Help diagnose Z-banding 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 &amp; 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:


slowing down print by 50&nbsp;% - no effect on banding.   
monitored hotend temperature with external probe on the heater block (-+5&nbsp;&deg;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
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.