How Hardness Became a Number on the Machinist's Dial

A micrometer will tell you a part is the right size. It can't tell you what the heat treater did to it. For about 200 years, the trade judged hardness by the colour of a tempering film, the colour of a spark and whether a file would bite, and even in 1967 a file was the only hardness check on the propeller part that brought down Lake Central Airlines Flight 527. This documentary follows hardness testing from Réaumur's scratch tests in 1722 and Joshua Rose's printed tempering-colour chart of 1878 to the diamond cones and load cells of today. Johan August Brinell shows his ball test at the 1900 Paris Exposition, the same exposition where Taylor and White showed high-speed steel, and runs into a problem: on hardened steel, the ball deforms. Albert Shore's scleroscope bounces a diamond-tipped hammer off the work instead. Then, at the New Departure bearing plant in Bristol, Connecticut, 2 unrelated men named Hugh and Stanley Rockwell file a 1914 patent for a tester "the ordinary shop workman" could use on a curved bearing race, reading depth on a dial after a lever buckles at a set force. Instrument salesman Charles H. Wilson turns the Rockwell tester into an industry standard with the diamond Brale and the 150-kilogram C scale. By 1939 there are 7,000 to 8,000 in use. Stanley Rockwell becomes a Hartford heat treater and dies in a yacht explosion in 1940. In the Second World War, Wilson testers ship to Russia on coded telegrams, and at Wright Aeronautical's Lockland plant the Truman Committee hears that hardness tests on engine gears were being falsified. The story then moves to the shop floor: Vickers and Knoop tests, case depth on carburized gears, why steel grows in heat treatment and gets finished after hardening, how hardness sets cutting speeds, and how CBN hard turning started replacing grinding in the 1980s. SAE writes down what "file hard" means in 1963. NIST finds in 1991 that American Rockwell test blocks don't agree and corrects the US HRC scale in 1998. The Bay Bridge anchor rods show what a single reading can miss. Subscribe to Trade Archive for more stories about the tools, machines and standards that built the trades:    / @tradearchive   Sources Benedicks, Carl. "J. A. Brinell." In Svenskt biografiskt lexikon, vol. 6. Stockholm, 1926. Brahimi, Aguilar, and Christensen. "Metallurgical Analysis of Bay Bridge Broken Anchor Rods S1-G1 & S2-A6." May 7, 2013. Buehler. "Hardness Testing Retrospective: A Century of Wilson." Advanced Materials & Processes, June 2020. DeVries. Technologic Paper T11. Washington, DC: US Bureau of Standards, 1912. Karmazinas, Lucas A. "Stanley P. Rockwell Co. Factory." Connecticut State Register of Historic Places nomination, 2016. Knoop, Frederick, Chauncey G. Peters, and Walter B. Emerson. "A Sensitive Pyramidal-Diamond Tool for Indentation Measurements." Journal of Research of the National Bureau of Standards 23, no. 1 (1939): 39–61. Low, Samuel R. Rockwell Hardness Measurement of Metallic Materials. NIST Special Publication 960-5. Gaithersburg, MD: National Institute of Standards and Technology, 2001. McKenna, Patrick. "Predicting Size Change from Heat Treatment." Production Machining, October 20, 2010. National Transportation Safety Board. Aircraft Accident Report SA-395: Lake Central Airlines Flight 527, Near Marseilles, Ohio, March 5, 1967. Adopted February 26, 1968. Oberg. Heat-Treatment of Steel. New York: Industrial Press, 1920. Ocel and Provines. FHWA-HRT-15-057. Federal Highway Administration, August 2015. Rockwell, Hugh M., and Stanley P. Rockwell. Hardness-Tester. US Patent 1,294,171, filed July 15, 1914, and issued February 11, 1919. Rockwell, Stanley P. Hardness-Testing Machine. US Patent 1,516,207, filed September 11, 1919, and issued November 18, 1924. Rose, Joshua, and Park Benjamin. Improvement in Chromatic Tempering-Scales. US Patent 205,578, issued July 2, 1878. Rushmer, H. W. "High Speed Steel." American Machinist, September 3, 1903. Shives and Smith. NISTIR 4531. Gaithersburg, MD: National Institute of Standards and Technology, February 1991. Shore, Albert F. Scleroscope. US Patent 962,790, filed November 26, 1907, and issued June 28, 1910. Smith, R. L., and G. E. Sandland. "An Accurate Method of Determining the Hardness of Metals, with Particular Reference to Those of a High Degree of Hardness." Proceedings of the Institution of Mechanical Engineers 102 (1922): 623–41. Society of Automotive Engineers. Surface Hardness Testing with Files. SAE J864. Issued June 1963; stabilized January 2018. Wilkinson, Stephan. HistoryNet, March 13, 2024. https://historynet.com/curtiss-wright.... Full numbered source list with links is published with the script.