COMMON FASTENING AGENTS 



253 



of the next is called the pitch, as P in Fig. 114. The pitch is 



always the distance from one thread to the next, no matter 



whether it is single, double, or triple thread. The distance 



that a screw enters 



a nut or hole for one 



complete turn is 



called the lead. For 



a single thread the 



lead is equal to the 



pitch, for a double 



thread the lead is 



twice the pitch, and 



for a triple thread 



the lead is three 



times the pitch. 



The point of a thread is the projecting end. The diameter 

 of a thread is the distance measured over it, and is the same 

 as the diameter of the bolt before the thread is cut. The 

 perpendicular distance from the top of the thread , to the 

 bottom of the groove is called the depth or height; twice 

 this distance is called double-depth. The root is the bottom 

 of the groove. The diameter at the root is the outside dia- 

 meter minus the double depth. This is called the root 

 diameter. 



FIG. 114. Double-Thread Screw. 



297. Measurement of Thread. 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^ per 

 inch it is best to measure the threads for 2 in., which would give us 

 23 whole grooves. Dividing 23 by 2 gives \\Y^ threads per inch. 



