POINTS TO BE OBSERVED IN SKETCHING.
1. Especial attention is to be paid to outlines—edges of plane surfaces are lines; when a line is made it represents the edge or outside of something.
2. Learn to be accurate before being rapid.
3. A sketch should be intelligible to any one, even if they are unacquainted with drawing.
4. Horizontal and vertical lines and a few curves will enable one to make almost any simple sketch.
5. It must be also remembered in making drawings from actual measurement that the instruments are not in the first place employed; the rough sketch is first made and then it is converted into a drawing. The draughtsman makes a rough sketch entirely by the hand and eye, measures the various parts, and jots down the measurements in his sketch; after this he reduces the whole to the desired scale, and proceeds to make his mechanical drawing.
6. Let the sketch book be the constant companion of the student; it may be advantageously filled with outlines of machine or other work suitable for preservation, to be made into finished drawings, or for reference. Sketches are often valuable for reference as aids in originating new designs.
7. A sketch, when possible, should have all the dimensions written upon it, but—
8. Sketches in shop practice should not take the place of working-drawings; the latter have a check upon them in being drawn to a scale—hence the figures written upon them and dimensions by scale must agree.
9. Place title and date on each sketch—no matter how seemingly unimportant—for future reference.
10. Practice sketching at every favorable opportunity. There is no necessity for detail at first—simply the outlines of the article and its parts.
11. Sketch-books, with paper bound in cloth covers, are utilized for bold, off-hand sketches by experienced draughtsmen, but a single sheet of paper, used on both sides, is not unworthy of service in an emergency—or even the blank side of a letter may be available. Sketching-blocks, or paper “pads,” 4 × 6, or more, in size, and containing 48 sheets, are sold by stationers, and are found to be most convenient to have in hand and for practical use. Portfolio-envelopes, made of extra length paper (manila) are useful in filing away sketches and drawings. The size 101⁄2 × 15 is used for United States Patent Office drawings.
The function of the pencil—in mechanical drawing—is to make a path for the pen to follow. If it were possible to make a drawing with all its lines ending at the proper place, at the first time, there would be no necessity for using the pencil. One is obliged, however, so to use the pencil that all lines pass beyond the actual ending place, thus making a distinct point for the drawing pen to stop at.
The pencil should be pressed to the paper just enough to make a clean, fine line, and no more; once over the path is sufficient, if the line is visible and true.
To sharpen drawing pencils, 1, use a fine file, after taking off enough of the wood with a knife; 2, make a conical point for the free-hand drawing pencils and a chisel point for ruling and marking distances.
Pencil compasses are instruments where one leg is provided with a pencil point. [Fig. 209] shows the mode of manipulation of those shown in [fig. 180] and [fig. 181].
The pencil compass is held by the projection above the joint between the thumb and first finger, which enables it to be rotated by a movement of the finger without causing any undue pressure on the points. Should much pressure be applied, there is a tendency to force the point or center through the paper, making an ugly center mark; at the same time the pressure tends to break off the pencil point.
These few illustrations from [fig. 212] to [fig. 219] are made designedly simple, so that they may be utilized in “free-hand” work, for which they are good practice, as well as serving for examples in mechanical drawing.
Figs. 212.
[Figs. 212], showing a spool or bobbin, exhibit three views, viz.: front elevation or plan, and section; both are drawn with simple lines, the end elevation by circles.
Fig. 213.—Fig. 214.
[Figs. 213 and 214] are two side views of a hexagon head bolt. [Figs. 216 and 217] are a square head bolt—two side views and end view. [Figs. 218 and 219] are a front and edge view of a forked or double joint.
Figs. 215.
[Figs. 215] show three views of a file handle; the front view and section are practice for compound curves and curved lines meeting straight ones; all these are capable of being produced by instruments.
Moreover, many of the views and illustrations used to instruct and explain machine tools and other devices in other parts of the volume, are drawn so that they may be used also as examples in advanced instrumental practice. This is a “hint” to the diligent and painstaking student worthy of remembering.
Fig. 216.—Fig. 217.—Fig. 218.—Fig. 219.
The designing and drawing of arcs and whole circles occupy a large proportion of space in nearly all mechanical drawings. The making of a complete circle is a matter of no great difficulty, but the beginning and termination of parts of circles require both judgment and considerable practice.
To aid the student these two illustrations of circles are introduced. To draw [fig. 220] with a pencil, using the upper edge of the blade of the T-square as a guide, draw a center line, A B, mark on it a distance of 4 inches, space this into half inches, using the dividers and making the points with it; then with the pencil compasses or bow pencil, which must be held as shown on page 209, and rotated from left to right, or clockwise, draw a series of circles through these points, tangent to one another or all touching at A, care being taken that the pencil lines exactly meet at A, and also cut the divided points as shown in the illustration. For [fig. 221] divide the center line as before, and draw the semi-circles on it A B, B C, meeting at B, and C D, D E, etc.
Fig. 220.
Fig. 221.
Now from center B draw circles A C, C E, meeting in C, and so on with the circles, arcs or segments; success in drawing this figure depends on the correct spacing of the center line in the first instance into equal parts.
The T-square should be used for drawing horizontal lines only. Its head should always be placed upon the left edge of the board. Vertical lines should be drawn by the use of a triangle placed upon the T-square and not by means of the T-square only; because the edges of a board are seldom at right angles to each other, and the blade of the T-square is often not at right angles to the head, so that lines at right angles to each other will not result from the use of the T-square upon all edges of the board. Only the upper edge of the T-square should be used, as the edges are often not quite straight or parallel.
The 45° triangle has two angles of 45° and one of 90°. The 30° and 60° triangle has an angle of 30°, one of 60°, and one of 90°. By placing these triangles upon the T-square, lines at any of these angles with a vertical or horizontal line may be drawn.
Drawings finished in ink are much more effective and desirable than pencil drawings; but as a good inked drawing cannot be made except upon an accurate pencil drawing, students should begin with the pencil, and should not use ink until they are able to produce satisfactory results in pencil.