I am trying to create a tyre that I can 3D print. I tried several methods from various tutorials, but they seem to be meant for animation rather than 3D printing and I always seem to end up with very complex, non-manifiold geometry. They usually involve creating the tyre tread, arraying it around a circle and wrapping it to a surface. The process I have adopted, which has been working, is to create the profile of the tyre from a curve, mirror modifying that over the Z axis, applying that modifier and then use a screw modifier to create the final 'donut'. These modifiers are then applied, so I have a nice, manifold mesh as my tyre carcass.
All good so far. I then created the scallops on the side, arrayed them using an Empty, applied the modifiers, Joined them into a single mesh object. I then applied a Boolean Difference modifier to the tyre carcass and that cut the scallops into the sidewall nicely.
Now, though, when I select neigbouring faces using Command +, I only get a small group of faces, not the entire ring. So it seems the Merge by Distance did not actually join adjacent vertices together. When I apply a Boolean Difference modifier to the tyre carcass using the tread loop as the target, I get no result.
I have tried all sorts of things, remeshing, convert to tris, separating and rejoining, converting normals to inside, converting normals to outside, everthing I can think of.
I have even tried separating the parts and then, one by one, joining them together with a Boolean Modifier, applying the modifier, saving the result and then joining the next tread part with another Boolean. Piece by piece. The result was even more confusing. When I applied the Boolean Difference modifier to the tyre carcass, some of the sections worked while others didn't! It still saw the joined sections as separate objects.
It seems to me I am missing some step that will unify the separated, arrayed parts into a manifold whole.
I would appreciate any suggestions as to what to try next!