Have lost confidence in mass properties calculations

[cwenner]

Just can't rely on results. Going to have to simply use spreadsheets (which is terrible... but reliable). I've had various problems over the years so yesterday I sat down and rely analyzed things.

Have an assembly set to "Geometry and Parameters", It has 3 subassemblies, each set to "Fully assigned". All models are set to IPS. When I run mass properties for the top assembly, the results DO add up properly when the last two models are suppressed. The results are also correct when just the last model is suppressed. However, when I resume the last model (ergo, all three models), the "total mass" is less than the sum of the three.

Even if this is some kind of "corruption problem" (it shouldn't be because, like the others, the last model is "fully assigned") I don't see any way to rely on Creo for mass properties when that kind of thing can happen. Perhaps for small models where errors may be evident... but not for large models like rockets, cars, boats, large machinery, etc.

If it can't even add masses correctly, I have no confidence at all in the inertial information.

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[TomD.inPDX]

Interesting observation and certainly worth reporting as a customer service case.

There has to be an explanation.

Just out of curiosity, are there interferences between the parts?

How do you account for symbolic vs actual screw thread volume differences?

[cwenner · Accepted solution]

The PTC customer support rep figured out what was going on... and an acceptable (I think) work around. For the masses of "fully assigned" components to properly roll-up (and sum up) to next level assemblies you MUST manually make sure that the product of density and volume equals the mass you want. (True for Creo 1.0. Don't yet know if fixed in 2.0).

I confirmed that doing this does NOT screw-up manually assigned inertias. I had feared that the software would change inertias based on this average density and the volume distribution. It does not make that mistake.

[TomD.inPDX]

This is indeed critical knowledge for those of us who use these properties.

How do you:

For the masses of "fully assigned" components to properly roll-up (and sum up) to next level assemblies you MUST manually make sure that the product of density and volume equals the mass you want.

?

...and what does it actually do to obtain the wrong data?

[cwenner]

You can literally type the values in the fields. I had been using 1 [for density] and 1 [for volume] for all objects regardless of the mass I had wanted to "roll-up" to next levels. The "mass field" values was being ignored and all objects were rolling up as being 1 lbf*s^2/in!!!.

Now I leave density at 1, and make sure that the value entered in the volume field is the same one entered in the mass field. That will make everything work just fine.

Incidentally, most "next assemblies" have some subcomponents that are "fully assigned" and others that are "geometry and density" and still others set to "geometry and parameters". For all that to get summed properly in these next assemblies (using the work around described in this discussion) you must be sure that the property setting for these next assemblies is set to "geometry and parameters".

[TomD.inPDX]

Thanks!