It's not a single technique, but rather a system of modeling techniques that work together. The core of it is:
- Using skeletons to define the design intent
- Creating Publish Geometries in the skeleton to define sets of geometry to pass on to the parts or sub assys
- Creating External Copy Geometries in the parts and sub assy skeletons to retrieve the data
- Careful, intentional reference selection that explicitly capture your design intent
A skeleton is a special part that is used to define your overall product's design intent. It has to be created in the context of an assy and you have to have the advanced assembly extension. The top level assy skeleton contains the geometry needed to be shared at that level. The main outside shape and major fastening or motion items, for example. Each sub-assy has it's own skeleton that contains more detailed information that needs to be shared between the parts in that sub assy. Depending on the complexity of the entire product, we typically only go 1 or 2 levels down with sub assy skeletons to keep things more manageable.
In the skeletons, we create a publish geometry feature for each part or sub assy that contains the geometry that item needs. A publish geom is like a bucket you put a set of geometry in for passing on to another part. In the sub assy skeletons and the parts, we create an external copy geometry that references that publish geometry, pulling the relevant skeleton model data in. It's like reaching in that bucket and pulling that geometry into that part.
In most cases, we use the default CS for both assembling the component and locating the external copy geom. In cases where default assy position doesn't make sense for a given component (the origin of the assy would be far outside the envelope of the part, for example), we create a CS for that component in the skeleton and use that CS both for assembling it and for the external copy geom. It's important that the assy CS and the external copy geom CS match.
We make very heavy use of the skeleton, putting in geometry for every instance where two parts or sub assys need to share geometry. Everything from the outside shape, parting lines and major mechanism features like pivots down to screw boss locations and heights go into our skeletons. Again, if it's shared geometry, it goes into the skeleton. Some will say keep them simpler, but we've had great success with our method.
A major consideration, however, has little to do with the actual TDD technique and more to do with the modeling and creation of individual features. If you are careful with reference selection and model to capture your design intent, you will be rewarded with very robust and very responsive models. Think as if you are baking your design intent into your feature construction.
We've been able to make major size changes to overall dimensions of very mature assemblies (in some cases live in a client meeting!) with very few feature failures. It's quite satisfying to change an early skeleton feature effecting 6 or 8 parts, regen your 3800 feature assy and have no failures. Make the whole product an inch shorter and still release for tooling on Tuesday? No problem.
There are some good KB articles on skeleton modeling, search for them. I cannot link to them because my company's maintenance agreement has expired. I searched a bit for videos, but didn't find any that show exactly what we are doing, but there are some out there that show the basics of creating a skeleton and creating publish & copy geometry features. Part of the problem is that it's very specific to your design. Learn the basic techniques and you can then apply it to what you do.
That's an intro (there's a lot more) but I hope that helps.
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Doug Schaefer | Experienced Mechanical Design Engineer
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