Z wobble only after a certain height (around 650mm) Design problem?

Hi everyone, originally I posted in the Printing area cause I thought it was a printing issue, but it seems more like a possible design problem.

So I've just finished building a printer size of 500 x 500 x 780mm ish, and was testing it. The first issue I had was z wobble, so then I changed from alum flex coupling to jaw/spider coupling, which helped just a tiny bit. After a few more tests, the digital dentist told me I should not be using 4 smooth rods while I have 3 lead screws. So I designed a coupling for the back on (and can't use LM12LUU due to clearance and using LM12UU) and that made a huge difference, so he was right. Wobbling was greatly decreased, and I decided to properly do a bigger print instead of the small like 2cm cubes which would sway badly itself at that height. So I printed a 300 x 300 x 700mm vase, and here are some pics:

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You'll notice there is still some slight z wobble, but only happens after 650mm height ish, and the most interesting is this, you'll notice in the pictures, some side don't have the wobble, the ones that doesn't are near the front where the two front side motors are, and the one at the back are where there's slight z wobble.

After the print I've noticed that the back side some screws were not as tight as they should be so I tightened them again.

Right now this is my setup:
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three z motors, 2 of them are connected in SERIES (the side ones) two a DRV8825 stepper driver, 1 in the back connected to a DRV8825 on the E1 that's being used as the 2nd z motor. they are connected by a 1700mm belt loop to keep them in sync. I'm wondering if that's the issue?
Please let me know what could be the issue here.

I originally was gonna go 4 lead screws and 4 motors at each corners, as some people seem to do that and I have a couple of extra lead screws at that length, but some research seems to be against that idea and triangle is better.

My problem is right now I'm in a really tight budget so I can't really buy anything more, I could print stuff though but no more buying extrusions or such etc.

Thank you very much!

Edited 1 time(s). Last edit at 02/04/2019 01:11AM by iamthebest22.

Why all these whip antennas ? I like the moving ground plane with the taps to adjust the resonant f though

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"A comical prototype doesn't mean a dumb idea is possible" (Thunderf00t)

huh? I'm confused to what you are saying >.<

In my experience, a single linear bearing on each z-guide was not sufficient to prevent z-wobble. I found if I preloaded the bearing to eliminate any play in the x-y worked, but I did not find that to be ideal. I added a second bearing to each guide and the problems went away.

Edited 1 time(s). Last edit at 02/07/2019 09:23AM by obelisk79.

Thanks obelisk, I'm doing that too, changed to a longer smooth rail for the clearance cause the original length did not permit me to use a second bearing, so I changed to a longer one and changed the smooth rod lower z holder to UNDERNEATH the extrusion instead of above like before. I also decided to use one longer lml12uu (57mm) instead of two LM12UU (30mm each)I hope that works just as well.

I have also tried to use only one motor as MKSA and others have said, unfortunately it seems like one isn't enough and it can't lift it, but lowering it is very smooth, so unfortunately I have to either change to a stronger motor which I don't have the money for, so for now, I'm using the two side ones which are connected in series and that is strong enough to pull the back motor with it. So with these two changes (longer bearing) and only two motors ( I wish it could only be one but for some reason just can't handle the weight) pulling the back one, will report back when this print finishes.

The reason you are getting the Z wobblies as you go higher is because the lead screws are under compression and will bend as the height increases. They are too thin to support the weight of the bed. You have 2 in front, so they are doing a bit better, but the one in the back is carrying more weight, so it buckles more as height increases.

You can do a few things to fix it :

1. Use lead screws of a larger diameter
2. Add more lead screws
3. Suspend the lead screws from the top (unfortunately this is not easy to achieve).
4. Only print to a height of 650 mm