ABSTRACT
Thread series are groups of diameter/pitch combinations differing from each other by the number of threads per inch (or thread pitch) applied to a series of diameters.
The current Unified coarse thread series (UNC) is patterned on the thread series introduced by Whitworth in the mid-19th century. The relatively coarse pitches he selected were probably chosen as much for being those which lent themselves to the manufacturing skills of the day as for any other single reason. Over the years, as production capabilities improved, it became economically feasible to produce threads to greater degrees of accuracy and with finer pitches. Many special purpose thread series were developed and the one now known as the Unified fine thread series (UNF) enjoys considerable popularity. As screw thread technology advanced it became evident that just two thread series, coarse and fine, would be inadequate to efficiently satisfy all engineering applications. Consequently, a number of constant pitch thread series were added to the system. Such series have a single thread pitch which is common for all diameters in the series.
