The failed promise of parametric CAD part 3: The direct solution


Pull-PushDirect modeling—a syncretic melding of concepts pioneered by CoCreate, Trispectives, Kubotek (and many others)–has shown the most promise to cure the parametric curse.

Direct modeling is today’s hot CAD technology. PTC, Autodesk, Siemens PLM, Dassault (CATIA, but not so much SolidWorks), IronCAD, Kubotek, Bricsys, SpaceClaim (and certainly some other companies I’ve forgotten) all have their own unique implementations of it.

The common thread in direct modeling is to use standard construction techniques when modeling, and feature inferencing (or recognition) when editing. It’s easier said than done. It’s taken about 35 years of industry research to get to the place we are today—where you can click on a face of a model, and the system will recognize that you’re pointing to a feature that has some semantic value. And that’s not even considering the tremendous amount of work that has been required by legions of PhD mathematicians to develop the math that lets you push or pull on a model face, and have the system actually edit the geometry it in a useful manner.

For the CAD software, figuring out which way to edit a selection is almost a mind reading trick: A user clicks and drags on a part of a model. What would they like to happen? In some cases it’s easy: Drag once face of a rectangular block, and the system will just make it longer or shorter. But if the block is full of holes, bosses, and blends, it becomes a lot more complicated. What should the system do if you drag a face so far back that it consumes another feature, and then pull it back to where it was? Should the consumed feature be lost forever, or should the system remember it in some way, so it can be restored?

There are no right answers. It seems that no two direct modeling systems handle the decision of what is a “sensible” edit in the same way.

While direct modeling absolutely solves the model brittleness problem inherent with parametrics, it does it by simply not using parametrics. Even with hybrid parametric/direct CAD systems, the answer to the parametric curse is still to not use parametrics when you don’t need to.

The solution of “use direct modeling when you can, and learn to live with parametric hassles when you can’t” just isn’t very satisfying to me.

Next: Going horizontal

5 Comments on “The failed promise of parametric CAD part 3: The direct solution

  1. Pingback: The failed promise of parametric CAD part 2: The problem is editing

  2. CADKEY (Now Kubotek KeyCreator) and CoCreate, among a handful of others, pioneered in what’s being called Direct Modelling a long time ago. KeyCreator today has the capability of dimensioning solid models and changing them by changing a dimension. A single dimension can control dimensional change in either of two directions, or bi-directionally, as well as controlling the effect on one or more features within the model and on other components in an assembly. The function name says a lot: Direct Dimension Editing. Coupled with its intelligent feature recognition selection system, it’s a powerful pair of tools for easily creating/editing components or assemblies. It ought to be called Complete Freedom Modelling.

  3. Not all Direct Modelers forgo parametrics. Again, Siemens NX allows for history based or non history based Direct Modeling operations. Additionally, in the history based mode, the values input by the user by dragging, placing dimensions, constraints, etc… are parameters in the history just like traditional modeling operations have always provided. These may be equated, math driven, spreadsheet, data from file, whatever somebody wants to do.

    In the non-history based scenario, parametrics are still present in the form of both “local features” such as a hole or fillet, both of which retain the edit options parametric users expect to see, and in the form of dimensions that operate right on the model, solved variationally.

  4. I have been using Cadkey/Keycreator since Cadkey 5. Direct modeling has proved to me it’s capabilities as free flowing and an aid to the creative process of modeling.

    Recently, our company was purchased by a larger company that has settled on Solidworks for it’s CAD system. The looks of utter contempt by the CAD people there when I expressed my preference for a direct modeler over a parametric CAD
    was palpable.

    I have been labeled a malcontent..good for me!

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