Print is "banded" in the Z dimension

Sounds like an SF movie title?

Anyway, I have a scratch built, large platform "Cartesian" printer that creates horizontal "banding". It gets a little worse as part height rises.
I have attached a photo of a hollow "Spannerhands" rocket that really shows this up well.
I have changed acceleration, jerk, etc. to no avail. The attached photo is done "vase mode" in Simplify3D, which will have no jerk or acceleration factors because it is a continuous circle with no stops, z-hops, retractions, or anything. It was done using a .4mm nozzle and .3mm layer.

I have attached a photo of the printer too, it is a 280mm cube build space. It uses three ball-screws (not threaded rods) to move the z-axis. I strongly suspect some kind of instability in the Z axis as the source, but I am not sure what to look for. A single stepper drives the "back" ball-screw, which is attached to two other ball-screws via GT2 belt and a tension idler pulley.

If anyone has any ideas, I'd love to learn.

Thanks,
DLC

Edited 1 time(s). Last edit at 02/17/2018 02:34PM by dlc60.

Is the pitch of the banding constant, and if so does it match the pitch of the ballscrews? Or does the pitch increase at the top where the model gets narrower?

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Doesn't seem to have any smooth rods on Z... just drive screws...

Quote
dc42
Is the pitch of the banding constant, and if so does it match the pitch of the ballscrews? Or does the pitch increase at the top where the model gets narrower?

It seems very regular, whether narrow or wide. It is kind of difficult to tell, but "eye balling" it, the banding seems pretty close to the ball screw pitch.

DLC

Dust made an important observation in his reply.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

How about the connecting pulleys? Do they wobble? They look printed, maybe they fit too loose on the ballscrew shaft?
Adding some linear guides would require a lot of changes around the ballscrews too. They should be only single side supported and the ball nut should be "floating" in X&Y direction. But that's gonna be difficult with the Z-stepper at the top end. Bearings, pulleys and motor should be close together.

Thanks all, interesting observations, which I did not understand at first.
I built this beast last year from parts a friend had cached, but realized that he would never actually build the printer, so he gave them to me. The only plans I had were a step-file from the creator of the printer. He did his design about five years ago when 3D printers were starting to become the "thing". The design works, it makes dimensionally correct prints. However, it is a bear to level and was two bears to build and tune. I have forever had this Z axis shimmy so have used this printer to make large pieces whose "perfectness" was not required; things like enclosures. In other words, it is my tinker printer. I printed all of the ABS pieces for it on my Folgertech 2020 delta printer and assembled it by trial and error. I have re-meshed several of the pieces and completely redesigned the print head assembly (I call it the FrankenStruder).
It works, but...

I have never been fond of the extremely long GT2 belt connecting all of the ball screws and the the tension idler to the drive ball screw. Recently the whole thing got out of sync and locked up. I then redesigned the tension idler to support both the top and the bottom of the bolt so that it won't be side-loaded in such an unbalanced manner (my bearing gave out). I then had to re-level the bed and adjust Z zero as well. It is working again, but alas the shimmy is still there and so far as I can tell, undiminished.

It sounds like the consensus is that the bed "shimmies" because there are no smooth rods to keep it in line. The ball screws are held at the top and the bottom, the bottom has thrust bearings to keep the screws from getting too much friction due to gravity. Where would one put the smooth rods, if I was to "upgrade" the assembly? It would be an interesting redesign because I would have to have some way to keep the ball screws "upright" if I was to eliminate the top bearing.
Clues and pointers appreciated. I _love_ a challenge.

DLC

Quote
o_lampe
How about the connecting pulleys? Do they wobble? They look printed, maybe they fit too loose on the ballscrew shaft?
Adding some linear guides would require a lot of changes around the ballscrews too. They should be only single side supported and the ball nut should be "floating" in X&Y direction. But that's gonna be difficult with the Z-stepper at the top end. Bearings, pulleys and motor should be close together.

