The student should understand the drawing of the threads, as previously explained; and every draftsman should be able to draw the true projection of a thread if he should have occasion to do so. It is evident however, that the process is complicated, and on a screw of small diameter and pitch it would be difficult to follow out the construction. To avoid this labor the construction shown in Figs. 73, 74, and 75 may be adopted, straight lines being substituted for the projections of the helix. Fig. 73 shows the conventional representation of the plain, single, right-hand V thread, the true projection of which was shown in Fig. 62. To make the conventional drawing (Fig. 73), draw the parallel lines AB and CD at a distance apart equal to the outside diameter of the screw, and draw the line AC perpendicular to these two lines. Along A, lay off the distances AE, EF, etc., each equal to the pitch. Along CD, lay off CH equal to ½ the pitch; and from H, lay off HI, IJ, etc., equal to the pitch. Draw lines from A to H, E to J, etc. Now, if the depth of the thread AS is known, draw the lines ST and UV; and beginning at L, perpendicularly under a point halfway between A and E, lay off LM, MN, etc., equal to the pitch. In like manner find the points 0, P, R, etc., and draw the lines LO, MP, etc.; also AL, LE, HV, HO, etc. The dotted lines should be left out in the finished drawing, but are put in the figure to show the construction.
Fig. 71. Enlarged U. 8. Standard Thread.
Fig. 72. Enlarged Whitworth Standard Thread.
If, instead of knowing the depth AS, we know the angle between AL and LE, the depth can be found by drawing from A and E the two lines AL and EL in such a way that they make the required angle with each other. To do this, the lines AL and EL should each make an angle with the line AB equal to 90° minus ½ the angle between AL and LE.
Fig. 74 shows the corresponding construction for the United States standard thread. Draw the lines AB, CD, and AC as in Fig. 73, and find the points E, F, G, H, I, J, K, etc., in the same way as in that case. Now draw the lines ST and UV so that AS and CU shall equal 65/100 of the pitch AE. On the line AB, lay off from A a distance A1 equal to 1/16 of the pitch; and on each side of E, F, G, H, etc., lay off E2, E3, etc., each equal to 1/16 of the pitch. From the points thus found, draw lines 1-4, 2-5, etc., making an angle of 60° with AB. The rest of the drawing is completed as shown by drawing in full lines those parts of the lines AB, CD, ST, and UV intercepted between 1-4 and 2-5, etc.
Fig. 73. Conventional Drawing for Right-Hand V Thread.
Fig. 75 shows the conventional representation of a square thread, and is drawn in exactly the same way as the true projection shown in Fig. 65, except that straight lines' are used instead of curves, and certain other minor lines omitted.
Square threads are seldom conventionalized more than as shown in Fig. 75, and V threads of coarse pitch and large diameter are usually drawn as in Fig. 73, whether sharp or U. S. standard. But for ordinary screws of small diameter and fine pitch, as are most frequently used, such a method involves too much labor, and the use of alternate long and short dashes across the body of the screw, as shown in Machine Drawing, Part I, Fig. 14, is universally employed.