Hello everyone!

please I need help concerning how to identify parts orign and coordinate in an assembly, I will be taken the CSWA test in few days but I am still having trouble on that. When I model the parts for any assembly questions, I always get the mass property, volume and surface area correctly and perfectly, but the center of mass is always wrong(usually one component is correct; eg x may be correct and the remaining y and z would be wrong). please help me.