LATHE OPERATIONS - OD1645 - LESSON 1/TASK 3
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 =
122
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