Help diagnose Z-banding

Question

The printer is "Monoprice MP Select Mini 3D Printer V2".

This is how my printed fan part looks like:

These printers are notorious for their Z lead screw assembly (M4 bolt & nut) so I installed flex coupler and aligned the M4 bolt and motor shaft so that the wobble is minimal. It is still there, but I don't think this amount of wobble would produce something like what I experience. Please check out the timelapse video at the end.

I also tightened up Z guide rails (no rattle, very little slop)

Moreover the banding appears to be less pronounced and have alot greater period for vase prints

(this is extruded ellipse printed along Y axis). I tried:

1. slowing down print by 50 % - no effect on banding.

2. monitored hotend temperature with external probe on the heater block (-+5 °C)

The banding appears to be almost exclusively in Y axis direction; for example, the same ellipse printed along X axis:

Now, same ellipse in layered mode (seam side) printed along Y axis

Answer 1

This differs from the traditional banding as observed from Z wobble induced banding as e.g. explained in this answer.

Your banding patterns clearly seem to form diagonal bands, this is most probably a combination of the lead and the full rotation of the stepper. The most logical explanation is that the layer shifts as a whole in X-Y direction (when seen against the print height, this movement is concentric seen from the top). This means that the next layer is positioned over the previous layer in a concentric pattern. This hints to some sort of defect in your X/Y-plane assembly and should be investigated further.

This is difficult to visualize, but this sketch shows the issue for some layers:

This could be related to the belts of the X and Y-axis, play on the drive pulleys, non-straight lead screw, guide rods with play, play in general, Z-stepper alignment to the threaded rod, etc. Considering the amount of Z-wobble fixes shared by the unofficial MP Select Wiki, the best place to look for is the Z-stepper to lead screw coupling.