Assembly mass properties: COG calculation when parts have assigned and computed weights

Question

Hi,
 
I have been struggling with the center of gravity of a large assembly which consist of parts and subassemblies. Mainly all the parts have density defined and Mass properties set as "Geometry and density". But then I have also some large weight parts with assigned masses and Mass properties set as "Geometry and parameters".
 
Now, I'm wondering when I'm doing the mass analyses for the assembly. I get the right mass for the whole assembly when I tap assigned in analysis mass properties but the center of gravity is not correct.
 
How does Creo handle the assigned weights when calculating center of gravity of larger assembly? Will it apply the assigned weight of a part to its center of gravity based on the (solid)geometry of the part? What happens if the part with assigned weight is only surface model? 
 
I tried to test these but got so mixed results in my large assembly. Maybe I need to create a simple assembly and test the things there..
 
Normally I work with assemblies in which all the parts have solid geometry and density defined. With these assemblies the analysis mass properties gives the center of gravity correct. Also in simplified presentations. Now when I applied simplified presentation to my assembly where I have parts with assigned weights, I feel that the center of gravity is calculated even more wrong.
 
Any thoughts? Have anyone of you experience of using mass properties analysis for calculating the center of gravity for an assembly that has parts with assigned and computed weights?

KenFarley · Answer

I don't know how you would go about calculating the mass and center of gravity of a large assembly. I'm not sure if it is possible in relations to parse through the components and sum up all the values, etc. The big base level problem I would have is how do you even define the center of gravity for a simply modeled, but complex in reality component? For example, if you have a model of a motor, getting the mass of the thing is relatively easy, it's often specified in the specifications for the thing. But the center of mass is another thing entirely. How or where would you get accurate data for that type of thing? It's not something that is readily available for models provided by manufacturers of such things as motors, electronic components, etc. Assuming center of volume is center of mass will most likely be incorrect.

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