Mass Properties Analysis: Assigned vs Computed

[jseymour]

Hello,

I have to provide moment of inertia and COG information for our products (avionics boxes), this was done by a consultant previously.  Since a number of circuit cards exist in the model, there can be thousands of parts that I don't want to provide materials to individually and instead I will weigh the card and provide an assigned mass to the assembly level of the card (a bit of corner cutting here).  My assumption is that Creo takes this mass and applies an even density through the assembly.  There are also other instances where I am working in other people's models and they have parts that should not be included in the analysis, but are parents to a number of parts that must be included.  Not wanting to fiddle with the model, I also assign negligible masses to these parts.  I'm assuming that this negligible mass is now practically invisible to the calculations.

My issue comes when I attempt to run the analysis with the assigned masses.  The total mass comes out accurate, and the center of gravity seems right however the inertia calculations are clearly wrong.  Based on some simple hand calculations I should expect values on the order of 20-100 lb*in^2, however I'm getting in the 10^7-10^9 range.  This is similar to what I get when running the standard computed mass analysis so I feel like creo is doing something with the default values and not the assigned ones.

How does Creo calculate these values when using the assigned values selection?  Is there a method I should use here instead for ensuring assigned values work properly?  Or am I missing something more fundamental?

Attached is a text file of the results of the assembly analysis, both the computed and assigned values versions. 

Thank You!

-Joe Seymour

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[Kevin · Accepted solution]

For an assembly Creo uses the calculated or assigned values from the part level. It also uses the density value not the mass value in the inertia calculation. Probably the easiest way to adjust the inertia values if you have the option to is to create an analysis feature for the card assembly that includes the inertia and mass values and write relations for the card assembly alternate inertia values. Doing this gets you in the range of 150 for Ixx and Iyy and 20 for Izz.

[dschenken]

I used to just adjust the part density so the mass came out correctly. This applies a uniform density to the part, which should allow a good close approximation for calculation of all other mass properties.

I would only use assigned properties if I was going to assign all of them.

[Aaronm87]

Another work around that I have used in the past when we received step files from customers with thousands of part is to set the density to 0 (or almost 0).Then, model a block with the assembly weight centered on the CG location.

[Kevin]

And if you do decide to change the density value for all parts you could use ModelCheck, if you have the option to, to do it instead of having to go into every model and change the value.

[Aaronm87]

Can you explain how to do this? We have some parts that need the density changed and they are all checked into windchill so changing individually will be painful.

[dschenken]

Model check works -before- checking into Windchill. No matter the method, all the parts will have to be checked out first.

[Kevin]

There are 4 files to setup: the config_init, condition, check, and start files. In the config_init you want to set the options MC_ENABLE and MU_ENABLED to Yes. For the condition file you set the conditions for the files to search, you can probably use the defaults and modify the check and start files used during the check. For the check file you can create a new file and set a check for RELATION_MISS. The columns relate to the mode in which modelcheck is used, the most common mode we use is the Interactive mode. Change the value in the Interactive column to E and when you run the check if it encounters an error it will update the model automatically. For the start file you can create a new file and add a part relation by picking through the drop down lists that are presented by picking part, then relation, and for the value type MP_DENSITY = desired value. For the condition file change the values for the check file and start file columns to the created files. When you run the check it should check to see if the relation exists and if not add it automatically. If you don't want to keep the relation you could write a check to remove the unwanted relation automatically and the value for the density should remain the same as the value you set.