Reference geometry with no weight

[BenLoosli]

We have some large structures that we need to show as reference in a sub-assembly. We want to bring in these larger structures in a manner that they have 0 weight in the sub-assembly file/

What is the best way to accomplish this?

[StephenW]

3 horrible ways I have done it in the past (And I still do these when I need that sort of installation type drawing)

1. I make 2D back drop geometry. I use a drawing, generate DXF geometry that is specific to the area(s) I need and add that 2D geometry (imported in to a part file) on a plane either on or at a distance from the default planes. This method works great when the project was done in Autocad and all the previous CAD was 2D only

2. I make 3-d surface geometry shrinkwrap of the larger assy.

3. when I really get desperate, I make 3D solid geometry shrinkwrap and set the density to something really small.

I hope you get better suggestions and I can learn from them.

[tbraxton]

Skeleton models should not contribute to mass property calculations. If you were to put your reference geometry into one or more skeletons that should resolve the issue and I think support the use of solid geometry rather than quilts/surfs.

Skeleton models should be ignored for assembly BOM generation as well which may also be useful in this context.

[Patriot_1776]

If skeleton models don't add mass and are automatically exempted from BOMS, that would be cool, I'll have to check those 2 things out since I am using a skeleton model in one of my assemblies

[KenFarley]

I've needed to show fixtures, parts, etc. in a drawing just to illustrate usage or placement techniques for the "target" sub-assembly. What's worked for me is creating another assembly that uses the target sub-assembly and all the needed reference stuff. When I create the drawing I use the target assembly for the majority of the views and to populate the data fields on the drawing sheets. For the views I need to show the reference stuff, I add that assembly to the drawing models and use it to create only those referencing views.

I don't know if people will hate this suggestion, since it now encumbers the drawing with an added assembly, but it's been pretty useful for me, despite taking longer to load, etc. Once everything is squared away, and if you're completely sure about the views, you could convert them to geometry only and get rid of the added assembly.

[StephenW]

Yes, I've done this too. It's another useful tool. 

It's important to state that your real assembly only contains the parts you have in it. Then you add that real assembly to the "drawing assy" along with your reference parts, so you are only maintaining one real assembly.

The rookie mistake is to make 2 assemblies and then someone modifies only one and there is a deviation.

[Patriot_1776]

It's been a long time, but I believe you can manually assign a mass to anything.  I seem to remember back in my aerospace days getting vendor parts as STEP files where we had to manually assign a mass and CG even, so all that could be calculated at the top assembly.  The mass and CG of a zillion little parts on something like the shuttle add up quickly!

I'd look into that avenue of manually assigning mass at the assembly level.

If that doesn't work, STEP it out as a single file, add a skeleton model to your target assembly, and when you bring the STEP file into the skeleton model, do it as surfaces instead of solids.  That will give you a 100% true representation of everything (unlike shrinkwrap), and zero mass.  The downside is now you'll have surfaces instead of solids so your dwgs may be more difficult.  Try manually assigning mass first.

Best of luck and let us know how it turns out and what it took!

[Chris3]

Step out the assembly, import as part using the hidden config option below (watch out! it can crash Creo - that's why it is hidden). Assign part a density of 1e-9

intf3d_in_as_part YES