How do you attach multiple unions together in an NPC model?

I've been working on creating a robot NPC for a while now, and I've been using solid modeling and unions to detail the NPC. I completed all the body parts and placed a Humanoid into my model, but unfortunately I can't seem to weld any of the Unions together and the NPC model falls apart immediately upon testing. Because the NPC is an enemy, I can't anchor it or it won't be able to move when using "MoveTo" . Is there any way around this?