Hi Sean, that's called a non-manifold juncture and yes you generally need to avoid that in many CAD programs. Solids in MoI are expecting to have an outer skin where each edge is joined between 2 surfaces. A juncture like that would break this rule, you'd have more than 2 surfaces trying to share a single edge.
> I could make it with a mill, 3d printer, or by gluing 2 pieces of material together.
Often times they won't be able to be 3d printed either. But none of this really makes any difference, non-manifold junctures just add a lot of complexity to CAD calculations. They can be ok to be present temporarily during some early stages of the boolean calculation, just as long as the final results are able to be sorted into volumes that do not have any self intersections. A non-manifold area of a single volume is basically a type of self intersection.
- Michael
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