Hi Scall, could you please post the 3DM model file as well so I could take a look at your geometry instead of just a screenshot? That tends to help a lot to give you more accurate answers, because a lot of things depend on how your geometry is structured and it's hard to tell some things just from a screenshot.

But most likely what has happened is that the 2 objects that you are trying to boolean union together are probably not closed solid volumes and are open surfaces.

If you are working on open surfaces, then you generally need to use the Trim command to cut them with each other, the boolean commands are focused on working on solids.

Both Trim and the booleans do a similar job of cutting things up, but with the Trim command you manually pick which cut up fragments you want to discard or keep, and then you can use Join as another step after that to glue together surface pieces that are touching.

Booleans also cut things up, but they automatically decide which pieces to discard and which to keep based on which volume the pieces are contained inside of, like with boolean difference pieces that are contained within the cutting volume are removed. But if your objects do not actually enclose a volume and have open ends or stuff like that then the boolean commands won't really be able to function properly.

It can actually be easiest to close off each separate object that you make to make it into a solid first before you work on combining them together, that way you will be able to use booleans to do the work and that usually saves some steps that you would have to do manually with Trim + picking pieces to discard + Join. Basically booleans are sort of like a batch mode operation of doing cutting plus discarding plus joining for you automatically but you have to have a solid in order to make use of them.

Hope this helps - if you can post the 3DM model file it would definitely make it easier to better diagnose what you are running into instead of just guessing though.

- Michael