Incorrect inner dimensions of 3D prints with Ender 3 I have problem with my Ender 3. Whenever I print, I have proper outer dimensions (accuracy usually better than 0.1 mm, sometimes a bit worse), but every hole, pit or any inside dimension is inaccurate (0.6 - 0.8 mm never less or more).
I have tried extruder calibration. Calibrated extruder voltage and e-steps (in Ender firmware), but this does not change a lot.
Here you can see test dimensions I modeled:

And it will come out with these dimensions:

Aaand the final print looks like this:

As you can see, my estimation (second picture) was almost perfect. 19.3 mm and results are weirdly consistent. Always holes are 0.6 - 0.8 mm smaller than holes in model.
Do you have idea why this is happening?
Specs:

Printer - Ender 3 with original circuit board
Printer software - Marlin 2.0.5
Model cutting software - Cura 4.8.0
Modelling software - Fusion 360
There are 3 effects at work, and you misread your micrometer: the measurement is 19.35 in the picture.
You have a little lip
There's a little lip at the top and bottom of the print. You'd need to clean that up with a sharp knife or sandpaper. That is the biggest part of the error you measure.
Movement errors accumulate
Errors also collect on the center of holes due to the order in which walls are usually placed, resulting in outer walls having the correct diameters but inner holes having a small offset.
Plastic shrinks when it cools
A smaller part of the error is the plastic shrinking as it cools, but that can be compensated for by the slicer - if your printer allows for it: Under Materials, there is a Shrinkage Ratio setting.
There's compensation for that in Cura
The option is under Shell and called Hole Horizontal Expansion. Setting that value to 0.6 mm to 0.7 mm should solve the hole sizing error.