The three-wire measuring method is an way of measuring external thread pitch diameter, which is indirect.

When measuring, put the three high precision, same diameter wires to the thread groove which will be measured. Using micrometer, mechanical comparison instrument,optical instrument, universal length measurement device etc calibration tools which used to measure external sizes, measure out the sizes. And then according to the pitch of the thread under test, the thread angle, and the diameter of the wires to calculate the pitch diameters.

The three-wire measuring method is a easy and quick way of measuring pitch diameter:

When measuring, put the three high precision, same diameter wires to the thread groove which will be measured. Two of which are placed in the same side of the groove, the other one is put  on the opposite side of the corresponding groove. With the lever micrometer, measure out M value the maximum distance from the wire, and then calculated by the following formula to find the measured diameter d2. Substituting on the above formula: for metric threads, with α = 60 °, then: d2 = M-3d0 + 0.866P where: d0 – wire diameter; P-thread pitch. In order to eliminate the influence of the half-angle error on the measurement results, the wire with the best diameter should be selected so that it is in contact with the mid-line of the thread side.

 

For metric thread

Measure the pitch diameter of the thread with the three-wire method, according to the following steps:

  1. Calculate and select the best wires diameter according to the thread pitch to be tested.
  2. Clean the thread to be tested and clamp it on the bracket.
  3. Clean the lever micrometer and adjust the zero level.
  4. Put the measuring wires into the thread groove, turn microdrum of the lever micrometer, contact the two probes with the wires, and then read the M value.
  5. Calculate the M value in two directions perpendicular to each other in the same section, and calculate the average value into the formula to calculate the pitch diameter.
  6. Judge whether the thread pitch diameter correct or not.