In accordance with the principle of Three-Line, this paper analyzed the correlation of every main parameter of internal
screw thread, and then designed a device to measure the main parameters of internal screw thread. Basis on the measured
value and corresponding formula calculation, we can get the internal thread parameters, such as the pitch diameter, thread
angle and screw-pitch of common screw thread, terraced screw thread, zigzag screw thread and some else. The practical
application has proved that this operation of this device is convenience, and the measured dates have a high accuracy.
Meanwhile, the application of this device's patent of invention is accepted by the Patent Office. (The filing number: