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Title: Position Measurement Of Internal Screw Threads
Author: Bruce Armstrong
Source: 1994 Measurement Science Conference
Year Published: 1994
Abstract: The most accurate method for measuring the position of internally threaded holes of an assembly has been debated recently. One method uses the Pitch Diameter of the internal threads as a datum for determining hole position. A seemingly more elegant method, which was previously advocated, uses the Minor Diameter of the internal threads as a datum for determining thread position. Using the Minor Diameter as a datum is initially easier to accommodate, and appears to be more straightforward. Unfortunately, using the Minor Diameter as a datum leads to false conclusions about threaded hole position. The manner by which threads are designed and dimensioned leaves the Minor Diameter with much larger size and positional tolerances than the Pitch Diameter. In addition, the Minor Diameter is a non-functioning surface that does not reflect the position of the mating male threads when assembled. It is acknowledged that to locate on the Minor Diameter as a cylindrical surface Regardless-of-Feature-Size (RFS) is easier than locating on the thread Pitch Diameter RFS. However, it shall be shown that locating on the Minor Diameter RFS is inherently less accurate for determining hole position than using Pitch Diameter as a datum with the seemingly less accurate Maximum-Material-Condition (MMC) criteria. This paper shall show that, considering the total process, locating on the Pitch Diameter with MMC criteria is more accurate and overall easier than using Minor Diameter as a datum RFS.

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