How machines learned precision

A history of how workshop practice and machine design produced the fine accuracy needed by steam engines and other machinery. Early engines used water-packed pistons that hid cylinder irregularities, but James Watt’s move to dry packing forced bores to be round and straight along the whole stroke. Cast cylinders and single‑ended boring bars produced crooked, out‑of‑round bores (Watt’s 1769 cylinder was off by ~3/8 inch in places). John Wilkinson’s innovation of a heavy boring bar supported in bearings at both ends let the cutter follow a true axis and produced cylinders whose error was no more than a worn shilling across a fifty‑inch bore - about thirty times better than earlier work. Lathes likewise improved: hand‑guided tools caused chatter, while Maudslay’s rigid slide rest and screw-cutting lathe put the forces into iron (the support bends ~100× less than a human arm), and by the 1850s Whitworth had measuring machines capable of detecting millionths of an inch.

Equally important were methods for making flat reference surfaces. Filing alone couldn’t reach the required precision, so fitters used scraping plus a transfer of marking color to remove high spots a few thousandths of a millimetre at a time. The three‑plate method - iteratively scraping three plates against each other - produced a true flat from rough castings without any preexisting reference. Scraped plates and straightedges let workshops check and scrape lathe ways, while planing machines removed bulk material before finishing; Maudslay’s shops had scraped plates by 1829 and Whitworth promoted these practices to spread precision manufacturing.