I have been using Fusion 360 to design a new chassis. However, every time I add a motor and create a joint to attach it to the frame, only the threaded insert or the cap appears to join. (See below image)
Typically, when an assembly is imported into a parametric CAD package (whether it be Fusion, SolidWorks, or whatever) from a step file, the various parts that make up the assembly are not actually mated to each other. This is why it “comes apart” when you try to mate a particular part. You can open the assembly in your CAD package and mate the various parts yourself, or you can get an “already assembled” motor from one of the libraries created by the community. The attached document has some likes to various parts libraries. CAD for VEX Robotics.pdf (299.7 KB)
You can also go to the file for the motor itself and make a rigid group. Just note that when you make joints, only the cap will join, but pressing ok will joint the entire motor