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Introduction
The three wire method is recommended for universal use in the direct measurement
of threaded gages and components. It has been found to be the most satisfactory
procedure for measuring the pitch diameter of straight threads. Pitch diameter is
the single most significant measurement which can be made on a thread because it
reflects the widest spectrum of potential defects in a threaded part or gage.
The
pitch diameter is the diameter of an imaginary cylinder which passes through the
thread profile at such points as to make the widths of thread groove and thread
ridge equal. The correct pitch diameter assures that the threaded product or thread
gage is within required limits in producing interchange ability and strength. Periodic
remeasurement of the pitch diameter is recommended to determine whether a thread
gage is worn below tolerance.
In Order to accomplish this measurement, three wires
of extremely fine tolerance and absolute identity are required. They are placed
in contact with the thread form and the measurement over wires is taken. Each set
of wires has a matched constant which, when subtracted from this reading, will yield
the pitch diameter. Van Keuren Thread Measuring Wires meet or exceed Federal Specification
GGG-W-366B for Grade A master Thread Wires in that Van Keuren Wires are made within
.000005" of the exact size of the wires. A matched constant is supplied with each
set.
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