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.
[Illustration: Fig. 190.]
[Illustration: 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.
[Illustration: Fig. 192.]
[Illustration: Fig. 193.]