Small "z-banding" and over-extruded first layer

[glx 26]

I printed some of these parts and noticed that there are very small layer variations on the surface.

To be honest this is whining on a pretty high level, the variations can barely be felt but are visible in certain light conditions, but I'm curious.

 

The parts are printed on an S5 with the "Visual Fine - 0.1mm" preset with Ultimaker PLA White.

 

Where do these variations come from? Variations in the filament-diameter?

 

I noticed another thing, the first layer is always over-extruded, resulting in a slightly too large first layer and visibly overlapping lines on the bottom. If I remember it on time I compensate that by setting the flow of the first layer to 90%, then everything is fine. Is the first layer over-extruded on purpose or is that some inaccuracy of the active leveling? That happens regardless of the material or profiles. I already had that same thing on my UM3, so I'm somehow used to it. The engineering profiles somehow seems to compensate that by setting the initial layer horizontal expansion to -0.2mm.

 

[libertyhouseplans 2]

I get similar Z banding on my models. It improved a lot after I tightened the pulleys (short and long), but it's still visible, though — on par with your photo.

 

There's a little forward-backward play in the right-side axle, and I aim to try that next. Is there any play in your axles? 

https://support.makerbot.com/s/article/1667417951326