Machine Design

CAD package pumps up the parametrics

NX CAD software goes one step beyond its already capable parametric features by including what's called "direct modeling."

Stephen Samuel

The surfboard initially had several mounting holes located at the rear. The Simplify command removed holes under the specified diameter of 2 in.

The Move Region command rotated the surfboard fin 30° about an axis located at the front of the fin.

The Local Scale command resizes the fin by a specified factor, in this case, three.

This capability is to solid surfaces as geometric constraints were to lines, arcs, and splines when first introduced years ago. In fact, direct modeling is one of the most important innovations in CAD modeling. It provides parametric modeling — on steroids, so to speak. For one, direct modeling saves the trouble of deciphering overly complex lists of features. It also allows thinking like an engineer as opposed to a programmer and, most importantly, parametrizing previously nonparametric models.

Users can thus alter the geometry of an existing feature and link it to a new variable. For example, users might create a Change Hole Size on the surface of a hole to override its original definition. The resulting feature now has a diameter, height, and location controlled by new expressions. It should be emphasized that NX's Direct Modeling tool works regardless of the part feature-construction technique. Thus, the part could be two united features, an extruded feature, or even imported "dumb" geometry without parametric-feature data, such as IGES and STEP. The command applies to any part feature, not just holes. Other commands in the tool's menu include Simplify, Local Scale, Resize Face, and Move Region. A few examples show how to use the commands.

Move Region lets users select any number of surfaces and apply a number of discrete tweaks. For example, I used it on a geometry model of a simple surfboard with a fin near the back by selecting the fin and rotating it 30° about an axis. The fin is thus modified after it's designed. I've added parametric qualities to an otherwise nonparametric feature. I also used Move Region to move the fin 2 in. to the right. Once users select a feature, the software allows moving it in any direction.

I then used the command Local Scale to resize the fin by a factor of three. The location where the back of the fin contacts the surfboard is a convenient reference point. The software resizes the fin in relation to the rest of the surfboard, which retains its original characteristics. Local Scale also comes in handy when designing parts where thermal expansion must be taken into account. For instance, engineers designing castings must deal with the fact that shrinkage is nonuniform. The tool applies " falsifications" to solid models in anticipation of shrinkage, which results in a more accurate model. Lastly, I use the Simplify command to remove mounting holes located at the rear of the surfboard. The command removes holes under a specified diameter, in this case, 2 in.

NX software comes from UGS Corp., 5800 Granite Pkwy., Suite 600, Plano, TX 75024, (972) 987-3000,

Stephen Samuel is president of Design Visionaries,

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