|Screw thread micrometers measure the pitch of the thread directly.
The screw thread micrometer has a 60-degree pointed spindle and a
double V-shaped swiveling anvil (Figure 1).
Figure 1. Screw Thread Micrometer
Figure 2. The pitch line indicates the imaginary thread
|The screw thread micrometer reading indicates the pitch diameter
of the thread. When the micrometer is set at zero, the pitch line of
the spindle and anvil coincide (Figure 2). When the micrometer is
measuring the thread it is measuring along the pitch diameter of the
thread (Figured 3).
|Fixed anvil screw thread micrometers are those types of screw
thread micrometers that can only measure thread pitches within a
limited range. It is important to make sure that the screw thread
micrometer that you will be using falls within the screw pitch range
of a particular thread. With fixed anvil screw thread micrometers,
seven micrometers will be needed to cover threads from 3 to 64
threads per inch.
Figure 3. The pitch diameter of the
thread is the most important dimension.
The type of screw thread micrometer shown in Figure 4 has
interchangeable anvils to cover a wide range of thread pitches. Whenever
you change the anvils on the thread pitch micrometer, check the micrometer
|Figure 4. 1–2 inch screw thread
micrometer with interchangeable anvils
||Figure 5. Set of interchangeable
anvils for an interchangeable anvil screw thread micrometer.