Machinery's Handbook, 31st Edition
1392
CAD/CAM
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Fig. 3. A 3D Surface Patch
Fig. 4. Isometric Drawing Showing Orientation of Principle Drawing Axis
Solid Images: A solid image is the ultimate electronic representation of a part, contain- ing all the necessary information about edges, surfaces, and the interior. Most solid-imag- ing programs can calculate volume, center of mass, centroid, and moment of inertia. Solid images may be sliced or sectioned on the screen to provide a view of the interior. This type of image is also useful for checking fit and assembly of parts. One method for building a solid model is to perform Boolean operations on simple shapes such as cylinders, cones, cubes, and blocks. Boolean operations are used to union (join), difference (subtract one from another), and intersect (find the common volume between two objects). Thus, making a hole in a part requires subtracting a cylinder from a rectangular block, in a process called constructive solid geometry (CSG). The bound- ary representation type of imaging program uses profiles of 2D shapes that it extrudes, rotates, and otherwise translates in 3D space to create the required solid. Solid images provide complete, unambiguous representation of a part, but the programs require large amounts of computer memory. Each time a Boolean operation is performed, the list of calculations to define the model becomes longer, so computation time increases. The greater the precision, the more time is needed for calculations, so compromises may be required to maintain reasonable productivity. Sometimes, combinations of different programs are used to attain a blend of flexibility, accuracy, and performance. Drawing Projections.— Different techniques are used to display objects to give an ac- curate 3D appearance. Several methods are commonly used in CAD drawings. Isometric drawings, as in Fig. 4, can be used to good effect for visualizing a part, be- cause they give the impression of a 3D view and are often much faster to create. Isometric drawings are created in 2D space, with the x and y axes being inclined at 30 degrees to the horizontal, as shown in Fig. 4, and the z axis as vertical. Holes and cylinders in isometric drawings become elliptical. Because of the orientation of the x, y, and z axes, the true length of lines may not be accurately represented in isometric drawings, so dimensions should not be taken directly from a print. Some CAD programs have a special set of pre- defined drawing axes to facilitate creating isometric drawings. In parallel projections , lines that are parallel in an object, assembly, or part being por- trayed remain parallel in the drawing. Parallel projections show 3D objects in a dimen- sionally correct manner. Though the resulting drawing may not appear as realistic as an isometric or perspective view, it can be used for accurate relative and scaled dimensions. A characteristic of perspective drawings is that parallel lines converge (see Fig. 5) so that objects that are farther away from the observer appear smaller. Perspective drawing techniques are used in some 3D drawings to convey the true look of an object, or group of objects. Because objects are not drawn to scale, dimensional information cannot be extracted from a perspective drawings. Some 3D drawing packages have a true perspec- tive drawing capability; others use a simulation technique to portray a 3D perspective.
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