Part 25

It is not always possible to get a view against a face or side of the object, but, with irregularly shaped bodies or under special conditions, it may be necessary to take a view of corners, sloping planes, curved or irregularly shaped surfaces.

Fig. 231 shows a hexagonal nut in the three normal projections; from the top view it is readily seen that the front view shows the side of the hexagon in a contracted scale, and that therefore the scaling and dimensioning for the horizontal direction have all to be done in the top or plan view; the front and side views convey, however, the dimension of height correctly, and these are therefore the right places for scaling and dimensioning in the vertical direction.

Fig. 231 shows how a cylindrical outline appears in the three views; the hole in the nut presents itself as circular in the top view, while it appears in the front and side views as a rectangle. For simple objects, it is unnecessary to show the edges of the planes, and the three views are grouped, as regards distances and positions, as most convenient for the execution of the drawing.

Where sloping surfaces are of irregular form, it may be necessary to employ help lines for their full determination in the three views. Fig. 232 shows how the surface that is produced by a slanting cut through a cylinder would appear in the three views. The help lines are placed in the top view in eight equal divisions around the circumference of the cylinder. These division lines are shown by dotted lines on the cylinder in the front and side views. Their intersections, with the sloping cut in the front view, furnish also the height of the corresponding points for the side view.

By progressive determination of points, lines and surfaces, even the most complicated bodies can be completely represented for their reproduction in any application to mechanical or industrial purposes.

The spur wheel shown on page 163 is an example of projection drawing.

The wheel is illustrated in three views: fig. 235 is a side view, or elevation; fig. 234 is a front elevation; fig. 233 is a section view on C D. The section is projected from the front elevation by drawing parallel lines from the points in front elevation where the lines are intersected by the center line C D, cutting the plane of view and showing the interior shape at C D.

The side elevation, fig. 235, is projected from the front elevation, fig. 234, by drawing parallel lines from the edges in the front view across its face.

In actual drawing practice the figures should be made about three times larger than the example, and as follows:

For the front elevation draw the center lines A B, C D, and from their point of intersection as a center, with the compasses draw the inner circle or hole, also the pitch line E E. With the dividers space this line into nineteen equal divisions; each point on this line will be the center of a tooth, and the distance from one point to the next one is the pitch of the tooth.

Now, with the dividers mark off the thickness of tooth at each side of these points on pitch line; with the compasses draw the outer circle for points of teeth, the inner circle for the root of teeth, and circles for thickness of rim and hub; also circles representing the fillets at rim and hub. For clearness and to prevent confusion these lines are shown on one-half the wheel only, terminating in center line C D; all the lines of the front elevation are now complete excepting the teeth.

In the drawing office it is unusual to delineate all the teeth in a gear wheel, the lines without the teeth as now completed being deemed sufficient, giving all particulars; however, to prevent the error of mistaking the circles it is well to represent one or two teeth on the drawing.

In the example, proceed and complete the entire wheel, taking on the compasses a radius equal to the pitch of the tooth; set them on the pitch line at the point G, already spaced for thickness of tooth, draw line H from pitch line to root of tooth; proceed similarly all around the circle, completing one side. Next, reverse the operation and draw the corresponding side of the root of tooth. Now take a radius equal to half the space between teeth and the thickness of tooth on pitch line, and, with center in pitch line, as shown at I, draw the outside or addendum of tooth, J; it will be apparent that the reverse addendum of the tooth next adjoining can be formed at the same time, with one setting of the compasses; finish all the teeth similarly; the front elevation will thus be completed.

Fig. 233 is an excellent example of sectional drawing, to be executed as follows:

First draw the center line L M of fig. 233, lay off at each side of it half the breadth of face and of the hub; now project, from center line C D, in the front elevation, fig. 234, with the T-square the upper and lower teeth, the fillets, the chamfers, the hub and center hole, also dot in the pitch line F F, take the radii and draw the fillets and chamfers; draw section lines and the view will appear as shown in fig. 233.

Fig. 235 is a side elevation of fig. 234, and is a fine sample of projection, to be executed as follows:

Proceed similarly as for fig. 233, projecting the lines from the outside edges of the front elevation, instead of from the center line C D as in last figure, and the end elevation will be as shown in fig. 235.

The student will be assisted in understanding this working drawing by consulting the pages under the heading of “Gearing.”

SECTION LINING]

“Inking In” Drawings.

When a drawing is completely finished in penciling, it should next be “inked in” for preservation.

Care should be used that the pen may be perfectly clean; the pen should be held nearly vertical, leaning just enough to prevent it from catching on the paper; the pen should be held between the thumb and first and second fingers, the knuckles being bent, so that it may be at right angles with the length of the hand.

The ink should be rubbed up fresh whenever it is about to be used, for it is better to waste a little time in preparing ink slowly than to be at a continual trouble with pens, which will occur if the ink is ground too rapidly or on a rough surface.

To test ink, a few lines can be drawn on the margin of a sheet, noting the shade, how the ink flows from the pen, and whether the lines are sharp. After the lines have dried, cross them with a wet brush; if they wash readily, the ink is too soft; if they resist the water for a time and then wash tardily, the ink is good.

Care must be exercised not to overload the pen with ink, and, like the pencil, the pen should always be moved from left to right and from the bottom to the top of the board. When inking, both “nibs” of the pen point must rest evenly on the paper and the pen be pressed only lightly against the T-square. Never ink any portion of a drawing until the penciling is complete.

In inking long, fine lines it is well to go over each line twice, without moving the T-square, trying not to widen the line on the second passage; also see that the pen contains ink enough to finish a line, as it is difficult to continue with the same width of line after re-filling.

To produce finished drawings, it is necessary that no portion should be erased, otherwise the color applied will be unequal in tone; thus, when highly finished mechanical drawings are required, it is usual to draw an original and to copy it. Where sufficient time cannot be given to draw and copy, a very good way is to take the surface off the paper with fine sand-paper before commencing the drawing; if this be done, the color will flow equally over any erasure it may be necessary to make afterwards.

The rules of procedure in drawing the lines in “inking” are, 1, ink in the small circles and curves; 2, ink in the larger circles and curves; 3, then all the horizontal lines, beginning at the top of the drawing and working downward; 4, next ink in all the vertical lines, commencing at the left and moving back to the right; 5, draw in the oblique lines; 6, all the center lines and dimension and reference lines. The figuring and lettering should be always done with India ink, thoroughly black; the last lines to be drawn are the section lines. The reason why irregular curves and arcs of circles are inked in first is, that it is easier to draw a straight line up to a curve than to take a curve up to a straight line.