I'm trying to follow the instructions here but I'm not sure if I understand this correctly. #2 bullet point specifies that the origin needs to be at the center of the mass. If I'm starting to build this part how can I tell where the center of mass if there is no geometry? I'm not shure how to do this. I know when I start a new part I get the global origin but I'm not sure how I can make this origin be at the same position of the center of mass when the part is completed.