Improving quality

Hi,

I just started out with 3D printing two weeks ago and I'm currently in the phase of improving my print quality.
The machine I use is a desktop CNC router, so it's bulky and causes some vibrations when moving quickly,

To tackle that problem my acceleration settings are set to 500mm/s^2. I still can see some vibrations if I put a glass of water next to the machine, but it got much better

The two problems I'm tackling now are shown in the attached photo. One is a what seems a small constant layer shifts. The two marvin figures both have some imperfections
which occur on the exact same layer. G-Code seems ok (can't attach it is to big)
From reading I'd say that is Z-wobbling or a similar mechanical problem. The Z-Axis uses a regular M8-screw threaded rod so I expected that to some degree. I've already ordered a proper trapezoidal threaded rod.
Are there other issues that could be causing this, or that could amplify the problem? Is there a way to verify my assumption before the new rod arrives?

The second problem only happens after the printer continues to print after a retraction. It doesn't seem to complete the full outer perimeter. What could be causing this problem?

I'd be happy for any advice or suggestions.

Could it be banding or backlash in X-Y direction, too?

I checked and found a loose pulley on both, the X and Y axis. I then went ahead and retightened the belts and the prints are way better now.
Both issues are gone. Thanks!

I've attached more recent prints (Owl, that tall S-shape and the marvin on the right, the one left is for reference). The two figurines look fine.

Beside the prominent z-banding the S-shape has some shift issues. Two were printed at the same time, both have this shifts at the same height.
The figurines are fine there. What could be causing the shifts? Could it still be vibrations?

How does your CNC router guide the z-axis? Is there any debris?
When you replace the M8 screw, you'd inspect everything carefully.

Nice improvement so far!

Edited 1 time(s). Last edit at 09/23/2017 02:12AM by o_lampe.

Again the pulley on the X-axis came loose, so that was backlash again... That's a dual driven belt axis might be sync problems there.
I just printed again and watched the side where the pulley was loose and noticed it skips steps.

Instead of fully moving the motor seems to tension the belt a bit.
I.e. when it tries to move, the part of the belt on the left side of the pulley becomes loose,
while the belt part on the right becomes very tight. It even started grinding the belt a little...

Now I'm not sure if that's an electrical or mechanical problem. But I guess that's my main issue.

I will try to use a separate driver for the two motors, so I hope to rule out an electrical problem,
but I need to wait for a new driver to arrive.

Dual driven means two motors for one belt? If so, you could try to run the printer without belt ( or loose pulleys ) to see if both motors are actually running.
It sounds like one motor has to do all the work...

Edited 1 time(s). Last edit at 09/24/2017 01:18PM by o_lampe.

Dual driven in my case means two belts, two motors. I removed the belt on the problem side to see if the motor was turning and noticed it was skipping steps.

Somehow grease from the rotor bearing creeped under the pulley that was driving the belt. That's why it was getting loose (the rotor does not have a flat part).
The more critical problem, however was that the pulley gradually moved toward the motor and at some point ended up pressing against it. That's the reason
it skipped steps.

I created a different topic over at the manufacturers forums, and hope someone there can help me to prevent this from happening again.

Thanks a lot for your help!