Figs. 18, 19, and 20 illustrate the common method of representing threads when they occur in pieces which are drawn in cross section. The front half of the piece is supposed to be removed and we are looking at the back half. Now the thread on the back side of a screw slants the opposite way from the way it slants on the front side, and of course the same is true of the thread in a tapped hole. Consequently, since it is the back side of the hole which is seen, the slant of the lines which represent the thread is opposite to the direction they would have were we looking at the front side of the screw which goes into the hole. We have just learned that for a right-hand thread on a screw the lines slant upward from left to right, and therefore for a right-hand thread seen on the back side of a tapped hole, the lines will slant upward from right to left. In other words, for a right-hand thread in a hole which comes in a cross section, the lines slant the same as they would on the front of a left-hand thread on a bolt; and for a left-hand thread in a sectioned hole, the slant is the same as for a right-hand thread on a bolt.

Fig. 19 is a piece which has a smooth hole through it and a thread on the outside. Here the entire thread is invisible, except at the contour of the cylinder, and must be indicated by the notches. These are drawn by spacing off the distance which is used for the pitch and from the points thus found drawing lines with the triangle which make an angle of 60° with the axis of the cylinder. For a Fig. 20 shows two ways of quarter-sectioning a threaded piece, the only difference being that on one the contour of the sectional part is drawn a straight line, while on the other the contour is notched. Either one may be used. The straight contour can, of course, be drawn much more quickly and in places where there is no danger of sacrificing clearness it should be used for that reason. If the drawing is somewhat complicated, so that without the notches it might not be quite clear that the piece was threaded, the notches should be used.

Fig. 18. Conventional Drawing for Threads in Sectional Pieces.

Table I. United States Standard Screw Threads

 -Diameter of Bolt Threads per Inch 1/4 20 5/16 18 3/8 16 7/16 14 1/2 13 9/16 12
 Diameter of Bolt Threads per Inch 5/8 11 3/4 10 7/8 9 1 8 1 1/8 7 1 1/4 7
 Diameter of Bolt Threads per Inch 1 3/8 6 1 1/2 6 1 5/8 5 1/2 1 3/4 5 1 7/8 5 2 4 1/2

Table II. Whitworth Standard Screw Threads

 Diameter of Bolt Threads per Inch 1/4 20 5/16 18 3/8 16 7/16 14 1/2 12 9/16 12
 Diameter of Bolt Threads per Inch 5/8 11 3/4 10 7/8 9 1 8 1 7/8 7 1 1/4 7
 Diameter of Bolt Threads per Inch 1 3/8 6 1 1/2 6 1 5/8 5 1 3/4 5 1 7/8 4 1/2 2 4 1/2

As has already been suggested, the student will doubtless find many other customs in the matter of drawing threads which are quite as good as the above. These have been given as ones which are common, and easily drawn. As a matter of convenience Tables I and II are given, which show the number of threads per inch on some of the most common sizes of bolts, according to the standard adopted by the United States Government, and the Whitworth or English standard.

Fig. 19. Conventional Drawing for Threads in Sectional Pieces.

Fig. 20. Conventional Drawing for Threads in Sectional Pieces..