Hi Dr.Sassi

Thank you very much for your response and your project files!
I didn't know about the "Connect" and "Thicken" tool. It's great to learn about it. Thank you again!

I'm modeling a simple mug cup using "Spline."
I used "Lathe" and "Cloth Surface" for thickness.
But when "Subdivision Surface" is applied it creates a hole.

I wonder whether there is way to fix the issue.
I attached screenshot and c4d file.

Thank you for your time and help.

mug_0001.c4d