0.072. The diameter of the wire to use for measuring an 8-pitch thread is

0.072.

The wires used in the three-wire method should be hardened and

lapped steel wires. They should be three times as accurate as the accuracy

desired in the measurement of the threads.

The Bureau of Standards has

specified an accuracy of 0.0002 inch. The suggested procedure for measuring

threads is as follows:

(a) After the three wires of equal diameter have been selected by using

the above formula, they are positioned in the thread grooves, as shown in

figure 49 on the following page. The anvil and the spindle of an ordinary

micrometer are then placed against the three wires and the reading taken.

To determine what the reading of the micrometer should be if a thread is the

correct finished size, use the following formula (for measuring Unified

National Coarse threads): add three times the diameter of the wire to the

diameter of the screw; from the sum, subtract the quotient obtained by

dividing the constant 1.5155 by the number of threads per inch.

Written

concisely, the formula is

Where m = micrometer measurement over the wires

D = diameter of the thread

n = number of threads per inch

W = diameter of wire used

Example: Determine m (measurement over wires)

For 1/2 inch, 12 pitch, UNC thread.

We would proceed to solve as follows:

Where W = 0.04811 inch

D = 0.500 inch

n = 12