On the Formulas for Three-wire Method of Measurement of the Pitch Diameter of the Type-2 Chased Worm Thread Screw with a Large Lead Angle

The present paper deals with the formulas to calculate the pitch diameters or the measurement over wires of the type-2 chased worm thread screw, which have one or multi-start threads and large lead angles, in measuring screw threads by the three-wire method. Furthermore, calculation has been made on the best wire diameter that enables the wire to touch the spiral on the screw surface whose diameter is equal to the nominal pitch diameter, together with the smallest wire diameter that provides the measurement over wires equal to the major diameter of an external thread. The approximate formula to calculate the pitch diameter of the type-2 chased worm thread screw with high accuracy is proposed, and the accuracy of the approximate formula is discussed. Some numerical examples are given for the application of the theory to practice.