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Composite Parts - how to use them?

S1monS1mon Member Posts: 2,104 PRO
I'm experimenting with using a composite part to represent a molded part with insert molded threaded inserts. We started with an assembly, but because no one in the supply chain ever orders the plastic part without the inserts, it started to seem like it would make sense to use a composite part.

A few things are a bit strange:
  1. I added two (of the same) inserts using the Point Derive FS. At first they didn't have their material set, so I set one to brass, and the second insert didn't pick up the material setting. I went to the part studio where the insert was created, and set the material, and then both inserts picked up the correct material. Is there a better way to manage this?
  2. With an open composite, the mass properties work in the part studio, but in the next level assembly, the BOM reports "no value" for the mass. I tried a closed composite, but that made things worse. Then there was no material defined in the part studio (and I assumed it would treat the composite as a single material, if I assigned it)
The fact that the mass doesn't roll up seems to defeat the purpose of an open composite. Is there some use for this? Is this a bug?


  • Evan_ReeseEvan_Reese Member Posts: 2,034 PRO
    I do use composites for this kind of thing as it can just be a bit cleaner (same for parts with overmolds or a 2-shot design), but it does limit some other things, like your ability to show it as an exploded view, and I suppose to get the mass (i've not run into this because it's not a critical part of our workflow). I'm curious where you end up landing with it. You could use the composite for most things and an assembly of it for other things, but I imagine that's too much duplicated work and clutter.

    As for how materials are handled for derived parts, what you describe is the behavior I'd expect. I'd plan on setting materials at the original part studio level. In this case, the studio with the insert.
    Evan Reese / Principal and Industrial Designer with Ovyl
    Website: ovyl.io
  • S1monS1mon Member Posts: 2,104 PRO
    I created an improvement request to correctly report the mass in BOMs.
  • eric_pestyeric_pesty Member Posts: 1,308 PRO
    Yeah the way the mass is handled does seem a bit odd. There are a number of things that I find a bit puzzling with the open vs closed behavior of composite parts.
    At least now you can set an assembly to hide it's sub-parts so it's less of a tradeoff to create this as an assembly for something like this (other than needing two tabs). 
  • S1monS1mon Member Posts: 2,104 PRO
    Yes. It's a bit odd. It seems more like what was happening in Solidworks land. "Let's add some kinda useful functionality, but don't think it all the way through."

    Unless we're just working on science fiction, all of our parts have mass. In my case, one of our product's key metrics is overall weight, so it's nice to track the mass of everything. I'm sure we're not unique.

    Your reminder of the subassembly options really seals this in for me for now. Until the BOM "no value" mass oversight (bug) gets fixed, we'll stick with assemblies, but our BOM and naming will treat it more like a part.

    My understanding of the closed composite behavior is that it's more useful for things where you have a ton of parts (e.g. a PCBA import) and want to simplify the load on Onshape by creating one dumb part. I'm sure there are other interesting uses, but that's the one place I've found it helpful.
  • eric_pestyeric_pesty Member Posts: 1,308 PRO
    If you think of a closed composite is basically as an alternate to a boolean union (for when an boolean union wouldn't work because of disjointed bodies and non-solid entities). Then it sort of makes sense that the mass would be set for the whole thing uniformly.

    The open composite is a bit of a weird one actually but it's the equivalent to a multi-body part in other CAD systems. What can be a bit confusing is that when you go to use it you have to be careful to not insert just a sub-part.

    It's almost like we would need a 3rd option that hides all the parts but retains the mass/appearances. However at this point it's basically an assembly (but without creating a new tab)...

    A good example of a dilemma is for models of sealing washer that we use on some our assemblies: a piece of SS steel with a rubber gasket bonded to it: it's nice to have a configured model so you can swap size easily, and also nice to be able to show them in their "raw" (exploded views for example) and "squished" (to see where the bolt actually ends up after assembly) configurations.

    Right now if I want the correct mass I need to keep the composite "open" but then you might grab the wrong thing when inserting them. If it's closed, then it's easy but you have to manually set the mass for each configuration.

    If it's an assembly then you have to know to look for an assembly when inserting so it's not ideal either...
  • S1monS1mon Member Posts: 2,104 PRO
    Having to know what to look for when inserting into an assembly is a bit awkward for composites as well. I kinda feel like the insert dialog should default to all parts/composite parts/sketches/assemblies and allow you to uncheck things to filter. I found myself momentarily confused why my composite part wasn't showing up in the insert part dialog at first.
  • eric_pestyeric_pesty Member Posts: 1,308 PRO
    Yeah, I also don't like how the "assembly" is an entirely different place in the insert. It would be nice if it behaved more like the tab manager (listing the filters for all the types of things contained in the document you are looking at).

    You already have parts, composite parts, sketches, surfaces, flat patterns, why not have just one more of assemblies rather than have them in a different "header". Good question about the default but it should probably include parts, composites and assemblies and then you could turn them off as needed to refine.

    Actually I just notice if you deselect all filters (i.e. deselect the part filter that is checked by default from the part studio header) it shows everything (I've always "added" the things I was looking for by clicking on them)!

    I would prefer "all filters off" to be the default but I also understand why it would be nice for "beginners" to only see parts when they go to insert something in an assembly so I'm not sure... 
  • Evan_ReeseEvan_Reese Member Posts: 2,034 PRO
    I wonder if some simple custom features could help here. Maybe one could read the mass before the composite feature, then the other could assign the recorded mass to the composite part. The actual Center of Gravity could be off, but the mass would be right. I'm actually not sure whether you can read the mass directly with a custom feature though. You might have to read the volume and manually add density, which would be a pain.
    Evan Reese / Principal and Industrial Designer with Ovyl
    Website: ovyl.io
  • S1monS1mon Member Posts: 2,104 PRO
    The bizarre thing is that in the part studio, the mass property tool works just fine on a composite part, but somehow it goes missing in the assembly BOM. It shouldn't require any custom features, just a fix from Onshape.
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