The ability to read drawings is a necessary part of the boy's education. To know how to use the tools, is still more important. In conveying an idea about a piece of mechanism, a sketch is given. Now, the sketch may be readable in itself, requiring no explanation, or it may be of such a nature that it will necessitate some written description.Fig. 95. Plain Circle
- In drawing, lines have a definite meaning. A plain circular line, like Fig. 95, when drawn in that way, conveys three meanings: It may represent a rim, or a bent piece of wire; it may illustrate a disk; or, it may convey the idea of a ball.
Suppose we develop them to express the three forms accurately. Fig. 96, by merely adding an interior line, shows that it is a rim. There can be no further doubt about that expression.
Fig. 97 shows a single line, but it will now be noticed that the line is thickened at the lower right-hand side, and from this you can readily infer that it is a disk.
Fig. 98, by having a few shaded lines on the right and lower side, makes it have the appearance of a globe or a convex surface.Figs. 96-98.
Shading or thickening the lines also gives another expression to the same circular line.
In Fig. 99, if the upper and left-hand side of the circle is heavily shaded, it shows that the area within the circle is depressed, instead of being raised.
On the other hand, if the shading lines, as in Fig. 100, are at the upper left-hand side, then the mind at once grasps the idea of a concave surface.
The first thing, therefore, to keep in mind, is this fact: That in all mechanical drawing, the light is supposed to shine down from the upper left-hand corner and that, as a result, the lower vertical line, as well as the extreme right-hand vertical line, casts the shadows, and should, therefore, be made heavier than the upper horizontal, and the left-hand vertical lines.Figs. 99-100.
There are exceptions to this rule, which will be readily understood by following out the illustrations in the order given below.
The utility of the heavy lines will be more apparent when drawing square, rectangular, or triangular objects.
Let us take Fig. 101, which appears to be the perspective of a cube. Notice that all lines are of the same thickness. When the sketch was first brought to me I thought it was a cube; but the explanation which followed, showed that the man who made the sketch had an entirely different meaning.
He had intended to convey to my mind the idea of three pieces, A, B, C, of metal, of equal size, joined together so as to form a triangularly shaped pocket as shown in Fig. 101. The addition of the inner lines, like D, quickly dispelled the suggestion of the cube.Figs. 101-104. Forms of Cubical Outlines
"But," he remarked, "I want to use the thinnest metal, like sheets of tin; and you show them thick by adding the inner lines."
Such being the case, if we did not want to show thickness as its structural form, we had to do it by making the lines themselves and the shading give that structural idea. This was done by using the single lines, as in Fig. 103, and by a slight shading of the pieces A, B, C.
|Fig. 105.||Fig. 106.|
If it had been a cube, or a solid block, the corners nearest the eye would have been most pronounced, as in Fig. 104, and the side next to the observer would have been darkest.
This question of light and shadow is what expresses the surface formation of every drawing. Simple strokes form outlines of the object, but their thickness, and the shading, show the character enclosed by the lines. Direction of Light. - Now, as stated, the casting of the shadow downward from the upper left-hand corner makes the last line over which it passes the thickest, and in Figs. 105 and 106 they are not the extreme lines at the bottom and at the right side, because of the close parallel lines.
In Figs. 109 and 110 the blades superposed on the other are very thin, and the result is the lines at the right side and bottom are made much heavier.
|Fig. 107.||Fig. 108.|
|Illustrating Heavy Lines|
This is more fully shown in Figs. 107 and 108. Notice the marked difference between the two figures, both of which show the same set of pulleys, and the last figure, by merely having the lower and the right-hand lines of each pulley heavy, changes the character of the representation, and tells much more clearly what the draughtsman sought to convey.
All drawings are made to a scale where the article is large and cannot be indicated the exact size, using parts of an inch to represent inches; and parts of a foot to represent feet.
In order to reduce a drawing where a foot is the unit, it is always best to use one-and-a-half inches, or twelve-eighths of an inch, as the basis. In this way each eighth of an inch represents an inch. If the drawing should be made larger, then use three inches, and in that way each inch would be one-quarter of an inch.
|Fig. 109.||Fig. 110.|
|Illustrating Heavy Lines|
The drawing should then have marked, in some conspicuous place, the scale, like the following: "Scale, 1 1/2" = 1'"; or, "Scale 3" = 1'."