LATHE OPERATIONS - OD1645 - LESSON 1/TASK 3
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
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