Machine Design
When Designs Are Dimension Driven

When Designs Are Dimension Driven

The ability to construct accurate, easily modified models is helping make manufacturers more competitive.

Dimension-driven design refers to a collection of solid-modeling capabilities that include variational, parametric, and feature based. Many solid-modeling packages today support elements of all three.

Feature-based modeling has, among engineers, rapidly become the preferred method of constructing solid models. In feature-based packages, solid models are constructed from geometric features such as slots, shells, bends, drafts, rounds, and so forth. The alternative is to construct models using mathematical geometric entities such as unions of spheres, cylinders, and boxes. One advantage of features is that they provide dimensions that correctly define how the feature behaves when dimensions change.

The classic example of this property is that of a hole drilled through a plate. In a feature-based modeling system, the geometry has enough embedded intelligence to know that the hole should go all the way through the plate, regardless of how thick the plate is. Thus, even if the designer decides to increase the plate thickness tenfold, the hole will still go through to the other side. In a model defined with older solid geometry schemes, the designer would have to manually lengthen the hole if the plate became thicker. Otherwise, the hole would stop within the plate. The formal way of referring to this property is to say that a feature is capable of producing many different geometric instances, depending on the dimension values that the designer spells out.

The most important aspect of feature-based techniques is that they capture design intent. In the drilled hole example, the designer intended to put a hole through the plate. This intent was maintained regardless of what changes were made in the plate dimension.

Another important property of feature-based modelers is the ability to let a feature reference the geometry of various models in an assembly. This referencing allows changes made in one model to propagate to other affected models. One example is where a metal housing has features that are dimensioned from other parts mounted to the frame. When these parts move or change shape, the housing updates as well.

Feature-based models have been likened to a recipe approach to building solid models. Once a design has been specified, it is possible can be created in simple tables. This offers the opportunity to create generic designs. For example, adding a macro program that prompts for inputs would enable the creation of a customized solid-model assembly, complete with drawings and related data such as a bill of materials, cutter paths, and so forth.

The variational approach couches the design in a mathematical model such that whenever the designer makes a change, the package recalculates the entire model. This capability makes for a flexible system and is most useful in early design stages where relationships between geometric constructions can change drastically.

Variational sketching is a capability used to let designers turn 2D profiles into 3D models. Profiles typically represent cross sections of extruded objects or sections used for complicated skinning and lofting operations. The designer typically creates only the part shape with no regard for final dimensions.

A separate facility allows the capture of geometric constraints for the profile. The package infers construction rules from the sketching process, letting the system capture and maintain design intent. Using a rubber-banding approach, geometric inferences are verified before the profile becomes part of a database. These constraints include relationships such as perpendicularity, tangency, and colinearity.

Parametric methods depend on the sequence of operations used to construct the design. The software maintains a history of changes in specific parameters. The point of capturing this history is to keep track of operations that depend on each other, so that, whenever it is told to change a specific dimension, the system can update all operations referenced to that dimension. For example, a circle representing a bolt hole may be constructed so it is always concentric to a circular slot. If the slot moves, so does the bolt circle. Parameters are usually displayed in terms of dimensions or labels and serve as the mechanism by which geometry changes. The designer can change parameters manually by changing a dimension, or by referencing them to a variable in an equation that is solved either by the modeling program itself or by external programs such as spreadsheets.

Parametric modeling is most efficient in working with designs where changes are likely to consist of dimensional changes rather than grossly different geometries.

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