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Is "explicit modeling" strict subset of parametric CAD?

  1. Feb 20, 2015 #1
    I am trying to make sense of these articles http://info.kubotek3d.com/3D-Engine...ubotek-Blog/?Tag=Direct CAD vs Parametric CAD and http://en.wikipedia.org/wiki/Explicit_modeling . To me this sounds like "if using parametric relations is too complex, let's stop using them and just hardcode every dimension" i.e. I don't understand why these opponents of parametric modeling cannot model in SolidWorks without specifying any relationships between parts, without using any special purpose CAD tool.

    Am I correct in the supposition that this is just marketing bullshit around a package with fewer features (and perhaps cost-effective low price)? Or are they in fact correct in that there are some "negative" inherent aspects of parametric modeling engine that their tools avoid?
     
  2. jcsd
  3. Feb 23, 2015 #2
    Well, that's hard to say. Direct modeling hasn't really been proven to be "better" or "worse". It's just a different tool for a different purpose.

    Feature-based modeling is nice when you know generally what you want and you're just trying to get it out. If you know generally where you want your holes or how about long a boss or flange needs to be, features help capture that intent and then later on others can see what you tried to do and fix it if they need to modify it. Plus, it helps the designer mimic the manufacturing process to a limited degree by making them think in a this-before-that kind of way. Features are more intuitive in a manufacturing sense.

    The real limitation in features is that it doesn't lend itself to something more free-form. In a concept generation stage, you might want to quickly visualize something like when you're doodling on paper. Figuring out what combination of features will yield what you want is harder than just manipulating it into that shape. As well, there are some industries where free-form would help. For a jet engine company, designing a fan blade by modifying the shape locally might yield better performance than tweaking features. This could also apply to optimizing structures for weight or strength considerations. Also, industrial designers would get to make those sexy shapes that us tech consumers love oh so much.

    Some other tools I know that do this already are the NX Synchronous Modeling "feature" and the T-splines workbench for Autodesk Fusion 360. From what I've seen, there's potential there to be a new way of designing parts. However, the limitation in free-form is that there's no way as of yet to constrain those points so that the surface is uniform or spherical, etc.

    EDIT: Technically, the definition of the surface control points is already there, so you're not really "hardcoding" dimensions to every point. You're just not letting the CAD system automatically place those points for you. Free-form or explicit modeling just allows the designer to place them.
     
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