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History, Nonhistory, or Both? (MCAD Modeling Column)
1 Mar, 2008 By: IDSA ,Mike HudspethModeling methodologies offer many choices to meet your needs.
When you import a model from someone else's CAD system, what do you expect to be able to do with it? Most people want to work with it and incorporate it into whatever they are doing. But depending on what software you are using, your expectations of the actual usefulness of that imported model will change. Why? Because of something called history. No, it's not about dates and places or somebody in old-fashioned puffed-out pantaloons. It's about modeling history. If you're using a history-based modeler, you'll see imported geometry slightly differently than someone who uses a nonhistory-based modeler.
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It's All History
What is history-based modeling? Simply put, it's the methodology of building your model in a particular order. When built that way, many features in your model will be dependent on those that came before them. Say, for example, you are applying some blends (figure 1). If you have them going around a rounded corner that is too small, they might not fit. The model could self-intersect, and most modeling programs would raise a failure notice. Likewise, if you make changes to your model that violate a relationship, your modeler isn't likely to allow it.
So why use history-based modeling then? Because creating relationships between features in your model also has an upside. You can build intelligence into your model that will be flexible for things such as what-if scenarios. You can organize your models so they make sense and are easy to change (figure 2).
Hard-line purists would tell you that you should always use history-based modeling. They see nonhistory-based modeling as a liability. After all, you can't change it, right? Not necessarily.
 Figure 1. With a history-based modeler such as SolidWorks, the order in which you add features makes a difference. With a nonhistory-based system, it doesn t. |
No History Here
I have to admit that using a nonhistory-based modeler is pretty liberating if you're used to parametric modelers. It can be a lot of work to establish all the relationships that a history-based modeler requires. Sure, history-based modelers have their advantages, but it's so nice to be able to just do what you want to do without regard to what came first — the chicken or the blend, er, egg. When you import geometry into a nonhistory-based modeler, you're essentially generating its topology — the general shape of the data, the outside envelope. Usually little or no additional information is included. So how useful is it? Plenty!
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