When Designs Are Dim. Driven or Feat. Driven

[mmccall]

Taken from "Machine Design" machinedesign.com

 

Take note of the last statement.

 

 

"The Features of Feature-based Modeling

 

Feature-based modeling refers to the construction of geometries as a combination of form features. The designer specifies features in engineering terms such as holes, slots, or bosses rather than geometric terms such as circles or boxes.

 

Features can also store nongraphic information as well. This information can be used in activities such as drafting, NC, finite-element analysis, and kinematic analysis. Furthermore, feature-based packages frequently record the geometric construction and modification sequences used in building the model.

 

Old method: Define slot as Boolean difference between the part and space.

 

The problem: Lengthening the part geometry turns the slot into a blind hole.

 

Feature method: Through-hole feature understands that it must pass through the part, no matter how the part changes. "

 

 

 

 

"When Designs Are Dimension Driven

 

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 extrude