Strange lines in prints and strange printer movements

[subgenius13]

Good morning all,
I have been having this strange behavior with my Simple Metal. I've been printing with it for almost 3 years now and I have never had this one. I don't recall making any setting changes to my slicer program, and I have tried Cura and Slic3r. I haven't changed firmware that I recall at this point. I am getting strange lines and gaps in my prints. When I observe the print motion it looks like the Z-Axis is picking up and coming back down and not moving in a continuous fashion. You know how you recognize the sound your printer makes, I'm not getting that. It's probably some setting that I have overlooked, but has anyone seen this?

[ktfergus]

When you manually raise and lower the z axis do you get the same behavior? The lines look like they repeat at regular intervals, so something physical might be slipping/interfering at a given z height. Go ahead & check the z coupler, make sure it's not slipping on the z motor shaft or the threaded rod. I've had my Simple Metal for roughly the same time & I've noticed the linear bearings on the z axis are getting a little worn out; after a tall print, when the hotend lowers The z axis jumps down in a few spots. However, on the way up it's still smooth. Wouldn't be a bad idea to lube the two upright rods. If all else fails, raising the current to the z motor might solve the problem, too.
-Kelly

[subgenius13]

20180403_001015.jpgI wanted to give an update on this and see if there may be more suggestions out there for other things to try. Here is the latest test prints:

It was kind of a long road to get back to this state. I replaced the Extruder stepper and when I got everything put back in its place it was severely underextruding. This took me down a rat hole of firmware, but I what I ended up settling on was this one found with the adapter on Thingiverse
https://www.thingiverse.com/thing:559657
While I had everything apart I did a couple things:
Replaced the y-Axis belt and flipped my Z-Axis threaded rod to see if there might be a road less traveled there.
Ultimately, I got back to where I started. Same lines no matter what infill pattern I use or what slicer I use to generate the gcode. I even went back and tried my old version of Cura to see if that mattered. It really all points to something mechanical I think. Then that begs the question of if it's worth replacing all the steppers and bearings.

Any suggestions would definitely be welcome.

Thanks!

[RetireeJay]

Are the blue sample and the gray sample oriented the same way, i.e. is the base at the right in both examples? And are both of these samples taken after all your rebuilding work?

I ask because although the pattern seems to repeat every 10 layers, it the samples are oriented the same way then the first skip in the gray sample is after layer 4 whereas the first skip in the blue sample is after layer 10. If both samples are oriented the same way, then this tells us that the "repeatable" pattern of a glitch every 10 layers has a non-repeatable initiation. That, in turn, seems to suggest that the problem is not a mechanical interference with the vertical motion in Z. However, it could be related to rotation of the Z screw.

Does your bed have more slope to it than it used to, causing the Z axis to move more and more to compensate (I'm assuming the Simple has a probe and does "bed leveling".)?

It might be worthwhile to check the coupling between the Z motor and your Z screw. Are the grub screws or other fastening components firmly seated, with the motor's coupling seated against the flat on the motor shaft? Is there anything interfering with smooth rotation of the coupling through a full 360 degrees of turning, like friction against a stationary object or a projection that runs into a stationary object? There should be at least a little clearance between the coupling and the motor body.

What kind of thread is on the Z screws on your Simple Metal? Have you changed (or verified) the steps per millimeter? Does the total height of your test pieces match what you specified in the model?