###I FOUND SOLUTION FIXED:###
My shelled object (cylinder was only filleted on the top surface. When I filleted the bottom surface too, and then applied the shell construct I have no problems.
Why is that?

Hello,

this problem drives me crazy because it makes no sense to me. I would like to create an object which is solid and has a thickness. It's a cylinder and I selected it and created a little shell.
Then I have some filleted rectangle spline and I want to create a hole in this cylinder object. (1 screenshot)
So the inside is empty and i have an intersection where one can see the "thickness where the spline cuts it. (Like in the 2nd screenshot -> these cooling holes)

Edit: Here is the MoI3D Project File
https://www.dropbox.com/s/7k6l3mpvbk6vmky/Shell_problem.3dm?dl=0

edit: correct screenshot because I can't attach it again after editing.:

https://ibb.co/MPxPT53



I tried the same thing with a sphere, created a little shell and when I do the same thing with the spline (extracting without caps into the sphere -> then sphere-boolean-merge-the extruded spline(the walls of it) i get the result I want (Screenshot1)

Thank you for the help in advance!

Hi Nikita,

re:
> When I filleted the bottom surface too, and then applied the shell construct I have no problems.
> Why is that?

Offsets are easier to calculate when the pieces meet smoothly.

An offset between 2 smooth pieces goes something like this:





With smoothly meeting pieces, the offsets of each piece automatically touch each other.

With pieces that meet at a sharp corner that isn't the case:





With sharp corners, the offsets of each piece do not automatically touch each other, a gap opens up between them. So there has to be a process of extending and intersecting the pieces which is especially difficult when it's surfaces involved instead of curves.

- Michael

Ah, thank you for that perfect explanation and the pictures!:)