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"Black Box"ing CAD Models for Assembly usage

azaex

New member
The extent of my CAD knowledge is really just my introductory university class and all the googling I've done on my own so far, so forgive me if I'm bringing up a concept that's already obvious to some. I help model for a college Formula SAE team and an issue that I've noticed so far is that many people are making references directly to part geometry, such as edges, faces, and points. The issue blows up when people adjust that round, or delete a face, add geometry, etc. etc., and it happens quite often given how fast Formula SAE teams iterate through designs. A way I thought up after sitting on this was to effectively "black box" communication between parts with a sort of datum layer sitting on top of the part model (or at the end of the part model, rather). All communication between parts would need to be handled through these datums, which are connected to the part geometries that the members would normally use. Effectively it's a sort of "buffer" between the part geometry and another part. When a design change is made that deletes a reference for the datum layer, the user would just disconnect the datum, float it around in free, space, make the change, and reattach the datum, thus never deleting the reference for another part. I figure this would be nice, because I don't need to ensure that everyone knows how to pick references from a list, or if they can even use Creo well.

Basically it takes the spaghetti references that we have now, and organizes it into a somewhat clearer hierarchy, since team members now also have to communicate about what links are being made. Additionally when one person updates their CAD, as long as they adjust their datums properly, the other person doesn't need to do anything; the design intent is preserved.

Is this already common in the real world? Is this something that people who use PDM's like Windchill have been doing for ages? We are also considering switching to Windchill, but I can't imagine how much worse the headache would get if references stayed the way they were on the team AND we had to pull things from a server to see full models :(
 
Windchill has nothing to do with managing assembly references.

This is basic good modeling practice. People have been doing it for decades.

You don't need any special modules although if you want to do top down design it's good to have the advanced assembly extension. With AAX you can create a skeleton model which defines things like the axis for the wheels and you push these common references down to the subassemblies and parts.

I prefer top up design as I like to do a lot of model reuse and I hate having out of date references from last years/decades model. So I always start my part models with at least 3 datum planes, and if it's mostly cylindrical an axis. Then add more datums to define where there will be interfaces to other parts. Next build your geometry referencing only the primary datums as much as possible. Then build the assemblies again only referencing the datums. With a little thought and work you will find you can completely blow away all the geometry of a model and recreate it with something new and it all the assemblies will still work just fine. Never ever reference surfaces or edges.
 
I was under the assumption that Windchill or PLM was some magic voodoo that managed assembly references from reading into other FSAE teams's practice, and the more I looked into it the more I was confused and didn't think this was the case. Glad to know that my hunch was right.

I've started to finally use datums a lot more in my models, and I wish our university modeling class would reinforce it more to the new students. The intro class here is more of an interface introduction, not a modeling practice introduction unfortunately. A lot of the models I see from people coming out of our intro CAD classes reference geometry edges or surfaces and makes assembly stability a total nightmare! I've started to pitch skeleton modeling to a lot more people to introduce them to using datums to drive models, and hopefully we'll be able to address a lot of our modeling issues this year.

Sort of an aside, but do you know if top down modeling with a skeleton is a paradigm used in by people proficient with other CAD packages like NX or CATIA? I haven't had much experience with them, though I can see the trickle down reference concept being applied in those packages.
 

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