The pulleys do not wobble. The tension idler did, then the bearing failed and I create a part that holds both ends of the idler bolt, it does not wobble now, but the shimmy is still there.
The pulleys are not printed, they are McMaster Carr, they didn't have set screws in them though so I drilled and tapped to add two on each one.

Thanks,
DLC

I'd add the linear rails to the side where the top of the "T" shaped bed frame is. Then use the stepper as belt tensioner, too. The amount of belt-wrap around the driving pulley is quite small. Here's my version of 3 ballscrews/two linear rails/one stepper. But tensioning the belt requires to move the whole alu-bar.

If not Z-wobble, this can also be caused by the bed flexing while trying to hold temp.

Quote
Dirty Steve
If not Z-wobble, this can also be caused by the bed flexing while trying to hold temp.

especially if you use bang bang heat control

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Dirty Steve
If not Z-wobble, this can also be caused by the bed flexing while trying to hold temp.

That's why I started by asking whether the pitch of the banding matched the pitch of the ball screws all the way up.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Quote
Dirty Steve
If not Z-wobble, this can also be caused by the bed flexing while trying to hold temp.

This thing has a 1/4" solid piece of T6 aluminum. While it isn't perfectly flat (I use a glass plate on top of it), I kind of doubt that it vibrates from heating. I have much thinner aluminum plates on other printers that don't. And, I don't use "bang bang" heating either. It is quite slow to heat up and cool down...

Thanks,
DLC

Quote
o_lampe
I'd add the linear rails to the side where the top of the "T" shaped bed frame is. Then use the stepper as belt tensioner, too. The amount of belt-wrap around the driving pulley is quite small. Here's my version of 3 ballscrews/two linear rails/one stepper. But tensioning the belt requires to move the whole alu-bar.

[attachment 102703 ballscrews_x3.jpg]

I am looking at this possibility. It shouldn't be impossible, just pretty difficult. But hey, why not? It is my only large platform printer and I'd like to keep it doing enclosures, but better than it is doing it now. I don't see any way to move the back screw's stepper to the bottom, but I can remove the top bearings on the front two if I add the guide rods next to the screws. That may make the difference. I am going to rebuild the print head gantry anyway, so lets just bung the whole thing up!

Thanks,
DLC

Would not be from vibrating, the build plate can swell and contract from heating, causing your Z 0 to physically change while printing, causing upper layers to squash. This can happen with even a minor swing high and low around set temp.

Edited 1 time(s). Last edit at 02/27/2018 12:57PM by Dirty Steve.

Loosen the z thread blocks at the built plate frame.

I'd bet you are getting plate wobble from there if they are rigidly attached. Let them float in the horizontal plane and lets see what it does.

Quote
Dirty Steve
Would not be from vibrating, the build plate can swell and contract from heating, causing your Z 0 to physically change while printing, causing upper layers to squash. This can happen with even a minor swing high and low around set temp.

It was running bang-bang on the bed (Marlin default). I corrected that, but no change in the banding.

thanks,
DLC

Quote
orbitalair
Loosen the z thread blocks at the built plate frame.

I'd bet you are getting plate wobble from there if they are rigidly attached. Let them float in the horizontal plane and lets see what it does.

If I do that, then there is nothing keeping the whole thing running vertically. There are no guide rods on this design, only the screws. Can your idea work when there are only the three screws, no smooth rods?

thanks,
DLC

After more thought, letting the anchor block float probably would not fix the issue.

Since I think the problem is lateral motion due to the harmonics of the threaded rods. Without a guide rod, the plate will still follow the wobble of the z threaded rods.

Say, a simple test would be to let 2 z blocks float, but keep one locked down (acting as a guide). What I would expect to see would be a definite wobble pattern that is strictly from the locked z guide block, but an overall reduction of the issue.

I think you will have to replace 2 z drives with guide rails or rods.

If you rebuild it be sure to look at Tech2C corexy build on youtube. He did some interesting tests of different materials with the goal of weight reduction at the head.