Override mass properties for imported IGES file?

rob_rowland

I need to calculate exact mass, center of gravity, and moments of inertia for a large assembly.    To do that, I need to ensure that mass properties have been accurately applied to all of the contained components.  

Many of the assembly components are CAD models given to me by my suppliers in IGES format.     IGES format imports into Onshape as hundreds of little surfaces.   I have included a section view screenshot so you know what I mean.  

My suppliers have already given me a mass and COG position for each component.    How can I override the mass properties of this imported geometry in Onshape (including the surfaces), to have the provided mass and COG?   I cannot accurately calculate assembly mass if the included components do not have an assigned mass.  

This is a critical feature that every single other 3D CAD tool I have used can intuitively accomplish (Siemens NX, Catia V5, Solidworks, etc.).  

rob_rowland

Screenshot of section view below.    This import is completed as 400+ surfaces and a few solid bodies.    I need to treat it as a single part.  

NeilCooke

Hi @rob_rowland the answer to your mass props question is not yet, but it is on the radar and hopefully not too far away (please don’t ask when). With regard to IGES, this is a notoriously bad transfer format that was last updated 24 years ago. Ask your supplier to give you Parasolid or STEP so at least you will be able to import the part into an assembly as one. 

owen_sparks

Hi,

If it were me (and ignore if this if it's just reinventing the wheel) I'd do the following, assuming you have mass and CofG:-

(a) Set the imported parts in the assembly to have no mass.

(b) Create a sphere, give it the total mass of the assembly, make it transparent.

(c) Transform this solid using the by XYZ method to move it to your supplier's provided CofG

(d) Group as sub-assembly.

The transform of the solid negates having to make a sketch at the CofG which without 3d sketches could be a few extra steps.

There are already custom features that will allow a target mass to be set for a given part but I'm not recalling something like this.  Seems a fairly simple FS, but it assumes you already know the CofG location...

Cheers,

Owen S.

NeilCooke

@owen_sparks that’s a good temporary solution and one I wouldn’t have thought of. 

owen_sparks

Thanks @NeilCooke

Being a bit late to the party in CAD terms I never know if I'm posting something blindingly obvious to others or something useful

rob_rowland

@owen_sparks
I had thought of doing exactly that, so thanks for the confirmation of it as a workaround.     Very tedious to do this for all my imports, but I guess that's the only option.   

@NeilCooke
Sad to hear that it can't do this but I understand.     If there is in an improvement request already in the pipeline, please please make this a priority.    It's such a critical feature for integrators and OEMs (like us) that import a lot of packaging CAD.   Having this feature will save my team huge amounts of time.    

Prachi

I've been using Owens method or pretty much the same.
Tried to hunt down the specific FS mentioned but not finding. If someone knows the name of the FS it would be appreciated.
Lots of files sent from vendors import as nothing but surfaces with no mass but CofG location given in drawing.
Request has been put in for CoG symbol in drawings. Please vote up.
https://forum.onshape.com/discussion/comment/23650#Comment_23650

MBartlett21

@glen_dewsbury
Here is my FS to set material + weight:
https://cad.onshape.com/documents/3ca261a95938af152088fca8

Prachi

Much appreciated. That does just what I need.