I'm having an issue where a shape is non-manifold, but I don't know why, or how to fix it.
I've taken this below shape from inside another design by copying the vertices and making a new part (it's effectively a flush mount for a screw).
However, when I select the top circle of vertices and fill (and the same for the bottom), the shape becomes non-manifold. I noticed this when I tried to use this shape to cut out of a solid body and the faces didn't cut out like normal with a boolean difference operator.
Now, this shape is simple enough that I addressed it by recreating from scratch by extruding a cylinder and then scaling the top section. But I'd love to know what I could do it differently using the shape here.
From searching, common culprits look like normals pointing the wrong direction or double verts. I checked and normals are all pointing out. I checked and couldn't find any double verts (merged by distance just in case).
Suggestions? I'm new to blender, so appreciate the help!
Edit: Solved - the issue was that I ended up with double faces, so after capping it top and bottom I had internal faces. Deleting the internal faces solved the issue. I didn't fully understand manifold or normals - so when looking at normals (not shown in the below views for the non-capped version) - I should expect to ONLY see external normals. But I had internal normals as well, indicating internal faces as well!