Repeating lines on a print

[tomato-icn 0]

Dear community, I've been fighting with this issue for a several days and I'm almost desperate:

the problem is - my print started to have strange repeating lines.

What I've done:

carefully checked the X-Y axes - both are orthogonal, not bent, belts are tight, long and short ones (they are all tight and nice) - moreover while filling the surface I can see everything's alright, I even tried belt tensioneer - but they only reduce quality, so I don't use them.

I also saw the wobbling Z-axis problem, so I reassembled and assembled Z-axis again, still no luck (checked the couplings and brass nut).

Any ideas please?

My prints now:

My prints were:

 

[illuminarti 18]

This is related to your z-axis wobbling more than the sliding nut in the z-stage can compensate for.

Make sure the z-screw is fully seated in the coupler, and centered. Try using Cura or pronterface to run the bed all the way up and down (I wouldn't use the Ulticontroller, if you have one, as its movement controls are a bit erratic to begin with). Move it fairly quickly, and watch what happens to the top of the z-screw, and to the nut in the z-stage, and indeed to the stage assembly itself.

I suspect you'll find that something isn't centered quite right, and the nut reaches its limit of how far it can slide to accommodate things, so that the bed itself moves back and forth slightly.

 

[tomato-icn 0]

thanks a lot for quick respond!

so the big brass nut should slide a little, please explain this moment, thank you!

I suspect also the problem is in big brass nut.

 

[illuminarti 18]

Yes, the brass nut is designed to be able to slide slightly left to right in the enclosure at the back of the z-stage. That way if your z-screw isn't quite straight and centered, the nut will move slightly sideways as it moves up and down, so that the bed itself doesn't get twisted.

The idea is that the nut is just responsible for setting the height of the bed. The other two rods keep it in the right position in x-y.

But if the screw is too far off straight, or the nut can't move, then the bed ends up being moved around as the screw rotates, and you get this banding pattern in your prints, matching the 3mm pitch of the z-screw.