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A Primer on MCAD Modeling Technology, Part 2: Design Intent is Not Necessarily in the Eye of the Beholder

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Full article is available for a fee

Paul Hamilton, July 31, 2007

A key difference between history-based and history-free CAD technologies (the latter is sometimes now called dynamic modeling or direct geometry editing) is in how the two systems capture and utilize design intent. In the context of 3D mechanical CAD, the term “Design Intent” refers to things such as parameters, constraints, dimensions, relationships and features that are added to the 3D model. Because of the nature of the CSG tree (see the first article in the series for details), Hybrid modelers make it easy to capture and store information about the geometry in their history tree. In order for a CSG or Hybrid modeler to work, it has to have this information. Through some great marketing by a few CAD companies, we were led to believe that history-based (hybrid) modeling is the only way to add this “intelligence” to 3D geometry in such a way that would allow, or disallow, modification based on our design intent. In reality, CSG and Hybrid modelers won’t work without it. This is not to say that Hybrid is the only technology that will allow for intelligence to be added to the geometry. It just provides the simplest method of doing so -- but this is quickly changing.


An important element in CSG and Hybrid modelers is the “feature,” a misleading term. For the most part, the term “feature” in the context of Hybrid modeling refers to a primitive in the tree.

For the users of history-free systems, features or primitives are not created with each modeling step and are not maintained automatically by the system. A “feature” is something that a user can define at any time by collecting faces into a group. These groups of faces can be named and are stored with the model. The groups/features can be edited anytime to keep up-to-date with design intent as the design matures. Features in this context can have significant importance to the design, rather than simply being a result of the modeling steps used to create the model.

Parent/Child Structure

Another important relationship captured in the Hybrid modeler is the parent/child relationship. You can’t avoid this relationship in these systems, so you must learn to manage it properly. There are a few advantages to having a parent/child relationship that history-free systems will never have.

There is one last thing to consider in the context of geometric relationships. You may have varying requirements depending on your business, your product development process, and the lifecycle of your products. For some, it may be important for relationship to remain constant while geometry changes (i.e. family-of-parts), for others it may be important for relationships to change while geometry remains constant -- something a history-based system struggles to manage. Users of history-based systems will find themselves re-modeling the part occasionally for this very reason -- the built-in relationships no longer fit the design intent.


  • What You See Is Not Always What You Get
  • Working with a History-Free Modeler

The full article is available for a fee at CADCAMNet.

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