DRAWING SCALES.
Scales are proportioned rules or mathematical instruments of wood, metal, etc., on which are marked lines and figures for the purpose of measuring sizes and distances. It is usual to make scales in the proportion of parts of an inch equalling a foot; the most generally adopted scale for machine drawing is one and a half inches, equalling one foot; that is, twelve-eighths of an inch (each eighth of an inch representing one inch); there is no fixed rule in the choice of a scale, as they are varied according to the coarseness or fineness of the parts of the machine to be drawn and the space or surface of paper to be utilized.
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.
[Fig. 190] shows such a scale broken. An explanation of the 1″ and 1⁄2″ side will suffice for all. Where it is used as a scale of 1″ to one foot, each large space, as from 0 to 12 or 0 to 1, represents a foot, and is a foot at that scale. There being 12″ in one foot, the twelve long divisions at the left represent inches; each inch is divided into two equal parts, so from 0 to one division at the left of 9 is 91⁄2″ and so on. The 1″ and 1⁄2″ scales being at opposite ends of the same edge, it is obvious that one foot on the 1″ scale is equal to two feet on the 1⁄2″ scale, and conversely, one foot on the 1⁄2″ scale is equal to six inches on the 1″ scale; and 1″ being equal to one foot, the total feet in length of scale will be 12; at 1⁄2″ to 1 foot the total feet will be 24.
In working to regular scales, such as 1⁄2, 1⁄8, or 1⁄16 size, a good plan is to use a common rule, instead of a graduated scale. There is nothing more convenient for a mechanical draughtsman than to be able to readily resolve dimensions into various scales, and the use of a common rule for fractional scales trains the mind, so that computations come naturally, and after a time almost without effort.
The protractor shown in [fig. 193] is an instrument for laying down and measuring angles on paper; it is used in connecting with a scale to define the inclination of one line to another.
Protractors have the degrees of a half circle marked upon them; as the whole circle contains 360 degrees, half of it will contain 180, one-quarter 90, etc. Hence, protractors showing 180° exhibit all that is needed. To protract means to extend, so this instrument is also useful in “extending” the lines of inclination at the circle.
Fig. 192.
Fig. 193.