http://www.sizes.com/tools/thread_american.htm
HistoryThe differences between American and British thread forms became a painful problem during the Second World War, especially in manufacturing and repairing airplane engines. In1948 representatives of Britain, Canada and the United States agreed on a Unified Standard. http://www.sizes.com/tools/images/Unifscrw.gifIn the compromise the British accepted the 60° thread angle, and the Americans accepted rounded roots and optionally rounded crests. Five classes of fit were defined. The new fasteners continued to fit, for most practical purposes, ones manufactured under the old American National Standards. NC became UNC, NF became UNF, etc. The UNS series is a catch-all category for threads which have the American Standard form, but whose pitches are not in the Unified Coarse (UNC) or Unified Fine (UNF) series.
2011-10-31 10:10
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