MEASURING SCREW THREADS MEASURING SCREW THREADS Measuring Screw Threads Machinery's Handbook, 31st Edition
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Pitch and Lead of Screw Threads.— The pitch of a screw thread is the distance from the center of one thread to the center of the next thread. This applies no matter whether the screw has a single, double, triple or quadruple thread. The lead of a screw thread is the dis tance the nut will move forward on the screw if it is turned around one full revolution. In a single-threaded screw, the pitch and lead are equal, because the nut would move forward the distance from one thread to the next, if turned around once. In a double-threaded screw, the nut will move forward two threads, or twice the pitch, so that in this case the lead equals twice the pitch. In a triple-threaded screw, the lead equals three times the pitch, and so on. The word “pitch” is often, although improperly, used to denote the number of threads per inch. Screws are spoken of as having a 12-pitch thread, when twelve threads per inch is what is really meant. The number of threads per inch equals 1 divided by the pitch, or expressed as a formula: 1 Number of threads per inch = pitch The pitch of a screw equals 1 divided by the number of threads per inch, or: 1 Pitch = numberof threads per inch If the number of threads per inch equals 16, the pitch = 1 ∕ 16 . If the pitch equals 0.05, the number of threads equals 1 ÷ 0.05 = 20. If the pitch is 2 ∕ 5 inch, the number of threads per inch equals 1 ÷ 2 ∕ 5 = 2 1 ∕ 2 . Confusion is often caused by the indefinite designation of multiple-thread screws (dou ble, triple, quadruple, etc.). The expression, “four threads per inch, triple,” for example, is not to be recommended. It means that the screw is cut with four triple threads or with twelve threads per inch, if the threads are counted by placing a scale alongside the screw. To cut this screw, the lathe would be geared to cut four threads per inch, but they would be cut only to the depth required for twelve threads per inch. The best expression, when a multiple-thread is to be cut, is to say, in this case, “ 1 ∕ 4 inch lead, 1 ∕ 12 inch pitch, triple thread.” For single-threaded screws, only the number of threads per inch and the form of the thread are specified. The word “single” is not required. Measuring Screw Thread Pitch Diameters by Thread Micrometers.— As the pitch or angle diameter of a tap or screw is the most important dimension, it is necessary that the pitch diameter of screw threads be measured, in addition to the outside diameter.
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Fig. 1. One method of measuring in the angle of a thread is by means of a special screw thread micrometer, as shown in the accompanying engraving, Fig. 1 . The fixed anvil is W-shaped to engage two thread flanks, and the movable point is cone-shaped so as to enable it to enter the space between two threads, and at the same time be at liberty to revolve. The
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