United States Standard Thread - History and Use

History and Use

William Sellers originally developed the USS thread, and set forth many of its details in his paper, "A System of Screw Threads and Nuts", presented in April 1864 to the Franklin Institute. In 1898, the standard for metric threaded fasteners was established. The metric standard used the same thread geometry as the USS standard but differed in that the dimensions and pitch were based on metric units. In 1906, the A.L.A.M developed what would be the SAE thread standard for threaded fasteners based on the USS standard but with a finer thread pitch.

A Unified Thread Standard UNC thread is mechanically interchangeable with a USS thread of the same diameter. However, there are tolerance and other differences between a thread compliant with the USS thread and a Unified Thread Standard UNC thread. The Unified Thread Standard for quarter inch and larger threaded fasteners was adopted on November 18, 1948, and was subsequently adopted for smaller thread series.