Editing Screw thread (section)

=== History of standardization ===
[[File:JFIScrewThread300.png|thumb|Graphic representation of formulas for the pitches of threads of screw bolts]]

[[File:American Machinists Handbook--2e--p21--v001.png|thumb|A good summary of screw thread standards in current use in 1914 was given in Colvin FH, Stanley FA (eds) (1914): [https://books.google.com/books?id=4Q8LAAAAIAAJ American Machinists' Handbook, 2nd ed], New York and London, McGraw-Hill, pp. 16–22. USS, metric, Whitworth, and [[British Association screw threads|BA]] standards are discussed. The SAE series was not mentioned—at the time this edition of the ''Handbook'' was being compiled, they were either still in development or just newly introduced.]]

[[File:Marks 1916 p667 American Screw Co standard.png|thumb|A table of standard sizes for machine screws as provided by the American Screw Company of Providence, Rhode Island, USA, and published in a ''Mechanical Engineers' Handbook'' of 1916. Standards seen here overlap with those found elsewhere marked as ASME and SAE standards and with the later Unified Thread Standard (UTS) of 1949 and afterward. One can see the theme of how later standards reflect a degree of continuation from earlier standards, sometimes with hints of long-ago intracompany origins. For example, compare the 6–32, 8–32, 10–24, and 10–32 options in this table with the UTS versions of those sizes, which are not identical but are so close that interchange would work.]]

[[File:Survey results on use of SAE standards Horseless Age v37 n9 1916-05-01 p353.png|thumb|Survey results on the use of [[w:SAE International|SAE]] standards (including screw size standards), reported in the journal ''Horseless Age'', 1916]]

The first historically important intra-company standardization of screw threads began with [[Henry Maudslay]] around 1800, when the modern [[screw-cutting lathe]] made interchangeable V-thread machine screws a practical commodity.<ref name="Screw thread">Quentin R. Skrabec, Jr. (2005). "The Metallurgic Age: The Victorian Flowering of Invention and Industrial Science". p. 169. McFarland</ref> During the next 40 years, standardization continued to occur on the intra- and inter-company levels.<ref name="Roe1916pp9-10">{{Harvnb|Roe|1916}}, [https://archive.org/details/englishandameri01roegoog/page/n39 pp. 9–10.]</ref> No doubt many mechanics of the era participated in this zeitgeist; [[Joseph Clement]] was one of those whom history has noted.

In 1841, [[Joseph Whitworth]] created a design that, through its adoption by many British railway companies, became a standard for the United Kingdom and British Empire called [[British Standard Whitworth]]. During the 1840s through 1860s, this standard was often used in the United States as well, in addition to myriad intra- and inter-company standards. In April 1864, [[William Sellers]] presented a paper to the [[Franklin Institute]] in [[Philadelphia]], proposing a new standard to replace the US' poorly standardized screw thread practice. Sellers simplified the Whitworth design by adopting a thread profile of 60° and a flattened tip (in contrast to Whitworth's 55° angle and rounded tip).<ref>{{cite web|url=http://anniversary.asme.org/2005landmarks3.shtml|title=ASME 125th Anniversary: Special 2005 Designation of Landmarks: Profound Influences in Our Lives: The United States Standard Screw Threads|website=asme.org|access-date=4 April 2018|archive-date=13 June 2005|archive-url=https://web.archive.org/web/20050613082757/http://anniversary.asme.org/2005landmarks3.shtml|url-status=dead}}</ref><ref name="Roe1916pp248-249">{{Harvnb|Roe|1916}}, [https://archive.org/details/englishandameri01roegoog/page/n328 pp. 248–249.]</ref> The 60° angle was already in common use in America,<ref name="Roe1916p249">{{Harvnb|Roe|1916}}, [https://archive.org/details/englishandameri01roegoog/page/n331 p. 249.]</ref> but Sellers's system promised to make it and all other details of threadform consistent.

