This article is based on a presentation at the 2002 PTC/USER World Event in Atlanta.

The technique outlined in this article allows you to drive a complex model with a series of 2D curves. It was developed while modeling complex sheet body stampings for a truck cab, a project that involved the need to combine design intent, a robust modeling scheme, and fully dimensioned drawings. The following text and examples illustrate how you can use 2D curves for more than just shaping and trimming surfaces.

Capturing Design Intent

Many models, especially in a conceptual mode, ramble along until some semblance of the design begins to come together. As in most design development, several iterations are necessary to finetune the model. But when this happens, all direction and integrity are lost. We have all seen projects where the final conceptual design has to be re-modeled to pass model check criteria, office standards, or even good modeling practice. And if there are weak constraints or redundant features, even the re-model can take a lot of effort.

Creating 2D curves of the desired part as the first set of features in the model, however, clearly establishes design intent up front. These primary curves are referred to here as “body curves.” By using multiple body curves in different planes, you can define the part. The sketches are then tied together through constraints and relationships, which establish the parent/child relationships of the curves.

Once you rename the curves with a descriptive title, a quick look at the model tree reveals which ones you need. This approach also provides a roadmap for downstream users to follow for changes or modifications. The body curves only need to have the essential driving dimensional information and the features they control. You can omit hole locations, fillets/rounds and other minor features to improve clarity and usability. Additional curves can be inserted any time during model construction.

Robust Modeling

By referencing the body curves, the construction of the surface, protrusion, or sheet metal first wall is driven by the body curves. This aids in refining the model, since you can quickly identify, modify, and regenerate the curves. Regeneration of a simple curve instead of a solid/surface feature quickly shows where a part might fail because of a modification. You can quickly make various design iterations simply by adjusting the set of body curves. The body curves can be projected to a surface for trimming (referred to as “trim curves”) and are fully controlled from the parent body curves.

When it comes to making drawings, some dimensions do not show up the way they are intended to. In fact, sometimes features are not modeled the way the dimensions need to reflect, requiring “created” dimensions on the final drawing. Most office standards require the usage of “shown” dimensions and getting them on the drawing can be difficult, whether your model allows it or not.

With the 2D curve-driven solid model, the dimensions defining the curve are those that appear on the drawing. Case in point: if you define two surfaces that intersect and are merged together, chances are you cannot define the intersection dimensionally. By establishing the intersections as a known that can easily be tweaked later, the dimensions forming that intersection will show up on the drawing because the 2D body curve is fully defined and the surface is driven from that set of body curves.

Increased Surface Manipulation

I always use the 2D-curve technique for creating surfaces. The body curves offer control over the design and the utility of a Thinkpad. To be able to modify a simple curve and drive some pretty complex shapes is really quick and I must admit, it is pretty cool!

(Click here for more tips on using curves for surface creation...)

How to Do It

Here’s a simplified list of steps required to create the 2D curves to drive complex models:

1. Sketch the body curves for the part.
2. Rename the curves for quick identification.
3. Create the first protrusion, sheet metal wall, or surface using the required body curves.
4. Make the required cuts in solids by referencing the body curves.
5. For surface trimming, use curves projected from the body curves.
6. Insert more body curves with the other body curves as the design progresses.