Just to clarify - it is not quite that everything has to be thick, but rather that the thickness between 2 objects to be unioned at a common edge must match.

For example - this is ok, 2 objects each without thickness can be unioned or joined:




Also 2 objects that have thickness (have a volume) can be unioned:




However, a mixture of thick and thin at a common edge will not work:



This would create a situation where the bottom circular edge is connected to 3 surfaces - the bottom plane, the hole from the thick part, and the cylinder sticking out the bottom.

The next beta will separate such objects into different pieces, keeping the upper volume as one piece - the only other choice is to allow non-manifold edges, but these can create problems when sending objects to other programs, so for a while I think it is a good idea to focus on not allowing them.

- Michael