Abstract
A new method of measuring pitch diameter of internal thread is devised, and its measuring principle and procedure are discussed fundamentally. Three balls are attached to a three-point internal micrometer, and each ball is inserted into a thread groove. From the geometrical condition of an equilateral triangle whose every apex is on a helix, a set of simultaneous equations is derived. Solving the equations, a reading of the internal micrometer is transformed into the diameter of the helix from the diameter of a circumscribed circle of the equilateral triangle. Numerical solution of the simultaneous equations can be obtained rather easily, and besides approximate solution is shown. A measuring instrument is manufactured practically, and accuracy of the three ball method is verified. The three ball method gives good agreement with results by a universal measuring microscope within difference of 3 μm.
