Invisible layer lines! How is it possible? (Page 1 of 2)

Looks like the object goes from almost square at the bottom to round at the top.

LIke Carl said, likely mechanical in that the motion components of your machine have some play that results in ringing when the machine vibrates.

The square shape at the lower layer requires more drastic directional changes resulting in vibration and ringing. As you go up ,the shape gets rounder and there are less and less jarring directional changes resulting in less vibration and ringing.

Thanks to everyone! I think you are extremely right.
I added a sort of guide on the top of the z-crew and results are better but not the best: I can still see some z-wobble.

Observing the printer I noticed that:
1. X-axes (where the extruder is mount) is not perfectly perpendicular to the bed
2. when the z-screw rotates the coupler has a not perfectly horizontal rotation

At the moment I don't have the right screwdriver to calibrate the bed, I will buy it tomorrow.

Do you think z-wobble is due to a bad bed calibration or it is a bad alignment of the z-coupler (or even worst a bad alignment of the z-screw motor.. I really hope not)?

Thank you all for your support

Thank you @heartless

The printer is 4 years old, but it is almost new with an extruder total time of 11 hours, and the z screw is well lubricated as well as all the rails. Belts are tight. Extruder runs smoothly.

I noticed the issue lately, printing cylindrical shapes, but looking at the old prints it seems the trouble is not new so I suppose the printer came out of the factory in this way.

I checked the alignment of the bed and it is totally horizontal (unfortunately!).
So I think the issue is due to the coupler or the z screw. Is it correct?

As I said the coupler spins in a not perfect way, so I will try to fix it. Is there some tips or trick I should know about this?

Talking about the z screw and the possibility of replacing it, are they standard?

the bed being perfectly horizontal has absolutely nothing to do with what is going on..

if the bed moves in that horizontal plane, when it should not, THAT is where the problem lies and that is what you need to correct

no, not all printers will have the same type or size of Z screw. there is no such thing as a "set standard" for 3d printers..
some use actual threaded rod, others use what are called "lead screws".. 5mm threaded rod is not uncommon, so it should not be that difficult to find.

here is one offering.. not sure how long it is, but I bet if you emailed them, they would tell you - chances are you would have to cut to the correct length, regardless.
https://folgertech.com/collections/line … i3-5mm-0-8

Moreover, if I know my screw is 5mm of diameter, and the maximum resolution is 0,001mm, the screw pitch has to be fixed, the same of other screws, right? I mean, there are thousand printers with a z screw of D 5mm and a resolution of 0,001mm, so there must be a sort of standard given by mathematics... Or not?

honestly, i would not recommend buying "cheap" printers to anyone, you get what you pay for.

this is a lead screw - has special "nuts"...

this is a threaded rod - takes regular nuts...

Perfect! Mine is threaded rod. So, if I know the diameter is 5mm and max resolution 0.001, what screw pitch is mine?

It is not so bent, do you think with a flexible coupler I can mitigate the trouble?

a well nut is not going to fix a bent rod, Tin.

5mm threaded rods are typically available with only one thread pitch.. the one I linked to earlier should work just fine.

no, a flexible coupler will not fix the wobble caused by a bent screw.

5mm threaded rod, 0.8 thread pitch should get you back in business.

This printer is driving me crazy hahahaha

I replaced threaded rod with a straight one. Vendor didn't know if the pitch was 1mm or 0.8mm but the rod perfectly fitted the nut. I mounted it but the results ware worst than before because it is a fully threaded rod and it is impossible to mount it perfectly vertical because of screws on the coupler push and pull it badly.

So I mounted again the old thread rod (that is not so bent) because it has a smoothed bottom part. I tried to adjust the vertical position with the screw on the coupler, screwing and little unscrewing those trying to find the right position, but it is almost impossible because of the tension caused by the screws in the coupler that push and pull the rod.

So I think the trouble is the coupler!

How can I solve this problem? Flexible coupler is a good idea?

Thank you for your time :)