Dear RubenDenolf,
When I look at your picture and the reference describing the Z-Axix wobble, I can see a fundamental difference:
Z-Axis Wobble is quite repeatable and"symmetric", but your print seems to be “irregular” in wobbling. That’s an issue I observe more or less on every print of mine. It is actually no problem for me, but I am thinking about a different reason:
When you take the Calculator for prusa printers for calibrating the stepper settings http://prusaprinters.org/calculator/, you find a section called “optimal layer height for Z-axis”.
Put in the targeted layer height (0.2mm), the motor step angle (1.8° or 200 steps per rotation) and the lead screw pitch (1.5mm for K8400), you will get a result which is showing you, that the 0.2mm makes an error on Z-Axis height.
Furthermore, the Z-Axis drive of the K8400 has the same stepper driver configuration like the x- and y- stages, which means, that the printer driver board is generating 16 “sub steps”. This means, by small shifts in the driving currents the stepper motor is forced to 15 intermediate position until the next “stable” hardware position is reached. This fine stepping can generate some differences depending on different mechanical forces.
To simulate this fine stepping you can introduce a gear factor of 16:1 in the calculator, and you will still see, that 0.2mm layer height will generate errors.
These errors must be compensated: when calculating the steps for each layer, sometimes a layer needs higher or lower step counts for the stepper motor to compensate the error. But this means, that the actual layer height is different to the nominal, but the extruder is still driving material for the nominal layer height. As a consequence the layer width (!) is changing complementary. And this change in width can be seen quite easily on the outer print line.
With this model I expect a wobbling impression, which is irregular and a little bit unpredictable, but still repeatable in a larger frame, since the error compensation will take place in a regular scheme, superposed by the positioning error of the sub steps…
I tried the improved target height of 0.2025mm an a few printouts, but the impression basically remained. So my conclusion was at that time that the “micro stepping” has the major impact on this effect.
