Z Axis Couplers on old Plus

Talk about modifications to the printer

Z Axis Couplers on old Plus

Postby RetireeJay » 2018-Feb-Mon-20-Feb

This may be applicable to other Printrbots from the 2012 - 2013 era.

I have a new Prusa MK3 on order, and in order to prepare to make a comparison with my existing Plus, I decided to print a Benchy. One thing that I noticed was that in some parts of the Benchy there were irregularities along the Z axis - where a wall or corner would be wavy in the Z direction instead of smooth. I played around with "the usual suspect" - namely the number of microsteps or full steps per layer in Z, but making these changed did not improve the appearance of the result at all.

Upon closer examination, I realized that the variations matched the pitch of my Z screw exactly (18 turns per inch). So this made me wonder if my couplers from the Z motors to the threaded rods were at fault, like not perfectly concentric or somehow causing slight unintended vertical motion superimposed on the desired motion. Using my dial caliper, I carefully measured the Z motion every 0.1mm for 3mm and compared the actual size of each step to the theoretical size of each step. I found that there was, indeed, a once-per-revolution pattern of going above and below the ideal step size.

So I designed and printed new Z axis couplers.

I was surprised when I disassembled the original couplers to see how the vinyl sleeve on the motor shaft was such a poor choice to establish good concentricity.
OriginalZ_Coupler.JPG

OriginalSleeveOnMotorShaft.JPG

NewZCoupler_1.JPG

NewZCoupler_2.JPG


After the upgrade, the standard deviation of the motion in the Z axis was roughly cut in half.
PrintrbotZIrregularity.JPG

And Benchy looks better, although still not as good as I'd hoped.
Benchy.JPG
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Printrbot Plus operational January 2013
Brass threaded rods (5/16" X 18) & nuts for Z axis
GT2 belts & pulleys
Cable chain to reduce probability of fatigue failure in wires
E3D V5 Hot End, 0.4mm nozzle, also 0.8 and 0.25 in use occasionally
PB fan mount + 40mm fan -- using printed mount adapter, not the E3D supplied fan
Injection molded extruder gears
Optical Z "endstop" (custom designed and built)
Have used many pounds of T-Glase filament. Now also doing some work with Ninjaflex SemiFlex
Print on glass with Scotch Craft Stick or other glue stick
"My next printer is..." Prusa i3 MK3
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Z Axis Couplers on old Plus

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Re: Z Axis Couplers on old Plus

Postby Mooselake » 2018-Feb-Wed-13-Feb

I used the so-called plum couplers, Lovejoy couplers here in the states. Mine came from eBay iirc. Prior to the purchased ones I used a pair of printed ones with a hot-glue insert (not sure if that's the exact one I used, thought I'd posted some pictures but can't find them), not sure if the fancy aluminum and red plastic version was any better.

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Modified KickStarter Classic Plus 7/2012
KS Thingybot Delta Pro 10/31/16
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