Читать книгу Computer Aided Design and Manufacturing - Zhuming Bi - Страница 74

A solid object consists of (i) geometric elements at different levels, from vertices to edges, faces, volumes, and features, and (ii) the topological relations of geometric elements. Geometric modelling is used to create computer representations of solid objects, while geometric modelling lays the foundation for using computer aided techniques. The commonly used modelling methodologies include wireframe modelling, surface modelling, space decomposition, constructive solid geometric modelling, and feature‐based modelling.

The information of geometric representation of an object from different modelling methods may be different. A wireframe model consists entirely of points, lines, and curves; it does not have the information for face or volume, and no topological data are needed in modelling. A surface model stores the topological information in corresponding objects, but a surface model can still be ambiguous in some cases. Both a surface model and a solid model can be utilized to identify a shading area and a solid model has complete, valid, and unambiguous spatial addressability. In addition, a lower‐level model (i.e. a wireframe) can be extracted from a high‐level surface or solid model.

The same geometry can be modelled in different ways. However, it is desirable to take into consideration the design intents in geometric modelling. Design intent is beyond the size and shape of features and can be extended to manufacturing features such as tolerances, manufacturing processes, and design constraints. The use of design intents is an effective approach to build a parametric model of a part that is fully constrained and easy to modify. By far, feature‐based modelling is the most advanced method, where a part model is modelled as a set of features as well as their topological relations. Modern computer aided software tools support feature‐based modelling methods.