REED LATHE TAPERS (Taper Per Foot = .6 In.; Taper Per Inch =.05 In.) Determining Number of Screw Threads Per Inch.-Set a scale a, Fig. 1, on the screw so that its end is in line with one thread point. Another thread point is opposite the first inch mark. Counting from the end up to and including the first inch mark, there are six thread points, as numbered; but the 2 FIG. 1 sixth point is really the first point in the next inch, so it is dropped. There are 5 points per inch, therefore, on the screw, or 5 threads per inch. Instead of a scale, a thread gauge b may be used. This is a more accurate means of measuring the number of threads per inch, as it shows fractional numbers of threads per inch as well as whole numbers. Pitch. The pitch of a thread is the distance from one point to the next, measured from center to center parallel to the axis. Lead. The lead is the distance that the thread advances in making one complete revolution, or, it is the distance from a point on one thread to a corresponding point on the same thread, after it makes one complete turn. (a) (b) (c) FIG. 2 (d) Single Thread.-A thread in which the lead equals the pitch, Fig. 2 (a). Double Thread.-Two separate single threads in which the lead equals twice the pitch, Fig. 2 (b). Triple Thread.-Three separate single threads in which the lead equals three times the pitch, Fig. 3 (c). Quadruple Thread.-Four separate single threads in which the lead equals four times the pitch, Fig. 4 (d). U. S. STANDARD, S. A. E., AND A. S. M. E. THREADS These threads vary in the number of threads per inch. See tables of tap drill sizes. |