Part 21

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.

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.

DRAWING-PENS.

A special pen called a drawing-pen, and also special ink, are required to ink a drawing; figs. 194 and 195 represent two sizes of drawing-pens--one being best adapted for fine work, and the other for coarse or heavy line work. The points, as will be observed in the illustration, are made of two steel blades which open and close as required for thickness of lines by a regulating screw.

A good drawing pen should be made of properly tempered steel, neither too soft nor hardened to brittleness. The nibs should be accurately set, both of the same length, and both equally firm when in contact with the drawing paper. The points should be so shaped that they are fine enough to admit of absolute control of the contact of the pen in starting and ending lines, but otherwise as broad and rounded as possible, in order to hold a convenient quantity of ink without dropping it. The lower (under) blade should be sufficiently firm to prevent the closing of the blades of the pen, when using the pen against a straightedge.

The spring of the pen, which separates the two blades, should be strong enough to hold the upper blade in its position, but not so strong that it would interfere with easy adjustment by the thumbscrew. The thread of the thumbscrew must be deeply and evenly cut so as not to strip.

An important requisite after the pencil lines have been put in is ink, with which to line the drawing. This should be of the best that can be procured. The pen is filled by dropping the ink between the blades, or nibs, while held in a nearly vertical position, as shown in fig. 196.

Liquid India ink can be procured in bottles with glass tube feeders, which are very good, and keep the hands and fingers free of the ink. Fig. 197 is a sectional view of such a bottle and “filler,” or feeder. This generally answers all requirements, but the dry ink of good quality, in sticks or bars, cannot be surpassed, although it requires skill for its preparation. Fig. 198 represents a sloping dish or “tile” for mixing, which should be done with little pressure, in clean, filtered or distilled water, care being taken to keep the liquid free of dust, which obstructs the free flow of the ink in the pens.

The bars of India ink are shown, as they are imported, in figs. 199 to 202.

Pure India or China ink is only made in those countries, because the special wood from which it is prepared is found only in those regions. So-called India inks, made of lampblack and animal glue, are only imitations; therefore India ink should be purchased from a reliable importing house--shape is little guarantee of quality.

Soft gray vulcanized rubber (fig. 203) should be used for cleaning drawing paper; for erasing any portion of a line in pencil, a piece of prepared white vulcanized rubber is the best, small in size and of rectangular shape (see fig. 205).

An ink eraser is made of a composition of rubber and ground glass, and it should be used as sparingly as possible on drawings, as it roughens the paper and removes the gloss from its surface (see fig. 204). Steel ink erasers are useful in removing defects, overrun lines, joint of lines if swollen, etc.; they have a fine point and can be used to advantage with a little practice; they are used with a scratching, not a cutting, motion (see figs. 206, 207).

DRAWING PAPER.

The first thing to be considered in selecting drawing paper is the kind most suitable for the proposed plan. Paper may be purchased in sheets 22 × 30 inches, that make four exercise sheets 11 × 15 inches; this may be of several grades and tints.

The qualities that constitute good paper are strength, uniformity of thickness and surface, neither repelling nor absorbing liquids, admitting of considerable erasing without destroying the surface, not becoming brittle nor discolored by reasonable exposure or age, and not buckling when stretched, or when ink or color is applied.

The sizes and names of paper made in sheets is as follows:

Cap 13×17 ins. Demy 15×20 „ Medium 17×22 „ Royal 19×24 „ Super Royal 19×27 „ Imperial 22×30 „ Atlas 26×34 „ Double Elephant 27×40 „ Antiquarian 30×53 „

For large drawings paper is made in rolls. “Detail paper” is especially made for marking out new designs; it is made in rolls 36, 42 and 54 inches wide; it has excellent erasing qualities and takes ink and color with facility.