This section is from the book "Applied Science For Metal Workers", by William H. Dooley. Also available from Amazon: Applied Science For Metal Workers.

It is important to be able to find the depth of a thread, for upon this depends the cutting of all threads and the size of all tap drills. By referring to Fig. 117, we see that the depth of the thread is the altitude of a right-angle triangle, since the angles are all 60° and the sides of the V or groove are equal to the pitch. Knowing two sides of a right-angle triangle we can easily find the other, since we know that the square of the altitude of any right-angle triangle is equal to the square of the hypotenuse minus the square of the third side. A simple problem in square root will give the correct figure for the altitude of depth.

An easier way to find the depth of a United States or V thread is to remember that the depth of a United States thread of 1 in. pitch is .65 in. and the depth of a V thread of 1 in. pitch is .86 in. Now if we wish to find the depth of any other thread we simply divide these figures by the number of threads to the inch. For example, we determine the depth of a United States standard thread 13 threads to the inch in this manner - .65 ÷ 13 = .05 in., and the depth of a V thread 4 threads per inch, in this manner - .86 ÷ 4 = .215. When figuring the size to bore or drill a nut or a hole to be threaded, subtract the double depth of the thread from the outside diameter of the thread on the bolt or rod.

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