Figure 115 shows how to measure the number of threads to one inch of a bolt. In this case the threads are an even 8 to the inch and we see that there are just 8 grooves from the end of the scale to the 1-in. mark. If a thread is an even number per inch it can be easily measured with the scale as described, but when we have fractional threads such as 11 1/2 per inch it is best to measure the threads for 2 in., which would give us 23 whole grooves. Dividing 23 by 2 gives 11 1/2 threads per inch.
Fig. 114. - Double-Thread Screw.
When a bolt is less than an inch long, it is necessary to count the grooves in 1/2 in. and multiply this by 2 to get the threads per inch. The best way to measure threads is with the thread or pitch gauge.
The number of threads per inch is the same on the same sized bolt whether the thread is cut single, double, or triple. If a double thread, 8 threads per inch is wanted, we ask for "8 threads per inch double"; if a triple thread, we say "8 threads per inch triple." Although to avoid any misunderstanding it would be clearer to say for the double thread, "1/4 in. lead, 1/8 in. pitch, double thread." There would then be no chance for mistake since we sometimes find an old print which calls for "8 threads per inch double," and means that a double thread, 16 threads per inch is wanted. With single threads the word "single" is not used, as it is understood. All single threads of coarse pitch weaken considerably the bolt or piece which is threaded. For this reason, multiple threads are used. With a double thread the groove is only one-half as deep as a corresponding single thread, and the bolt will advance just as far for one turn as it would if cut single. Figure 116 shows a triple thread with its corresponding single thread dotted.
Fig. 115. - Measurement of Screw Threads.
Fig. 116. - Triple-Thread Screw.