Fig. 9. U. S. Standard Thread, Thread Tool, and Gage
Cutting a U. S. Standard Thread.—The method of cutting a U. S. standard thread is the same as described for a V-thread, so far as handling the lathe is concerned. The thread tool must correspond, of course, to the shape of a U. S. standard thread. This tool is first ground to an angle of 60 degrees, as it would be for cutting a V-thread, and then the point is made flat as shown in [Fig. 9]. As will be recalled, the width of this flat should be equal to 1/8 of the pitch. By using a gage like the one shown at G, the tool can easily be ground for any pitch, as the notches around the periphery of the gage are marked for different pitches and the tool-point is fitted into the notch corresponding to the pitch wanted. If such a gage is not available, the width of the flat at the point can be tested by using, as a gage, a U. S. standard tap of the same pitch as the thread to be cut.
When cutting the thread, the tool is set square with the blank, and a number of successive cuts are taken, the tool being fed in until the width w of the flat at the top of the thread is equal to the width at the bottom. The thread will then be the right size provided the outside diameter D is correct and the tool is of the correct form. As it would be difficult to measure the width of this flat accurately, the thread can be tested by screwing a standard nut over it if a standard thread is being cut. If it is being fitted to a tapped hole, the tap itself is a very convenient gage to use, the method being to caliper the tap and then compare its size with the work.
A good method of cutting a U. S. standard thread to a given size is as follows: First turn the outside of the blank accurately to diameter D, and then turn a small part of the end to diameter r of the thread at the root. The finishing cut for the thread is then taken with the tool point set to just graze diameter r. If ordinary calipers were set to diameter r and measurements taken in the thread groove, the size would be incorrect owing to the angularity of the groove, which makes it necessary to hold the calipers at an angle when measuring. To determine the root diameter divide 1.299 by the number of threads per inch and subtract the quotient from the outside diameter. Expressing this rule as a formula,
| ( | 1.299 | ) | ||||||
| r | = | D | - | ——— | ||||
| N |
in which D equals outside diameter; N, the number of threads per inch; and r, the root diameter. The number 1.299 is a constant that is always used.
