I created nested elliptical cylinders in two different ways recently, and neither of them work. 1. Thin Feature Extrude. 2. Extrude the cylinders after sketching them together in 2D. Interestingly SW assumes that the user wants a hollow cylindrical cylinder While a third method works, which is sketching and extruding the larger cylinder, then positioning the sketch of the smaller one on it and doing an extruded cut. What I mean by 'work' is there are no geometrical peculiarities and when I import the CAD file into COMSOL, it behaves--it meshes ok. But when I import the nested cylinders into COMSOL using the first two methods, I get a gap and the geometry won't mesh.

If you run Tools/Interference Detection on the cylinders, the gap shows up in red on the right. My question is why the gap? And is there a fix for it? Is this an example of what Jerry Steiger told me in "Mate a rod curving in 3D to a curved surface," that SW can get confused about which bits are in or out of two bordering parts so experienced users often create an over-sized part and cut it back with a surface?

Thank you.