The theoretical principles for shade lines, already given in Mechanical Drawing, Part III, cannot be exactly applied to working machine drawings without involving an excessive amount of time and labor. The conventional rule, therefore, has been established that shade lines may be used for all lower and right-hand projecting edges. By "projecting edges" are meant edges of surfaces which are not flush with adjoining surfaces, but which project above them, or are in a plane nearer the eye. Alt views of an object are treated alike, the ray of light casting the shadow being supposed to come from the upper left-hand corner of the drawing. The contour lines of cylinders, cones, and other rounded surfaces, if projecting, are shaded the same as sharp edges.
Shade lines, when used, are for the specific purpose of relieving the flatness of drawing, and represent a purely conventional means of indicating to the eye projecting surfaces, i.e., surfaces which are in different planes parallel to the eye. Whether the surfaces be curved or flat, as long as they are projecting, or in front of other surfaces, is of no moment, for the effect desired is the same for both, namely, the separation of the surfaces. Applications in Practical Work. Few drawing offices allow shade lines to be used on regular detail machine work on account of the extra labor required and the loss of accuracy in the drawing by the use of a wide line. For general or "show" drawings, where the effect of separating the surfaces is desired, thus bringing out more clearly the relation of the parts, the use of shade lines is occasionally permitted. The draftsman should know how to apply them when required, and Figs. 11 to 16 illustrate their use.
Fig. 11 shows the assumed direction of the ray of light in each of the three views. The piece should be inked in with the usual standard width of line, then gone over the second time, making the extra width for the shaded lines on the inside of the proper lines. This leaves the outside measurement of the piece unchanged for possible scaling. Some draftsmen claim that they can make the heavy shade lines as they go along, thus avoiding the second inking, but in the long run it will be found that time will be saved, more uniform lines and fewer blots made, if the process of shading be accomplished by a second inking.
Fig. 11. Direction of Light Rays and Location of Shade Lines.
Fig. 12. Location of Shade Lines for Piece with Hole.
Fig. 12 is similar to Fig. 11 but with a hole instead of a lug, and the difference in shade lines should be noted.
Fig. 13 is the same as Fig. 11 with a round boss and the lower right-hand corner rounded.
Fig. 14 is a plain washer, Fig. 15 a common hexagonal nut.
Fig. 16 is a washer or disk with a shaft in it. The right-and left-hand views are shown to bring out the point that the shaft projecting on the right has its end shaded, while on the left, being flush with the face of the disk, it is not shaded.
Fig. 13. Location of Shade Lines for Rounded Corner.
Fig. 14. Location of Shade Lines for Circular Piece.
Fig. 15. Location of Shade Lines for Hexagonal Piece.