Shading Drawings.
To produce an effect, drawings are shaded; that is, shadow lines about twice the width of the regular line are drawn according to a recognized rule, which always represents the same peculiarity of form in the same way.
In working drawings light lines only are permitted; shade lines are wider than the working lines, and in reading scale measurements the extra thickness of line would make a difference.
Instead of representing the shadow as it is really cast by the object, the edges which cast the shadow are determined, and all the views are treated as if the light came from behind and from the left, downwards, at an angle of 45° to the horizontal line, as shown by the arrows in figs. 245 to 248.
Fig. 245.—Fig. 246.—Fig. 247.—Fig. 248.
The lower and right-hand outlines of projecting parts will cast shadows, and the student should make them of extra width.
Fig. 249: In shading curves, divide the center line as before (see page 146) and describe circles from center, D; these lines are not to be shaded in penciling, but when inking. The figures represent two rings, A and C, and spaces, B and D. The outside of a surface is shaded according to circle 1, and the inside surface according to circle 2. This will give the desired shading, but it makes a drawing incorrect, and therefore shading is not used in working drawings.
This shading is accomplished by inking the circle first with regular width of line; then with the same radius remove the point of compasses from the true center, placing it outside, according to the desired position of the shaded line, and describe an arc of a circle.
In fig. 250 divide center line as before; with the 45° triangle or set square, draw through the center the diagonals shown by dotted lines; and through the points A, B, C, etc., draw the perpendiculars, cutting the diagonals; from the points of intersection draw the horizontal lines, completing the squares.
Fig. 249.
Fig. 250.
Now, take a radius of half an inch, and in the corner of each square draw with the bow-pencil a circular arc meeting the pencil lines exactly; with the bow-pen ink in the arcs first, then ink carefully the lines joining with the arcs; all lines must be of the same width; put in the shade on arcs and lines as in fig. 249; and, finally, erase the pencil lines at corners, etc.