Exact drawings for the helix forming the thread of a screw are shown in Machine Drawing, Part II. These not only are difficult to draw, but they consume considerable time to produce accurately, therefore draftsmen have adopted certain conventions to represent the thread on working drawings. Some of these conventions are shown in Fig. 17. Here a represents a single, right-hand, square thread; b, a single, right-hand, sharp V thread, and its modifications, the United States Standard or Seller's thread, and the Whitworth thread; c represents a left-hand, sharp thread; d is the most common convention for any thread of a V-shaped cross section; e for any thread on a very small bolt or set screw; f is a modification of d, there being no slope to the thread, which convention is preferred by some draftsmen; g represents a standard pipe thread, the taper on the sides of the pipe being neglected.
Fig. 17. Conventional Representation of Screw Threads.
There are other conventions for threads in use, but the above are the most important ones. These certainly can not be mistaken for anything else, which is the real test for any conventional representation of an object.
The pitch of a screw thread is the distance between corresponding points on two successive threads measured parallel to the axis. A small axial section is shown at D on the thread a. The square groove, which gives the thread its name, has a depth equal to about ½ the pitch. Starting at the bottom, and following the edge of a thread in making one turn around the bolt, or from A to B on the front and B to C on the rear, we find that the thread advances parallel to the axis a distance AC, or the pitch. As we can see but one half of a turn it will be noted that a single right-hand thread advances a distance equal to ½ the pitch along the right-hand side, and similarly for a single left-hand thread, the distance advanced would be ½ the pitch on the left-hand side. The slope is, therefore, upward and to the right in the first case, and upward and to the left in the second case.
To draw the thread, space off the sides of the bolt with the dividers set to ½ the pitch, determine the slope, whether for single, double, or triple thread, and fill in the tops of the threads. The depth of the thread, say ½ the pitch, should then be laid off on each side and the lines drawn which show the visible portion of the bottom of the thread, thus completing the view for the ordinary convention. When the slope is considerable, as in this case, a small portion of the rear thread becomes visible, and may be shown.
For the thread at b it will be readily seen that an axial section would give V grooves, and as the standard angle for the grooves of the thread in this country is 60°, the projections are equilateral triangles. As before, if we follow a thread around the bolt, or from E to F on the front, and F to G on the back, we find for the single thread screw that the visible portion of the thread EF advances along the right-hand side a distance equal to ½ the pitch. Hence starting at the bottom, laying off the distance KF equal to ½ the pitch on the right-hand side, and connecting E to F, we have the slope of the thread determined. Spacing the pitch on the left-hand side for such distance as is required, we may then draw through these points lines parallel to EF, or the top lines of the thread. The V's may then be drawn with the aid of a T square and 30° triangle, after which the bottom of the threads may be connected. It will be noted that the top of the thread on one side is directly opposite the bottom of the thread on the other side. The left-hand V thread at c is drawn in a similar way to 6, the thread advancing on the left-hand side instead of on the right.
Considerable care is required to get the V's uniform, and the more practical, usual, and in nearly all cases satisfactory method of showing the thread is given in the remaining figures.
The spacing for the conventions d, e, and f approximates the pitch of the thread, and time can be saved by the draftsman learning to space with the eye rather than with the dividers. The light lines representing the top of the thread should be drawn first, the heavier lines for the bottom of the thread are then drawn midway between the light lines, stopping a short distance from the edges of the bolt. Often no difference in width is made between the lines representing the top and bottom of the threads, thus still further simplifying the conventional representation. It is well for the beginner to draw pencil lines limiting the bottom of the thread, so that the ends of the heavy lines will not be ragged or irregular. It rather improves the appearance of the thread to have the slant exaggerated in d, e, and g.