A dimension is a numerical value expressed in an appropriate unit of measure. It is indicated on drawings along with lines, arrows, symbols, and notes to define the size and specification of an object.
Dimensioning is thus a process of incorporating numerical values onto a drawing to enable the sizing of different elements and the location of parts of a building or object. Drawings should be fully dimensioned so that a minimum of computation is required and all the parts can be built without having to scale the drawings to determine an object's size. Duplication of dimensions should whenever possible be avoided unless it adds clarity. Figure 4.1 shows examples of different types of dimensions.
Distances may be indicated with either of two standardized forms of dimension: linear and ordinate. Linear Dimensions are used for displaying and measuring length along the X or Y axis. As the name suggests it can only be aligned along the X or Y axis. These dimensions are typically used to show absolute lengths along the X or Y axis (Figure 4.1A). With linear dimensions, there are two parallel lines, called "extension lines," that are spaced at the distance between two features and shown at each of the features (Figure 4.2A). The dimension line consists of a line perpendicular to the extension lines; it is shown between the extension lines and terminates at these lines typically with arrows, slashes, or dots. (For examples of different endpoints see Figures 4.2A, 4.2B, and 3.8C.) The distance is indicated numerically at the midpoint of the dimension line, either adjacent to it or in a gap provided for it.
Ordinate dimensions are used for measuring the length along any X or Y axis and displaying length as a text with the use of a leader. Generally, these dimensions are used to show lengths of entities using leader lines (Figure 4.1B).
Radial Dimensions are used for measuring the radius of arcs, circles, and ellipses and displaying it with a leader line. Radial dimensions often use an "R" followed by the value for the radius (Figure 4.3); Diametral dimensions generally use a circle with a forward-leaning diagonal line through it, called the diameter symbol, followed by the value for the diameter. A radially aligned line with arrowhead pointing to the circular feature, called a leader, is used in conjunction with both diametral and radial dimensions. All types of dimensions are typically composed of two parts: the nominal value, which is the "ideal" size of the feature, and the tolerance, which specifies the amount that the value may vary above and below the nominal.
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