FIGURE 48.

THREAD GAGES.

(4) The three wire method is another method of measuring the pitch

diameter of a screw thread. It is considered the best method for extremely

accurate measurement.

Figure 49 on page 124 shows three wires of the

correct diameter placed in threads with the micrometer measuring over them.

The pitch diameter can be found by subtracting the wire constant from the

measured distance over the wires. It can be readily seen that this method

is dependent on the use of the "best" wire for the pitch of the thread. The

"best" wire is the size of the wire that touches the thread at the middle of

the sloping sides, in other words, at the pitch diameter.

A formula by

which the proper size of the wire may be found is as follows: Divide the

constant 0.57735 by the number of threads per inch to be cut.

If, for

example, 8 threads per inch have been cut, we would calculate 0.57735/8 =

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