The 200-Year Precision Argument Over How Tight A Fit Should Be

In Manchester in 1857, Joseph Whitworth showed the Institution of Mechanical Engineers a steel ring bored to .5770 of an inch and 2 plugs, one .5770 and one .5769. Dry, the larger plug fit tight and the smaller one rattled. With a drop of oil, they swapped. This documentary follows the history of engineering fits from before shops measured them at all to the codes on drawings today. It begins with a trade that fitted by feel. In 1776 Matthew Boulton praised a 50-inch Wilkinson cylinder for erring no more than the thickness of an old shilling, and by 1830 a fitter who could work to a sixteenth was counted a good workman. Whitworth asked what any man could mean by "a bare sixteenth," defined a good fit as true surfaces plus a proper allowance, and built a measuring machine that worked by touch. A generation later Brown & Sharpe found that Whitworth's own master plugs and rings varied in fit, and Pratt & Whitney needed the Rogers-Bond comparator to settle the length of a foot. The film follows the interference fit through its oldest form. Coleman Sellers pressed a 4-inch axle on at 30 to 35 tons in 1872, and William Sellers & Co. built its keyed wheels around the forcing fit. In a Farnham wheelwright's shop in 1884, George Sturt watched a blacksmith measure an iron tyre with a traveller and shrink it onto a wooden wheel until the spokes snapped home. In 1886 the American Railway Master Mechanics Association put locomotive tyre shrinkage in thousandths, and railway shops took a wheel off again if the press gauge read low. It then covers the turn of the century, when the names arrived. Newall Engineering of Warrington surveyed English, Continental and American practice and averaged it into force, driving, push or keying, and X, Y and Z running fits. Britain issued BS 27 in 1906. The 1908 American Machinists' Handbook defined the shop words and showed how machinists set calipers for a running fit by side play. The 1924 Machinery's Handbook printed the 5 classes, the personal factor, and tests showing a shrink fit gripping 3.66 times harder than a press fit. The middle of the film is the naming fight of 1924 and 1925. Germany named 22 fits, Britain's BS 164 offered 154 combinations and recommended against saying "push fit," and America's B4a settled on 8, from loose to heavy force or shrink. Irvin Fullmer of the Bureau of Standards lined them up in 1927. Carl Edvard Johansson's 20 °C reference became the international standard by a voice vote in 1931. The video follows the names into the RC, LC, LT, LN and FN classes, decodes ISO 286 fits from H11/c11 to H7/u6, and ends on the transition fit, where a single drawing allows a pair to land on either side of Whitworth's line. Subscribe to Trade Archive for more on the measurements, machines and shops behind the trades:    / @tradearchive   Sources Ahlström, Göran. "The Edison of Sweden: C. E. Johansson and the 'Standards of Standard'." Lund Papers in Economic History No. 71, 2001. Ahrons, E. L. Steam Locomotive Construction and Maintenance. London: Locomotive Publishing Co., 1921. Amesweb. "Preferred Fits and Tolerances (ISO 286-1, ANSI B4.2)." BSI. "BS 27:1906, Report of BS Systems of Limit Gauges for Running Fits." BSI Knowledge. Colvin, Fred H., and Frank A. Stanley. American Machinists' Handbook and Dictionary of Shop Terms. New York: McGraw, 1908. Doiron, Ted. "20 °C—A Short History of the Standard Reference Temperature for Industrial Dimensional Measurements." Journal of Research of the National Institute of Standards and Technology 112, no. 1 (2007). Fullmer, Irvin H. Comparison of American, British, and German Standards for Metal Fits. Bureau of Standards Technologic Paper No. 344. Washington, DC: Government Printing Office, 1927. Grace's Guide to British Industrial History. "Newall Engineering Co." Institution of Mechanical Engineers Archive. "Sir Joseph Whitworth, 1856–1857." International Organization for Standardization. ISO 286-1:2010, Geometrical Product Specifications (GPS)—ISO Code System for Tolerances on Linear Sizes—Part 1. Geneva: ISO, 2010. Kent, William. The Mechanical Engineer's Pocket-Book. New York: John Wiley & Sons, 1895. Lobben, Peder. Machinists' and Draftsmen's Handbook. New York: D. Van Nostrand, 1910. Machinery's Handbook. New York: Industrial Press, 1924. Roe, Joseph Wickham. English and American Tool Builders. New Haven: Yale University Press, 1916. Science Museum Group. "Internal and External Limit Gauges, Newall Engineering Co., 1907." Object 1907-41. Sturt, George. The Wheelwright's Shop. Cambridge: Cambridge University Press, 1923. USA Standards Institute. USA Standard B4.1-1967, Preferred Limits and Fits for Cylindrical Parts. New York: ASME, 1967. Whitworth, Joseph. Miscellaneous Papers on Mechanical Subjects. London: Longman, Green, Longman, and Roberts, 1868.