AutoCAD Training

AutoCAD 3D Solids



Constructive solid geometry is a building-block approach to construct 3D solid models. The system provides tools to construct solids of simple geometric shape as building blocks and provides Boolean operations to combine the solids. Any complex solid model is constructed by combining simpler solids together. The simplest solids are called primitives. The six primitive types available are box, sphere, cylinder, cone, wedge, and torus. See Figure 1.

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Figure 1  Box, sphere, cylinder, cone, wedge, and torus

The repertoire of complex solids that can be composed from these six types of primitive solids is very limited. To extend the scope of model creation, You can make solid objects by extruding a 2D object along a line or a path and revolving a 2D object about an axis. The 2D object can be either a region or a closed polyline. Figure 2 shows a revolved solid, a solid extruded along a line, and a solid extruded along a path.

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Figure 2  A revolved solid and extruded solids

To compose two solid objects together to form a complex solid, you can use one of the three types of Boolean operations union, subtraction, and intersection. A union of two solids creates a solid that encloses all the volume enclosed by the first and the second solid. A subtraction of two solids creates a solid that encloses the volume of the first solid but not that of the second solid. An intersection of two solids creates a solid that encloses the volume of the first solid that is also contained in the second solid. Figure 3 illustrates the effect of these Boolean operations on a solid box and a solid cone.

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Figure 3 Boolean operations

As a further refinement to a solid, you can chamfer or fillet the edge. Figure 4 shows a solid box, a chamfered box, and a filleted box.

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Figure 4  Solid box, filleted box, and chamfered box

Isolines and Silhouette Display

The appearance of 3D solid objects in a computer is affected by the setting of two system variables, ISOLINES and DISPSILH. Isolines are series of lines on the curved surfaces. The ISOLINES variable controls the density of the isolines. If you set it to zero, there will be no isolines. If you set it to a higher value, there will be more isolines. The DISPSILH variable controls the display of silhouette curves of solid objects. It has two settings. If it is 1, the silhouette edges appear. If it is 0, the silhouette edges do not show. Silhouettes provide a better picture of the 3D object. However, regeneration of the display will take a longer time. Figure 5 shows a box, cone, and cylinder displayed with ISOLINES = 5 and DISPSILH = 0.

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Figure 5  ISOLINES variable