CATIA presents an approach to modeling called 'feature-based design'. A feature is a tool that lets you create and shape a solid model. There are many different types of features. A feature-based solid model is a collection of these features, which makes describing the solid much easier because it is broken down into the individual features used to construct it.
CATIA assigns a feature number to each feature making up a solid model, as shown in the Specification Tree.
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The feature order describes the way the part is built; it gives the history of the solid body's construction. In the example above, a pad is created first, followed by a mirror and then a hole.
Definition of Parametric Modeling
Design intent is captured using parameters. Parameters are all of the necessary dimensions that describe the various features in a solid model.
For example, observe the pad and hole combination in the figure.
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The following dimensions describe the size of the pad:
Diameter: 7.9
Height: 25.0
These dimensions are called 'feature parameters'. You can locate the pad relative to the edges of the part. The dimensions that locate the boss are called 'constraints'.
Dimension from edge: 33.3
Dimension from edge: 56.4
These parameters describe the pad in the solid model. When a change to the solid model is necessary, the feature-specific parameters are also modified.
CATIA assigns a name to each parameter. The main defining parameters on the pad in this example are:
PartBody\Sketch.4\Diameter.1\Diameter = 7.9mm (Diameter)
PartBody\Pad.2\FirstLimit\Length = 25mm (Height)
PartBody\Sketch.4\Offset.4\Offset = 33.3mm (Horizontal Position)
PartBody\Sketch.4\Offset.5\Offset = 56.4mm (Vertical Position)
Modifying Feature-Based Models
Specifying design intent in a feature-based model pays off when you need to make modifications. You can make changes to the model very quickly by changing parameters. Explicit models, however, can be very time-consuming to modify.
In order to demonstrate feature-based design modifications, consider a moderately difficult design change. Imagine that most of the design in the model already exists, taking into account fillets and material thickness. Now there is a last-minute change in the design due to the results of a finite element analysis. You determine the wall thickness needs to increase to prevent failure. The size of the fillets must also increase to make the design more aesthetically pleasing.
If the model consists of curves and surfaces, the changes could take many hours to implement. You would probably need to delete and recreate objects. With explicit solid modeling, these changes could also take hours because you need to recreate solids or manually edit faces. In feature-based design, you make the change in minutes by altering the parameters used to define the features.