Direct Modeling is Better at Capturing Design Intent

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If there’s one thing I’ve learned over the past five years, it’s that debate gives you new perspectives. Such was the case coming out of a webinar I presented in that was titled The Pros and Cons of 3D Modeling Paradigms: Direct Modeling and History-based Modeling. Brian Thompson of PTC and Dan Staples of Siemens PLM discussed which modeling approaches best fit the concept design and detailed design phases. The whole webinar is good, but the Q&A session at the end is where the real discussion began. That got me thinking about this topic.

What is Design Intent?
Terminology in this industry is difficult. There are often many definitions for the same term. If you’re not careful, you can find yourself in violent agreement with someone because of such confusion.

For the sake of this post, my definition of Design Intent is the dimensional controls and rules that a designer or engineer want to enforce. These critical variables impact the form, fit and function of a design.

An outstanding question, of course, is how many such controls and rules are needed for most designs. I find that most engineers only need a few that really matter. Obviously, this varies by the complexity of the design and the components that are manufactured.

Capturing Design Intent with Parametric Modeling
This is a pretty easy topic to cover, right?

With parametric modeling, dimensions define practically every aspect of a 3D model. Every sketch must be fully constrained. Every extruded, revolved, swept or blended solid or cut is minutely detailed with parameters. Every round, hole, chamfer and whatnot are defined.

The good news is engineers are assured that the dimensional aspect of their design intent is captured in some form or fashion. Let’s be honest, you couldn’t avoid it if you tried. Furthermore, having all those embedded dimensions makes drawing creation pretty easy.

Therein lies the problem. Engineers are forced to define design intent for every aspect of a 3D model. Why is that bad? If you walk away from a model for a month or two, how do you go back and find those controls in the model that represent your critical design intent? It’s like finding a needle in a haystack. Furthermore, we all know that such models can fail easily with even small changes.

Capturing Design Intent with Direct Modeling
You can’t use dimensions in direct modeling, right?

Well, that’s where there is a big misconception. It’s true that geometry creation with direct modeling doesn’t require any parametric controls. When you create 3D geometry, you can use variables to define its extents. Such controls used in direct modeling methods can be persisted, however. You can use them to make changes to geometry long after you initially create it.

That is one of the biggest contrasts with parametric modeling. You don’t have to define a ton of dimensions. That means engineers can only create the dimensional controls that truly represent their design intent. Coming back to such models months later is also very easy. There are far fewer controls to interpret. And where there are no controls, you can use direct methods to push, pull and drag geometry.

What are the cons? Well, you don’t have all those dimensions to automate the creation of drawings. Furthermore, direct modeling can’t approach the complex and progressive kind of intelligent automation that parametric modeling offers. You wouldn’t want to use it for design automation or configuration.

Recap and Questions

  • Parametric modeling forces users to dimensionally define every aspect of a 3D model. This outstrips the design intent needs for most engineers. In fact, it makes it harder for engineers to define and later find their design intent.
  • Direct modeling doesn’t require any dimensional controls. However, users can manually designate parameters that should be persisted. This allows engineers to selectively capture their design intent in the few dimensions that truly matter.
  • Make no mistake: Direct modeling is a poor fit for design automation and configuration. Those tasks are better left to parametric modeling paradigms.

Well, folks, that was one of my big takeaways from The Pros and Cons of 3D Modeling Paradigms: Direct Modeling and History-based Modeling webinar. Am I on target? Am I off base? Do you think direct modeling is better for capturing design intent? Sound off and let me know what you think. Looking forward to it.

Take care. Talk soon. Thanks for reading.

Editor’s Note:  For more on direct modeling, find out how PTC Creo Direct can help organizations achieve faster design cycles by enabling more users to access 3D CAD data. And be sure to download your free trial of PTC Creo Elements/Direct Modeling Express 4.0. The software allows users to create and modify design data in real time; edit work with direct cut-and paste, push-and-pull, and drag-and-drop techniques; and much more.

This blog post has been licensed for hosting by PTC. The concepts, ideas and positions of this post have been developed independently by Industry Analyst Chad Jackson of Lifecycle Insights.

This entry was posted in Creo Technology, Reinventing Design and tagged , , , . Bookmark the permalink. Trackbacks are closed, but you can post a comment.

One Comment

  1. Satish Ramavat
    Posted Mar 17, 2014 at 1:44 pm | Permalink

    I think direct modelling can also b used for design automation like parameteric modeling. Both have its pro-cons related to design automation.

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