Abstract
The present paper deals with the formulas to calculate the pitch diameters or the measurements over wires of the Archimedean worm thread screw and the involute worm thread screw, which have one or multi-start threads and large lead angles, in measuring screw threads by the three-wire method. Furthermore the best wire diameters which contact in the pitch diameter of worm, and the smallest wire diameters to provide the measurements over wires equal to the major diameter of an external thread are calculated. The approximate formula to calculate the pitch diameter of the Archimedean worm thread screw with high accuracy without using the method of successive approximations is obtained, and the accuracy of the approximate formula is discussed. The theoretical formula to calculate the measurements over wires of the involute worm thread screw is derived, and the equation to calculate the tooth thickness of the involute worm threads from the measurements over wires is obtained.
