When objects are of moderate proportions they may be represented full size; but when large, the drawings must be smaller. Standard scales for mechanical drawings are 1⁄2, 1⁄4, 1⁄8 and 1⁄16 full size. These scales are often written 6″ = 1 ft.; 3″ = 1 ft.; 11⁄2″ = 1 ft., and 3⁄4″ = 1 ft.
Fig. 190.
Fig. 191.
Instead of selecting one of the scales named or one found upon the ordinary scales used by draughtsmen, drawings may be made to any scale whatever. Thus, if any object is to be represented in a certain space, a scale should be constructed which will cause the whole of the object to be shown.
Drawing to Scale.—The meaning of this is, that the drawing when done bears a definite proportion to the full size of the particular part, or, in other words, is precisely the same as it would appear if viewed through a diminishing glass.
The two-foot rule shown in [fig. 192] is the most useful instrument for the comparison of linear dimensions—it can be used as a scale of one-twelfth, or 1 inch equal to a foot, 12 inches = 12 feet, it being divided into portions or spaces, each of which is subdivided into halves, quarters, eighths and sixteenths; frequently in the latter class of two-foot rules there are graduations of scales, and it is then also called a draughting scale.
[Fig. 190] represents a flat scale, graded so that one inch represents a foot—1⁄12th size—etc., as shown.
[Fig. 191] represents a triangular scale (broken). The triangular scale should read on its different edges as follows: Three inches and 11⁄2″ to one foot, 1″ and 1⁄2″ to one foot, 3⁄4″ and 3⁄8″ to one foot, 1⁄4″ and 1⁄8″ to one foot, 3⁄16″ and 3⁄32″ to one foot, and one edge read sixteenths the whole 12″ of its length.