ender 3 not printing concentric walls?

[jackfrost311]

Ok so here is my machine setup:

ender 3 modified

y axis linear rail setup
silent board
microswiss direct drive and all metal hot end
th3d firmware (latest)
bltouch
cura slicer (latest)

Here is my problem: Im printing cylinders and my inner and outer walls (shells) are not printing concentrically. any ideas on what could be causing this issue? Full disclosure i bought this machine used so i do not know what happened before i got it. my prints are still coming out... but my inner and outer wall thicknesses are not consistent. and the entire part could be more of an oval... hard to know.

things i plan to check (or i have checked) after this latest print.
1. check the machine to make the x and y axis are square to each other
2. re check belt tension (looks pretty good)
3. model is good and the preview looks good in cura
4. create a cylinder that i can check with a cylindrical gauge
4. ?????

there is some "stiction" in the linear rail blocks but its not awful. nothing that i would think is causing repeatable non concentric walls. i would expect the stiction to cause random discontinuities in the print surface.

any help would be very appreciated.

[xayoz]

I don't have the linear rails, but if they are sticking, would suspect the bearings need lubrication. Otherwise the best advice I could offer is to slow it down. See if you get the same issue when printing at lower speeds.

[jackfrost311]

That’s good advice. So I pulled it back apart this morning, and the rails are pretty free without the belt attached.

I did note that because of the increased height of the plate the belts are now entering the plate at a higher angle, so instead of the belts pulling in axis, they now have a downward component to them.

That additional downward force is causing more Stiction in the bearings

I created a shim to move the belt down so it was more in plane, and would reduce that vertical force. I’m also not going to the tension the belt as tightly.

I’m going to re lubricate the bearings as well, stay tuned