topic Re: Derived part are including Base Comp. mass even though they are surfaces. in Inventor Forum

Derived part are including Base Comp. mass even though they are surfaces.

CamperUnhappy — Mon, 30 Apr 2018 21:40:57 GMT

This is not a new bug/behaviour but it needs fixing already... I've run into it before and thought it was an isolated case because it was on pieces that I had published to the Content Center, but these are all standard components and just basic sheet metal.

I have a bunch of sheet metal plates that bolt pieces of a square HSS structure together. I built them using Derived Parts and brought in the appropriate pieces of tubing into my part as surfaces. The problem is....the part (which is made of steel) should weight around 4 lbs is showing a total mass of 420 lbs. I've had this problem in models before because I usually sort my parts in a BOM by size/mass. The only way I can seem to get my mass to come in correctly is by suppressing the link to the base parts and leaving the link suppressed......which means that that parts are not dynamic. Is there absolutely no way to design parts in Inventor that relate to other parts that isn't absolutely broken? Projecting geometry doesn't work, derived parts doesn't work. I think the only way is to build parts off of an Excel Spreadsheet.......

Why are pieces in my assembly that are 100% surfaces (ie. non-solid) showing a mass or why is Inventor pulling the mass from the base part? ANY body/feature/region/surface/etc that does not create a SOLID should NOT have mass associated to it.

Re: Derived part are including Base Comp. mass even though they are surfaces.

johnsonshiue — Wed, 16 May 2018 22:42:27 GMT

Hi! This behavior is indeed as designed to accommodate Shrinkwrap Substitute workflow. The composite option retains the mass prop from the assembly so that the substitute part can preserve the source assembly's mass prop.

There is actually a workflow allowing you to zero out the mass prop in the Derive Assembly. Create a new assembly, place the target source assembly in the new assembly. In the new assembly, right-click on the target source assembly -> BOM Structure -> set it to Reference. Now the new assembly will have zero mass. You just need to derive this new assembly to the part. It will still retain the geometry but there is no mass.

Could you try it out and see if it works better for you?

Many thanks!

Re: Derived part are including Base Comp. mass even though they are surfaces.

CamperUnhappy — Wed, 16 May 2018 23:09:15 GMT

I absolutely could include that as part of my workflow, but unfortunately that is not a solution, that is a workaround.

For anyone who has ever used any other 3D software (and I have used a few SolidEdge and SolidWorks), objects that are not solid never have mass unless you override them. It just doesn't make any sense to have the mass come and go if your suppress/unsuppress the derived links. The first time I ever saw this issue I thought I was going crazy. The idea of having to stuff a part/assembly I want to use to design another part from into another assembly and then derive that assembly is just not an elegant solution. If anyone ever had to revise one of my models if I got hit by a bus, they're think I was not OK in the head. Could we not just be given a checkbox inside the window where you select what parts you want to include that would allow you to include/exclude the mass of the derived items??

Re: Derived part are including Base Comp. mass even though they are surfaces.

johnsonshiue — Wed, 16 May 2018 23:22:22 GMT

Hi! Please note that Composite is actually not a surface body, although the Part icon shows surface. It is actually a hybrid collection of surfaces and solids. It is like a container encompassing all body types.

To be fair, I have heard similar complaints before. We did have internal discussion about providing a document level switch allowing users to include or exclude the carryover mass properties. However, it could also be confusing when the option was not set right.

Another workflow you may consider is to derive the assembly as a solid. Then delete a face from the solid so the derived geometry becomes surface. Then the surface body will not participate in mass prop compute.

Many thanks!

Re: Derived part are including Base Comp. mass even though they are surfaces.

draftsman3 — Wed, 03 Apr 2019 19:52:45 GMT

I am having the same issue.

I am trying to make a derived component using a multiple piece assembly (standard parts, weldment, and sheet metal) imported as a surface. It wants to maintain the mass of the assembly and I do not see anywhere I can change my reference assembly B.O.M. to "reference" as mentioned as a workaround. I really need to reference the assembly and I do not see a workaround that will let me get it to zero mass.

Re: Derived part are including Base Comp. mass even though they are surfaces.

johnsonshiue — Wed, 03 Apr 2019 21:26:58 GMT

Hi! There are a few ways to control the mass prop. Here are the options.

Option1: Create a new assembly and place the assembly you are trying to derive. Right-click on the subassembly -> BOM Structure -> Reference. Check the mass. The entire assembly should have zero mass because the subassembly is a reference. Now, derive the new assembly to a part. The part mass will remain zero.

Option2: If you don't want the mass of the derived assembly part to be counted within another assembly, you can simply change the derived assembly part's BOM Structure (Tools -> Doc Setting -> BOM Structure -> Reference). The mass of the part by itself will not be zero. But, if you place the part in an assembly, the part will not contribute to mass prop.

Option3: Derive the assembly as a solid. Then use Delete Face command to delete a small face from the body. Now the solid body becomes a surface and the mass prop will be zero.

Many thanks!

Re: Derived part are including Base Comp. mass even though they are surfaces.

draftsman3 — Wed, 03 Apr 2019 22:57:38 GMT

Thanks johnsonshiue.

Option 1 - this worked, but requires an additional assembly file and an additional part file just to be able to do something that, in my opinion, should be handled when you use the "surface body" option.

Option 2 - this did not seem to make the mass zero, but rather N/A, this then carried through to the derived part, and I was unable to assign a material to the derived part.

Option 3 - this also worked and only required one additional file - the derived solid with a deleted face.

The BEST option would be for Autodesk to make the "surface body" option default to NOT bring in any mass information for assemblies. I do not understand the requirement to bring a mass in with a surface. I think for my purposes Option 3 would be the best workaround, but it is extremely cumbersome to require additional parts be made for such a simple task.

Re: Derived part are including Base Comp. mass even though they are surfaces.

johnsonshiue — Thu, 04 Apr 2019 20:13:47 GMT

Hi! For Option3, it should not require an additional file. You can derive the assembly as a solid as usual and then delete a face so that the body becomes a surface body.

Yes, it would be nice to have some control over the mass property compute beyond geometric level. I believe we have ideas like that on Ideas forum.

Many thanks!