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In April 1955 representatives of Britain, Canada and the United States agreed on a standard for miniature threads, which the Americans had never addressed. In the manufacture of watches the situation of the 1850s lived on. The Waltham Watch Co. had a metric series of 12 screws peculiar to itself, and the Elgin National Watch Co. produced 27 miniature screws based on the inch, replete with left-hand thread sizes.
The scheme adopted for the Unified Miniature Threads was based on the meter and on the recommendations of the ISO regarding sizes. Unlike other metric screws, the designation is the major diameter of the screw, in millimeters, followed by UNM.
nominal major diameter
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B1.10-1958, R1988. American Standard for Unified Miniature Screw Threads.
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Last revised: 11 August 2004.