The Sellers thread, easier to produce, became an important standard in the U.S. during the late 1860s and early 1870s, when it was chosen as a standard for work done under U.S. government contracts, and it was also adopted as a standard by highly influential railroad industry corporations such as the [[Baldwin Locomotive Works]] and the [[Pennsylvania Railroad]]. Other firms adopted it, and it soon became a national standard for the U.S.,<ref name="Roe1916p249" /> later becoming generally known as the [[United States Standard thread]] (USS thread). Over the next 30 years the standard was further defined and extended and evolved into a set of standards including ''National Coarse (NC), National Fine (NF),'' and ''National Pipe Taper (NPT).''

Meanwhile, in Britain, the [[British Association screw threads]] were also developed and refined for small instrumentation and electrical equipment. These were based on the metric [[Thury thread]], but like Whitworth etc. were defined using [[Imperial units]].

During this era, in continental Europe, the British and American threadforms were well known, but also various [[Metric system|metric]] thread standards were evolving, which usually employed 60° profiles. Some of these evolved into national or quasi-national standards. They were mostly unified in 1898 by the International Congress for the standardization of screw threads at [[Zürich]], which defined the new international metric thread standards as having the same profile as the Sellers thread, but with metric sizes. Efforts were made in the early 20th century to convince the governments of the U.S., UK, and Canada to adopt these international thread standards and the metric system in general, but they were defeated with arguments that the [[capital cost]] of the necessary retooling would drive some firms from profit to loss and hamper the economy. 

Sometime between 1912 and 1916, the [[SAE International|Society of Automobile Engineers (SAE)]] created an "SAE series" of screw thread sizes reflecting parentage from earlier USS and American Society of Mechanical Engineers (ASME) standards.

During the late 19th and early 20th centuries, engineers found that ensuring the reliable interchangeability of screw threads was a multi-faceted and challenging task that was not as simple as just standardizing the major diameter and pitch for a certain thread. It was during this era that more complicated analyses made clear the importance of variables such as pitch diameter and surface finish.

A tremendous amount of engineering work was done throughout [[World War I]] and the following [[interwar period]] in pursuit of reliable interchangeability. Classes of fit were standardized, and new ways of generating and inspecting screw threads were developed (such as [[:File:ThreadGrinder1K.jpg|production thread-grinding machines]] and [[optical comparator]]s). Therefore, in theory, one might expect that by the start of World War II, the problem of screw thread interchangeability would have already been completely solved. Unfortunately, this proved to be false. Intranational interchangeability was widespread, but international interchangeability was less so. Problems with lack of interchangeability among American, Canadian, and British parts during World War II led to an effort to unify the inch-based standards among these closely allied nations, and the [[Unified Thread Standard]] was adopted by the Screw Thread Standardization Committees of Canada, the United Kingdom, and the United States on November 18, 1949, in [[Washington, D.C.]], with the hope that they would be adopted universally. (The original UTS standard may be found in ASA (now ANSI) publication, Vol. 1, 1949.) UTS consists of Unified Coarse (UNC), Unified Fine (UNF), Unified Extra Fine (UNEF) and Unified Special (UNS). The standard was widely taken up in the UK, although a small number of companies continued to use the UK's own British standards for Whitworth (BSW), British Standard Fine (BSF) and British Association (BA) microscrews.

However, internationally, the [[Metrication|metric system was eclipsing inch-based measurement units]]. In 1947, the ISO was founded; and in 1960, the metric-based [[International System of Units]] (abbreviated ''SI'' from the French ''Système International'') was created. With continental Europe and much of the rest of the world turning to SI and ISO metric screw thread, the UK gradually leaned in the same direction. The ISO metric screw thread is now the standard that has been adopted worldwide and is slowly displacing all former standards, including UTS. In the U.S., where UTS is still prevalent, over 40% of products contain at least some ISO metric screw threads. The UK has completely abandoned its commitment to UTS in favour of ISO metric threads, and Canada is in between. [[Globalization]] of industries produces market pressure in favor of phasing out minority standards. A good example is the [[automotive industry]]; U.S. auto parts factories long ago developed the ability to conform to the ISO standards, and today very few parts for new cars retain inch-based sizes, regardless of being made in the U.S.

Even today, over a half century since the UTS superseded the USS and SAE series, companies still sell hardware with designations such as "USS" and "SAE" to convey that it is of inch sizes as opposed to metric. Most of this hardware is in fact made to the UTS, but the labeling and cataloging terminology is not always precise.