Hi Oskar, so your boolean thing is because one of your pieces there was an open surface and not a solid.

The booleans are more oriented towards using solids, the different kinds of booleans (like difference vs union) are about different ways of combining the "insides" or "outsides" of the shapes and if you do not have a solid volume there is not a well defined inside or outside volume to it.

So it can be good to solidify objects before doing booleans on them, so that they work in the most expected way.

To do that for your case, select the tall body piece and then run Construct > Planar - that will cap off the open square bottom by putting a plane in there, and then it will be a solid. After you have done that, then you should see that doing a boolean union gives you the proper result right away.

In MoI v2 there is an object type readout in the upper-right corner of the screen that will tell you if the selected object is a solid or not:



If it does not say Solid there (either Joined Srf for more than 1 surface joined together but not closed, or just "Surface" for an individual surface that is not closed) then it means that there is still a gap or hole in the object somewhere.


The booleans will still try to cut things up when not working on solids, but usually you will want to use Trim on non-solids instead. Trim does not try to discard things based on volume but rather it lets you pick the pieces you want to get rid of.

- Michael