16 AutoCAD and Its Applications—Advanced Copyright Goodheart-Willcox Co., Inc. Like surface modeling, solid modeling represents the shape of objects, but it also provides data related to the physical properties of the objects. Solid models can be analyzed to determine mass, volume, moments of inertia, and centroid. A solid model is not just a skin, it represents a solid object. Some third-party programs allow you to perform fi nite element analysis (FEA) on the model. Solid model fi les can also be exported for use in stereolithography, rapid prototyping, and 3D printing. These processes can produce a plastic or polymer prototype for analysis and testing. This is discussed in Chapter 14. In addition, solid models can be rendered. Most 3D objects are created as solid models. In AutoCAD, solid models can be created from primitives. Primitives are basic shapes used as the foundation to create complex shapes. Some of these basic shapes include boxes, cylinders, spheres, and cones. Detailed shapes and primitives can be created using 3D mesh primitives, mesh modeling techniques, and surface modeling. A 3D mesh object, which is a type of surface model, can have a free-fl owing shape because the size of the mesh can be adjusted to achieve various levels of smoothness. Mesh objects can be converted to solids for use in model construction. See Chapter 9 for a detailed discussion on 3D mesh modeling. Solid primitives also can be modifi ed to create a fi nished product. See Figure 1-18. Surface and solid models can be exported from AutoCAD for use in animation and rendering software, such as Autodesk 3ds Max®. Rendered models can be used in any number of presentation formats, including slide shows, black-and-white or color prints, and animations recorded to video fi les. Surface and solid models can also be used to create virtual worlds for entertainment and gaming applications. 3D Object Snaps 3D Object Snaps The construction and editing of a 3D model can be more effi cient with the use of 3D object snaps. These work in the same manner as the standard 2D object snaps and can be set using the 3D Object Snap tab of the Drafting Settings dialog box. If you use 3D object snaps, turn on only those options you need to construct the object. See Figure 1-19. The Vertex, Midpoint on edge, Center of face, Perpendicular, and Nearest to face 3D object snap modes should be familiar to you from your work in 2D. The Knot 3D object DSETTINGS Type DSETTINGS DS SE DDRMODES A B Figure 1-18. A—The two cylinders and the box are solid primitives. B—With a couple of quick modifications, the large cylinder becomes a shaft with a machined keyway.