mass property and COG

: "solid3ddesigns" wrote : Can someone advise on how to apply a mass and a center of gravity to a : imported assembly (STEP) consisting of a few hundred parts. I don't : want to individually apply a material to each component. :

If all of the parts just happened to be the same material or at least their densities were close, the only problem would be getting the hundreds of parts to act as a single part. You could do this by creating a shrink wrap model or possibly, by importing the STEP file as a part instead of an assembly. In both case I think you can make a merged solid to which you can apply a material or at least a density and do a model mass properties analysis. It would be nice though to have a check, like knowing the actual weight. If the mass calculation is within

5% of measured weight, the cog should be very close, as well. If you need to get closer to measured weight, adjust the density number.

This might also work where some of the component densities vary greatly from an average. Before making the shrink wrap, suppress the components with extreme densities and just apply density to then separately. You'd cut down the work compared to doing them all individually.

David Janes

Thanks for your reply. The imported parts are of multiple materials, but I know the mass of the imported assembly and I know where the center of gravity (COG) is located. I want to apply these properties to the imported assembly so that I can determine the mass and COG of the complete assembly consisting of all my components and the imported assembly, but I just don't know how to do it. Any information would be greatly apprecieted. HB

Now you can do an analysis on the top level assembly and use the values from the subassembly. It will interrupt the calculation to ask how you want to treat the subassembly with the input mass props. You have a choice between Assign or Calculate. Leave it on the default Assign and hit Close. It will generate the usual Mass Props analysis, including average density, mass, volume and surface area plus assign the whole thing a cog. Record the cog and try it again, this time selecting Calculate for the imported assembly. The cog should shift as the one you input in Mass Props will be ignored.

David Janes

: > If all of the parts just happened to be the same material or at least their : > densities were close, the only problem would be getting the hundreds of parts to : > act as a single part. You could do this by creating a shrink wrap model or : > possibly, by importing the STEP file as a part instead of an assembly. In both : > case I think you can make a merged solid to which you can apply a material or at : > least a density and do a model mass properties analysis. It would be nice though : > to have a check, like knowing the actual weight. If the mass calculation is within : > 5% of measured weight, the cog should be very close, as well. If you need to get : > closer to measured weight, adjust the density number. : >

: > This might also work where some of the component densities vary greatly from an : > average. Before making the shrink wrap, suppress the components with extreme : > densities and just apply density to then separately. You'd cut down the work : > compared to doing them all individually. : >

: > David Janes

Thanks for your response, David, you've been very helpfull ! Looks like it's working now. Is there a way to display the COG on the model ?

Thanks again, HB

: Is there a way to display the COG on the model?

Yes, the analysis you did can be captured in a feature which stays with the model. When you are at the point of doing the top level analysis, go to 'Insert>Datum feature>Datum Analysis'. Pick Model Analysis/Next and Compute/Done. You can now concretize any or all of the values of the analysis as parameters by simply picking the analysis from the list and clicking the radio button for Yes. To get the COG, click Next again and specify either a CS or Point to identify the COG.

Because this is added to the model as a feature, it remains in the model and continues to be visible, unless blanked with a layer.

David Janes