Is this "Z-Wobble"

Here are 3 Prints I've done, there seems to be a bit of a line-i-ness to the prints. Is this from Z-Wobble? What can I do to improve the prints? I also lubricated my rods prior to the print. Machine is a RepRap MendelMax with a Budashnozzle 2.0 hot end, .35mm nozzle, printing at .2mm layer height, 3mm filament from UltiMachine.



Edited 1 time(s). Last edit at 09/02/2013 07:16PM by isonoob.

That looks like a definite case of z-wobble. Measure the distance between bands in your prints, I bet it matches the pitch of your z lead screws. Do you have the z lead screws constrained on both ends? Also, what diameter are your z lead screws and z smooth rods?

I'm using 1/4" Precision Leadscrews w/nuts and springs, for the green print I had it constrained at the top with zip ties to try and straighten the rod, but for the red prints i took it off. both z-rods are using flexible couplings on the bottom to connect to the steppers. the smooth rods are 10mm

I am presently encountering z wobble on a printer I am building. I am using 10mm lead screws with shaft turned down to 6.35mm (0.25 inches) and I am using 5mm to 6.35 mm aluminum helical couplers at the bottom of the shaft.

The issue I am seeing is due to the fact that the set screws of the couplers push the couplers off center. I can see them wobbling, and if I move the axis over a great distance, I can watch the x end wobble.

I am waiting for delivery of spider couplers from musumi. Instead of setscrews, they clamp onto the rods. I'm hoping this, together with the fact that the spider design decouples the two halves, makes a difference.

The point of this post is to emphasize that the alignment of the couplers is not only very important, but also easily overlooked

Here's a very interesting article on Z axis artifacts written by whosawhatsis.

Taxonomy of Z axis artifacts in extrusion-based 3d printing

But I would lie if I said I knew what to do after reading that... I'm still very much a newbie.

If the set screws push the couplers off centre then the bore is too big.

For example pulleys have H7 tolerance holes and motors have h7 shafts. The maximum the set screw can offset it would be 24um if the hole was top tolerance and the shaft bottom. Couplers need to be made to the same sort of precision. Is there any tolerance specification for them.

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[www.hydraraptor.blogspot.com]

hmm interesting read and nophead, i dont have any specifications for them unfortunately. they came with my 1/4" precision rods to be attached to the stepper. They are the metal flexible couplers, do you think switching them out for a different type would help the issue?

Yes - I agree that the bore might be too big, but I also think the problem is exacerbated by the fact that the motor shaft has a flat ground into it. That being said, though, I was careful to make sure the set screw was perpendicular to the flat. Still had wobble.

As far as specs on the couplers - I don't have any. They came from trinity labs if you want to look at them.

The flat shouldn't matter as long as the first grub screw is tightened onto it. Might be worth asking Trinity labs for a spec and getting them replaced if they are out of tolerance.

Are you sure your motor shafts are 5mm? Some US motors have imperial shafts which are only about 4.75mm.

I personally don't think those type of couplers are right for the job. I am currently experimenting with different types of printed couplers to cope with motors that have flats on the shafts.

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[www.hydraraptor.blogspot.com]

Hey,

I had a very similar issue. In my case (not a mendelmax), the z motor mount was not exactly parallel to the z-axis (only slightly off). The flexible coupler (clamp style) compensated for the misalignment, however, there was no additional bearing on the motor side and i got z-wobble. Improving my z-motor mount completely solved the issues.

Victor

According to lulzbot, the motor shaft is 5mm.

I agree that these are not the best couplers. I have ordered and can still awaiting a set of misumi split spider couplers. They are due to ship today so hopefully I should get them in a few days. As I mentioned before, they clamp onto the shaft instead of using a setscrew. Hopefully this will allow them to attach with better accuracy - to both the motor shaft and the lead screw. Then the spider in between the two halves should allow for some misalignment. I hope to have constraints on both the top and bottom of the lead screw (they have 8mm turndowns meant to fit into a skate bearing) so that they will be exactly where I need them, and then connected below the bottom constraint with the spider coupler to the motor.

I don't see how split couplings will work well on a shaft with a flat. That is the very problem I am trying to fix with these couplings which were designed for round shafts but the motors now come with flats. A set screw should be more accurate as long as the bore is accurate.

You don't want to constrain both ends of the lead screw although with a spider coupling I think you will need a bottom bearing. Constraining the top causes any eccentricity to fight with the smooth rod. Better to allow the top to wobble if it wants to.

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[www.hydraraptor.blogspot.com]

I swapped out my helical aluminum couplers for printed 2 piece solid couplers, z-wobble resolved for my machine.

Do you have a link for your couplers?

Mine are similar to nophead's couplers on thingiverse, [www.thingiverse.com], but mine are sized for a 5mm motor shaft and a 5/16" threaded rod. They bolt together with M3 screws and nuts.

Also posted on thingiverse. [www.thingiverse.com]

Edited 3 time(s). Last edit at 09/08/2013 07:00PM by Dirty Steve.