Depth of Whitworth thread = 0.6403 ÷ No. of threads per inch.
The movable point measures all pitches, but the fixed anvil is limited in its capacity, for if made large enough to measure a thread of, say, 1/4-inch pitch, it would be too wide at the top to measure a thread of 1/24-inch pitch, hence each caliper is limited in the range of threads that the anvil can measure. When measuring the “angle diameter” of a thread, the micrometer should be passed back and forth across the thread, in order to make sure that the largest dimension or the actual diameter is being measured. If the micrometer is placed over what seems to be the center of the screw and the reading is taken by simply adjusting in the anvil or point against the thread, without moving the micrometer back and forth across it, an incorrect reading may be obtained.
Fig. 26. (A) Testing Size of Thread with Ball-point Micrometer.
(B) Testing Size of V-thread by the Three-wire System.
(C) Testing the Size of a U. S. Standard Thread
If standard threaded reference gages are available, the size of the thread being cut can be tested by comparing it with the gage. Micrometers having small spherical measuring ends (see sketch A, [Fig. 26]) are sometimes used for this purpose. The ball points are small enough to bear against the sides of the thread and the diameter, as compared with the reference gage, can be determined with great accuracy.
Three-wire System of Measuring Threads.—A method of measuring threads by using an ordinary micrometer and three wires of equal diameter is illustrated at B and C, [Fig. 26]. Two wires are placed between the threads on one side and one on the opposite side of the screw. The dimension M over the wires is then measured with an ordinary micrometer. When the thread is cut to a standard size, the dimension M for different threads is as follows:
For a U. S. standard thread:
m = d - 1.5155p + 3w
For a sharp V-thread: