238 AutoCAD and Its Applications—Advanced 238 AutoCAD and Its Applications—Advanced There are a number of workfl ows in AutoCAD available to the 3D designer. The following approaches can be considered depending on the nature of the work or the requirements of a specifi c application: • Creating 3D models as solids, meshes, procedural surfaces, or NURBS surfaces (procedural surfaces and NURBS surfaces are discussed in the next section). • Using Boolean operations on solids to create composite solids. • Slicing composite solids using surfaces. • Converting solids to mesh models. • Converting solids to surface models. • Converting surface models to NURBS surfaces. These are just a few of the possible workfl ows. Editing techniques are also avail- able and often play a signifi cant role in surface modeling. Understanding Surface Model Understanding Surface Model Types Types There are two basic types of surface models in AutoCAD: procedural surfaces and NURBS surfaces. A procedural surface is a standard surface object without control vertices. By default, a procedural surface, when created, is an associative surface. This means that the surface maintains associativity to the defi ning geometry or to other surrounding surfaces. Editing the defi ning geometry of an associative surface, or an adjacent surface in a “chain” of associative surfaces, modifi es the surface. A NURBS surface is based on splines or curves. The acronym NURBS stands for non-uniform rational B-spline. NURBS surfaces are based on a mathematical model and are used to create organic, freeform shapes. NURBS surfaces have control vertices that can be manipulated to edit the shape of the surface with great precision. Unlike a procedural surface, a NURBS surface cannot be created as an associative surface. A third type of surface in AutoCAD is a generic surface. A generic surface has no associative history and no control vertices. The type of surface model created is controlled by the SURFACEMODELINGMODE system variable. The default setting, 0, creates procedural surfaces. If the SURFACEMODELINGMODE system variable is set to 1, NURBS surfaces are created. When creating a procedural surface, the SURFACEASSOCIATIVITY system variable setting determines whether an associative surface is created. The default setting, 1, creates associative surfaces. This system variable has no effect when creating NURBS surfaces. Surface models can be created from either closed and bounded geometry or open profi le geometry. When using modeling commands such as EXTRUDE, REVOLVE, SWEEP, and LOFT, the Mode option determines whether a surface model or solid model is created. Open profi le curves always create surfaces, regardless of the Mode option setting. The advantage to a procedural surface is the ease with which the surface can be created based on common shapes. In addition, working with procedural surfaces allows the designer to take advantage of associative modeling. Based on the design intent, the designer can use profi le curves such as lines, circles, arcs, ellipses, helices, points, polylines, 3D polylines, and splines as the basis for the model. A procedural surface model can then be created using commands such as EXTRUDE, REVOLVE, SWEEP, LOFT, or PLANESURF (as discussed in previous chapters). If created as an associative surface, the model is linked to the defi ning geometry and can be modifi ed by editing the geometry. The commands used to create surface models are located in the Surface tab of the ribbon. See Figure 10-1. As discussed in Chapter 6, selecting a command from the Create panel in the Surface tab automatically sets the model creation mode to