Surface Modeling Overview

When creating parts, Mastercam lets you create three different types of surfaces. Surfaces are created through functions on the Surfaces tab. Each type of surface describes a method that the system uses to calculate and store mathematical data about the surface. For most surface options, you can choose from all three surface types. However, for Coons, Swept, and Blend surfaces, you can choose only Parametric and NURBS.

The parametric method of storing surface information is analogous to the parametric method of storing spline information. However, a parametric surface expands each curve segment in a second direction resulting in a patch. A patch is an area of the surface bounded by four segments of generating curves. In addition, parametric surfaces have the following characteristics:

  • Compatible with IGES
  • Not associated with their generating curves
  • Require a large amount of data storage

The NURBS method of storing surface information is analogous to the NURBS method of storing spline information. However, a NURBS surface expands the string of control points in a second direction resulting in a grid. In addition, NURBS surfaces have the following characteristics:

  • Require less data storage than parametric surfaces, but with slightly more computing time
  • Compatible with IGES conversion

  • Can be output to VDA file format (if created in Mastercam)

    NOTE

    Due to limitations with the VDA file format, surfaces that you bring in from another source using the File, Converters function can be output to VDA only if they are bicubic (mathematical degree 3 by 3). You can analyze a surface to determine if it is bicubic.

  • Not associated with their generating curves

When the system stores a surface as a curve-generated surface, it stores a direct reference back to the original curves. In addition, curve-generated surfaces have the following characteristics:

  • Require less data storage than parametric or NURBS surfaces
  • Associated with their generating curves
NOTE

Curve-generated surfaces store exact data about their generating curves (rather than approximations).

Surface Associativity

Association refers to the dependent relationship between one entity and a second entity or group of entities from which the first entity is generated. To understand the relationship between dependent entities, think of the original, or generating, entity or entities as the "parent" and the resultant, or generated, entity as the "child," which depends on the parent for its definition. Surface association occurs between the following entities:

  • Between curve-generated surfaces and their reference curves
  • Between offset surfaces and their generating surfaces
  • Between trimmed surfaces and their generating (base) surfaces
  • Between surface curves and the surfaces on which they lie

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