Ok, after reassembling the whole Z axis the following changes were done:
- Making some space for the brass nut of the Z screw to enable it to move.
- Checking the stepper placement and the rods and the whole table for some problems and reassembled everything properly.
- Additionally we rebuilt the heated bed to make the layers (heat insulation, heater PCB, glass plate) fit tight together, to prevent movement from there.
And finally we decided to update the firmware as it´s almost a year old now and meanwhile checked the maximum acceleration of the Z axis which is now set to 200 mm/sec^2
The funny thing is, it did not change anything in the printing results, the random wobble is still there and did not even changed. (Well that on the other side means that our Z axis was pretty well built and in order )
Afterwards, in desperation I played around with cura settings and tried to change flow rate and following the advise of gr5 tried different layer heights. It´s rather strange but the random wobble effect seems to disappear at a layer height of 0.24, as seen on the attached picture. (Layer height of 2.4 vs. 0.1)
It´s to late for me now to play around more with the settings, but I wonder if it something like perimeter that are to close together and build up to a specific height where they kind of collapse and produce these kind of effects on thin layers.
I would pretty much like to print in thin layers without the effect and will probably have to find the right settings in Flow, Wall Thickness + Layer Height.
Does that make sense to you?