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Z layer wobble

Posted by ikilledkenny 
Z layer wobble
September 04, 2012 10:05AM
I am having some Z layer wobble issues that have just started recently.
I have a Prusa printer and started to notice ridges forming in the Z direction, but only on one half of the print. So far I have removed the second M8 and spring between then on the X ends, as well as installed this screw isolation [www.thingiverse.com] but with no improvement.
I have attached a recent print. It seems that the wobble or squish is more on the rear half of the printer then the front half.

Does anyone have any suggestions? I am tempted to try ScribbleJ's isolation method [www.thingiverse.com]
Attachments:
open | download - 20120904_064010.jpg (161.8 KB)
open | download - 20120904_064028.jpg (155.4 KB)
Re: Z layer wobble
September 04, 2012 10:40AM
reduce your flow rate by 1-5% wobble effect should minimize. it looks like wobble, but to me it looks like it could be the filament is being swished wider than the nozzle surface. if anything reducing flow rate will confirm if it is really wobble.
Re: Z layer wobble
September 04, 2012 11:10AM
jamesdanielv Wrote:
-------------------------------------------------------
> reduce your flow rate by 1-5% wobble effect should
> minimize. it looks like wobble, but to me it
> looks like it could be the filament is being
> swished wider than the nozzle surface. if anything
> reducing flow rate will confirm if it is really
> wobble.

By flow rate do you mean extrusion multiplier (using slic3r)? If so I have altered the flow rate from 100% to 88% (12% reduction), but have the same result.
Re: Z layer wobble
September 04, 2012 12:33PM
If that's wobble, it should be visible during long Z axis moves. Try that and see. If you can't see the X ends wobble during a Z move, look into making sure your flow rate is correct, (as already suggested) and also make sure your Z layer height is an even multiple of your Z axis step (mm per whole step). If you don't have this, your steppers may not be producing the correct layer heights. The error will run in cycles, resulting in a banding effect that look like Z axis wobble. Another way to discriminate between the two problems is to see if the ridges follow a helix around the part (wobble) or if they simply go around at the same layer (Z error). Z wobble will also match the pitch of your screws.

Someone around here made a calculator for computing preferred layer heights based on screw pitch and step/rev. If you can't find it, that's OK. The calculation isn't very difficult.
Re: Z layer wobble
September 04, 2012 03:10PM
Z steps are set to a nice round number 2560 (I believe this is the calculator you where talking about [calculator.josefprusa.cz]) I have seen some wobble during long Z moves (0 to 100 or 100 to 0). There the ridges to repeat themselves about every 6 layers or so. This would match up with my .2 layer height and .8 x 1.25 threaded rod.

Any suggestions on how to remove/reduce the wobble?
Re: Z layer wobble
September 04, 2012 05:04PM
is it possible z axis is slipping. can you turn down z axis feed rate, just again to test? also are all your frame nuts tight?
Re: Z layer wobble
September 05, 2012 09:12AM
Update:
Played around with both speed rate as well as the current limiting pot. I noticed no change, but what I did notice was that once per rev the lead screws would bind. I confirmed this by sound (during long Z movements the sound would very about once per rev) as well as visual (during long Z movements the carriage would slow down once per rev). Conclusion - the threaded rods are bent to a degree preventing full Z movement once per rev. Next step will be to see how I can keep the rods inline.

Thanks everyone for their help.
Re: Z layer wobble
September 05, 2012 03:35PM
Try greasing your z-rods. If that doesn't work, loosen up the motor screws a bit.
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