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cbrinker

UM1 - Backlash still even w/a direct drive

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I need some help identifying the source of my constant backlash issues.

I have had my UM1 for over 2 years now, in that time I have had persistent issues with backlash that results in shells that consistently don't touch infills. I have found various workarounds for this issue to generally get good results out of the machine (hotter extrusion, more material, overlap etc) but these techniques are fiddly and cause other problems like dimensional accuracy and I would like to be done with them.

After a few interactions on the mailing list, and being told it was probably my short belts, I took drastic measures to make sure that it wasn't: I removed them entirely and made my machine direct driven. 2 new shafts, 2 flexible couplers and 2 printed motor mounts, and I have a much more straightforward machine. Even after all of that, I am still to this day seeing backlash issues.

A clue I recently uncovered was that the X motor runs _much_ hotter than the Y motor. For example, 80minutes into the current print job, I am seeing X motor ~= 118F vs Y motor ~= 95F. What would cause one dimension to require significantly more energy to print? Was there a bad batch of UM1s that had axis alignment issues that could be the fundamental problem with this unit? Anyone have any similar experiences with backlash and direct drives?

Thanks,

Chris

 

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First be aware that only one axis has to have backlash in order to get gaps between infill and border. This post gives you some ideas of why (see post #7 and #8):

http://umforum.ultimaker.com/index.php?/topic/1872-some-calibration-photographs/?p=14474

Backlash can be caused by too much friction or too much play. Too much friction because there is still quite a bit of pulling in the belts when the head grinds to a halt and changes direction without getting to the goal. Too much play causes it because the motors are getting to the right spot but not the nozzle.

Turn off power and test for too much friction. Push the head around with your fingers. It could be your long belts are too tight or more likely your end caps that hold the rods are pushing too hard against those rods and they are having trouble spinning. Consider loosening a few if friction seems high. You should also oil all the rods (6 rods in the top area and 2 vertical Z rods) with a light petroleum oil hopefully with very few additives (WD40 will work but I recommend you find something without powerful additives). Just a light oil. Even baby oil is probably better than WD40.

Check for play. Hold your coupler still if possible and push on the head and also push on the nozzle itself. Sometimes the nozzle is loose in the head. Sometimes one of the 2 linear bearings in the head doesn't fit right and there is a bit of play. The amount of play you are looking for should be visible and you should be able to feel it. It's the same amount of gap you are seeing on your infill-to-wall-gap.

It could be something as simple as the head being too loosely put together and need some tightening down.

It could be loose long belts. It could be many many things but it shouldn't be too hard to find.

 

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My understanding is that stepper motor temp isn't very representative of the energy it's consuming while printing in a traditional M=AV sort of way, but more of the amount of current being used as holding force in order to make sure it isn't skipping steps when trying to move at all. Which is to say I suspect your motor is just hotter because the stepper driver potentiometer for your X axis is simply tuned for a higher current than the one for the Y axis.

Any uneven amount of friction in X vs Y is usually pretty easily diagnosed by moving the printhead by hand when the machine is off.

Oiling axes works well but it's also short lived solution in my experience - whether you use great or mediocre lubricants, it eventually ends up as sticky gunk on your axes after a few months, either because of collecting dust or because of hardening itself. I've now washed everything in acetone and live with the slightly higher but rarely problematic friction of dry axes and bushings, just letting the bronze and polished steel do their thing. Lubrication can be a good diagnostic tool for isolating the source of gantry resistance, though - lubricate one motion point at a time until you find the problem spot.

 

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