Do you have suggestions for finding the volume inside an assembly with a shell that has multiple parts inside? The problem would be much simpler except that the shell is designed to be made of metal parts that are modeled with clearances for weld prep, so it is by nature not watertight in the model.
I created a simple test assembly just to show the concept.
I "found" a solution to my problem that works wonderfully. Since I am working with assemblies with weld gaps, I saved the assembly as a weldment and used Inventor to weld all the gaps. This solidifies the model and makes it watertight (once properly welded). This allows you to create a block around the assembly like the tutorial shows, create a derived part to subtract out the assembly from the block, delete face with lump/void to delete the excess material on the outside of the assembly, and you are left with exactly what fills the assembly. Then you can place the derived fill into your normal assembly (or the weldment) and change its iproperties so that it accurately represents the fill material. Now you can do mass and CG calculations,etc.with what should be a more accurate model.
Doing the welding does take some time but I found it took less time than modeling all the nooks and crannies (even while teaching myself how to weld as I went).
The answer to filling in my weld gaps was to use...weld! Who knew? : )
Now I guess I may have to take back a few of those things I said about Inventor...
There is another way as well... Dont know if this helps but it worked for me when calculating the volume.
Lets say you need to calculate the volume of a square 10x10x10 whit a thickness of 1mm.
Just draw the inner measures like 8x8 extrude 8... go to iProperties then choose physical, select material (dont think u have to btw) then select update. And you will have the volume of the mass. Which will be the same as the volume of the space in your 10x10x10 square.
Hope this helps, and u understand what i mean, even tho my bad english.