Cura seems to not deal well with walls that are an exact multiple of the Line Width.
Example:
This is a simple object with 3 walls of 2mm thick. With a Line Width of 0.4 and Wall Line Count set to 3, it should just use 5 lines to fill the wall.
Instead it does this! (Also known as a printer-killer!)
If I set the Line Width to 0.39 I get the desired result:
But I shouldn't have to do that. This is a rounding/calculation BUG in Cura.
Also, printing a 0.4mm wall (for instance for creating custom supports) does not work.
This is the same object, but I added a cross in the hole with walls of exactly 0.4mm thick.
It now does not matter if I set the Wall Width to 0.4mm or 0.39mm. Cura doesn't want to print that as a single line.
(Yes, the Print Thin Walls option is on)
If I change these thin walls to 0.41mm this, Cura does print them as a single line.
So here is the object with 0.41mm walls. And Cura set to 0.39mm Line Width.
Again, I shouldn't have to do this to work around issues in Cura.