Machinery's Handbook, 31st Edition
CAD/CAM
1393
Fig. 5. Perspective Drawing of Three Equal-Size Cubes and Construction Lines An axonometric projection is a 3D perpendicular projection of an object onto a surface, such that the object is tilted relative to its normal orientation. An axonometric projection of a cube, as in Fig. 1, shows three faces of the cube. Today’s CAD systems are adept at using this type of view, making it easy to see an object from any angle. Drawing Tips and Traps.— Exact decimal values should be used when entering point coordinates from the keyboard, if possible. Fractional sizes should be entered as fractions, not truncated decimals. For example, 5 ⁄ 16 should be entered as 0.3125 or 5 ⁄ 16 , not 0.313. Accumulated rounding errors and surprises later on when parts do not fit are thus reduced. However, drawing dimensions should not have more significant digits or be more precise than necessary. Unnecessary precision in dimensioning leads to increased difficulty in production, because the part has to be made according to the accuracy level of the drawing. Snap and object snap commands make selecting lines, arcs, circles, or other drawing entities faster, easier, and more accurate when picking and placing objects on the screen. Snap permits only points that are even multiples of the snap increment to be selected by the pointer. A 1 ⁄ 8 -inch snap setting, for example, will allow points to be picked at exactly 1 ⁄ 8 -inch intervals. It is important to set the snap increment to the smallest distance incre- ment ( 1 ⁄ 4 inch, 1 foot, etc.) being used in the area of the drawing under construction. It also is useful to reset the snap increment frequently. The snap feature can be turned off during a command to override the setting or to select points at a smaller interval than the snap incre- ment allows. Some systems permit setting a different snap value for each coordinate axis. The object snap selection mode is designed to select points on a drawing entity accord- ing to predefined characteristics of the entity. For example, if end-point snap is in effect, picking a point anywhere along a line will select the end point of the line nearest the point picked. Other object snap modes include point, intersection, midpoint, center and quad- rants of circles, tangency point (allows picking a point on an arc or circle that creates a tan- gent to a line), and perpendicular point (picks a point that makes a perpendicular from the base point to the object selected). When two or more object snap modes are used together, the nearest point that meets the selection criteria will be chosen. Create Blocks of Frequently Used Objects: Once created, complete drawings or parts of drawings can be saved and later recalled into another drawing. Such objects can be scaled, copied, stretched, mirrored, rotated, or otherwise modified without changing the original. When shapes are initially drawn in unit size (i.e., fitting within a 1 × 1 square) and saved, they can be inserted into any drawing and easily scaled. One or more drawing elements can be saved as a group element, or block . Block properties vary, depending on the drawing program, but are among the most powerful features of CAD. Typically, blocks are uniquely named and, as with simple objects, may be saved in a file. (Blocks are ideal for creating libraries of frequently used drawing symbols.) Blocks can be copied, moved, scaled, rotated, arrayed, and inserted as many times as is required in a drawing and manipulated in a drawing as one element. When scaled, each object within the block is scaled to the same degree.
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