenbok 
Re: Mendel wobble in z-axis of prints
July 11, 2012 09:24AM
Karl_Williams Wrote:
> After a few months of getting very nice prints, my
> mendel has developed the same problem. Will
> replacing the captive nuts solve this problem? It
> seems that the nuts are over constrained in the
> mendel design. Is there a way to let the z-axis
> nuts move slightly in the holder since it is the
> smooth rod that holds the axis in place?
> [www.thinkbotics.com]

Something like this:

It seemed easier to me than getting a straight threaded rod and coupler.
Z-axis wobble is caused by eccentricity of the Z threaded rods so when they rotate they produce circular movement in x-y plane, which is transferred to the print head. Therefore each layer is slightly shifted from previous. After full turn Z threaded rods are again in the starting position, completing full cycle.

Move print head some distance in Z direction (e.g. 50 mm) and look at the threaded rod while rotating. If there is a movement in x-y plane, that's it. Cause could be bend of threaded rod - needs replacement, or improper fixture - try to adjust position of the Z-axis motor.
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