Very bad lateral surface - need help

Hello there,
at first sorry for my bad english!
I have a Prusa i3 Steel with MK8 Direct Drive extruder, Nema 17 motors 1.2A, RAMPS 1.4 Board with Marlin firmware.
In the beginning I had problems with small waves on the lateral surface. Then I swapped the cheap China LM8UU bearing against IGUS RJ4JP. On the X and Y axis.
It was slightly better, but still not good.
Then I swapped all the motor drivers from A4988 to TMC2100. Also for the extruder.
Nevertheless, I have no nice surface finish. The surface seems somehow "turbulent". It looks like small micro waves, but they do not feel with your finger.
The belt I have rotated on the idler side so that the smooth surface runs on ball bearings. Also does not help.
Look at the pictures, please. You can see what I mean. The small cube is printed with 0.2mm layer height, the great with 0.3mm.
Maybe you have a tip for me?

My hard and software:
Prusa i3 Steel
MK8 dd extruder
0,4mm Nozzle
1,75mm PETG from Extrudr.eu
230°C nozzle
93°C bed with PEI plate
Nema 17 1,2A motors
RAMPS 1.4
Marlin
Slic3r

Speed Perimeter: 35mm/s
Speed extern Perimeter: 70%
Speed Infill: 40mm/s


Best regards from germany
Steffen

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Doesn't look bad to me for .2 resolution.

If you mean the ripples near the corners that's called corner ringing and it has to do with the extruder changing direction and the steppers trying to rectify the over/under shoot. This can be almost entirely cancelled out by:

- turning on acceleration
- modifying acceleration parameters
- slowing down print speed
- decreasing moving mass (eg. removing second extruder if dual)

This is a basic physics issue. Nothing wrong with the LM8UU bearings.

What Sebastian said. Alternatively, add a chamfer on the corner : it will help too because you won't have such sharp accelerations/decelerations.