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The Evolution of Screw Threads

by on 10th July 2012  •  In Technical Information

Some believe that the evolution of screw threads first began around 400BC by a contemporary of Plato, Archytas of Tarentum. The screw principle was applied to pressess for the extraction of oils from olives and juice from grapes. Over the centuries advances on the principle continued but still depended on the skill of the craftsmen to cut them by hand. Around 1750 Antoine Thiout introduced the innovation of equipping a lathe with a screw drive allowing the tool carriage to be moved longitudinally semi-automatically. However advances in industry required screw threads with a fine pitch, such a thread could only be  cut by a lathe, in 1770 Jesse Ramsden made the first successful screw-cutting lathe. The demand for threaded fasteners increased considerably as industry continued to develop, however, the lack of thread standardisation made fastener interchangeability problematical. Screw Threads To overcome the problem, the English engineer, Sir Joseph Whitworth was the first person to create a thread standard in around 1841.  Whitworth became British Standard Whitworth, abbreviated to BSW (BS 84:1956) and the British Standard Fine (BSF) thread was then introduced in 1908 because the Whitworth thread was too coarse for some applications. The thread angle was standardised as 55°, and the depth and pitch varied with the diameter of the thread (i.e., the bigger the bolt, the coarser the thread). Spanners for Whitworth bolts are marked with the size of the bolt, not the distance across the flats of the screw head. Whitworth screw sizes are still used, both for repairing old machinery and where a coarser thread than the metric fastener thread is required, but the most common use of a Whitworth pitch nowadays is in all UK scaffolding. Additionally, the standard photographic tripod thread, which for small cameras is 1/4″ Whitworth (20 tpi) and for medium/large format cameras is 3/8″ Whitworth (16 tpi). It is also used for microphone stands. Whitworth Screw Thread Form

A later standard established in the United Kingdom was the British Association (BA) screw threads, named after the British Association for Advancement of Science. Screws were described as “2BA”, “4BA” etc., the odd numbers being rarely used, except in equipment made prior to the 1970s for telephone exchanges in the UK. This equipment made extensive use of odd-numbered BA screws, in order—it may be suspected—to reduce theft. BA threads are specified by British Standard BS 93:1951 “Specification for British Association (B.A.) screw threads with tolerances for sizes 0 B.A. to 16 B.A.”

BA Screw Thread

In 1864 in America, William Sellers independently proposed another standard based upon a 60 degree thread form and various thread pitches for different diameters.The Unified Thread Standard (UTS) is most commonly used in the United States of America, but is also extensively used in Canada and occasionally in other countries. For most size screws there are multiple TPI available, with the most common being designated a Unified Coarse Thread (UNC or UN) and Unified Fine Thread (UNFor UF).

Unified Thread Diagram

  Around the same time metric thread standards were being adopted in continental Europe with a number of different thread flank angles being adopted. These progressed to the ISO Metric Standards that we know today.

Iso Metric Threads

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