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Re: Apparent Z-Banding Driving Me Crazy

Posted: Mon Aug 13, 2018 6:32 pm
by mwester
The two lead screws will move in tandem at all times, even when leveling. There's simply no mechanism to permit otherwise (there's only one motor driver, and the two motors are in parallel). So just grasp one of the screws, and gently rotate it 1/4 turn, and see if the banding is less obvious.

Re: Apparent Z-Banding Driving Me Crazy

Posted: Tue Aug 14, 2018 7:19 am
by mikehill85
mwester wrote:
Mon Aug 13, 2018 6:32 pm
The two lead screws will move in tandem at all times, even when leveling. There's simply no mechanism to permit otherwise (there's only one motor driver, and the two motors are in parallel). So just grasp one of the screws, and gently rotate it 1/4 turn, and see if the banding is less obvious.
Okay, that makes sense. For some reason I thought they moved independently but if that were the case it would change the angle of the nozzle relative to the bed. I'll give it another shot. Lulzbot support had me trying to dial in my Z-offset height. I have no idea how they think not having your Z-axis dialed in could cause recurrent surface imperfections that match your Z screw pitch.

Re: Apparent Z-Banding Driving Me Crazy

Posted: Wed Aug 15, 2018 12:28 pm
by RCBrust
Is it possible to improve banding by using layer heights that are even steps of the Z stepper motors? If, for example, you have a layer height that requires 10.5 steps, the layer height will sometimes be a little shy and other times be a little large as the errors accumulate. I can see this producing banding.

Does anyone know what layer heights to use to get even steps?

Randy

Re: Apparent Z-Banding Driving Me Crazy

Posted: Wed Aug 15, 2018 7:03 pm
by mikehill85
RCBrust wrote:
Wed Aug 15, 2018 12:28 pm
Is it possible to improve banding by using layer heights that are even steps of the Z stepper motors? If, for example, you have a layer height that requires 10.5 steps, the layer height will sometimes be a little shy and other times be a little large as the errors accumulate. I can see this producing banding.

Does anyone know what layer heights to use to get even steps?

Randy
For the mini the screw pitch is 2mm. The motors are 200 steps per rotation and we are using 1/16 microstepping. Therefore the steps per mm is (16*200/2mm) = 1600 steps/mm. So there are 1600 steps per mm.

In other words, Steps = Layer Height x 1600. As an example, 160 steps = 1600 x 0.1 or 320 steps = 1600 x 0.2 etc.

Personally, I think the banding is caused by a very slight bend in one of my threaded rods. I suspect it is the one furthest from the power supply since it appears that there is less protection to banging it in shipping there and the banding seems to originate on that side (i.e. the banding is deepest on that side).

Re: Apparent Z-Banding Driving Me Crazy

Posted: Thu Aug 16, 2018 4:07 pm
by mikehill85
RCBrust wrote:
Fri Aug 10, 2018 1:16 pm
I get the exact same thing. If you get anywhere with this issue, please let us know.

Randy
I'm finally making some progress. I was totally incorrect about adjusting one of the Z axis screws not doing anything. I gave one of the silver couplers ~a 1/16th turn with the motors turned off and went from the 1st photo to the 2nd photo.
IMG_1414.jpg
IMG_1415.jpg

Re: Apparent Z-Banding Driving Me Crazy

Posted: Thu Aug 16, 2018 7:05 pm
by kcchen_00
Glad thats helping. Just be careful not to skew the gantry too much. It could bind at taller heights.

Re: Apparent Z-Banding Driving Me Crazy

Posted: Fri Aug 17, 2018 7:56 am
by mikehill85
kcchen_00 wrote:
Thu Aug 16, 2018 7:05 pm
Glad thats helping. Just be careful not to skew the gantry too much. It could bind at taller heights.
Yeah. Thanks for the tip! I haven't had much luck getting it any better than shown above. It isn't terrible though and only really noticeable if go out of your way to look for it. That said, do you know if there is any theoretical sweet spot for the rotation of the screw relative to each other. I was thinking maybe 180 degrees "out of phase" in terms of thread orientation would be ideal. I tried that and it was worse than the second photo. Any thoughts?

Also, do you know the best way to test if your lead screw is bent? I have a sneaking suspicion that lead screw on the side furthest from the power supply is slightly bent, since the banding is at its worst on/seems to originate from this side. Also, this side isn't as protected and could easily have been subjected to damage during shipping. Honestly, I'd be surprised if Purolator didn't destroy any precision instrument that it acted as a carrier for.

Any thoughts on whether I'm chasing ghosts trying to get better quality than shown in the second photo above? Could this simply be the limit of the precision of lead screws? Since no lead screw is perfectly straight, there will always be slight wobble.

Re: Apparent Z-Banding Driving Me Crazy

Posted: Fri Aug 17, 2018 6:44 pm
by kcchen_00
To be honest, I think you've got it pretty close to perfect. At one point, I thought a heavier lubricant might stabilize the travel even further. But decided not to try anything too drastic.

Somewhere along the research path, I read that all lead screws have a slight bend due to manufacturing. After reading that, it really did seem like chasing ghosts.

Testing for a bend... I would resort to the "pool stick" test, lay it on a flat surface and roll it. A bend should be fairly apparent. Of course, that would mean a lot of disassembly. Unfortunately no ideas how to test on-machine.

Ultimately, post processing and finer layers would be the only way to get smooth "walls". Personally I don't mind the slight banding... on my machine its only apparent in certain lighting on curved surfaces. A SLS machine would be the only other way to eliminate the banding.