Prior to the 18th century, devices such as guns were made one at a time by gunsmiths, and each gun was unique. If one single component of a weapon needed a replacement, the entire weapon either had to be sent back to an expert gunsmith to make custom repairs or discarded and replaced by another weapon.
Around 1778, Honoré Blanc began producing some of the first firearms with interchangeable parts. Blanc demonstrated in front of a committee of scientists that his muskets could be assembled from a pile of parts selected at random. Other inventors who began to implement the principle included Henry Maudslay, John Hall, and Simeon North.
In the U.S., Eli Whitney saw the potential benefit of developing "interchangeable parts" for the firearms of the United States military, and thus, around 1798, he built ten guns, all containing the same exact parts and mechanisms, and disassembled them before the United States Congress. He placed the parts in a large mixed pile and, with help, reassembled all of the weapons right in front of Congress, much like Blanc had done some years before.
The Congress was immensely impressed and ordered a standard for all United States equipment. With interchangeable parts, the problems that had plagued the era of unique weapons and equipment passed, and if one mechanism in a weapon failed, a new piece could be ordered and the weapon would not have to be discarded. The hitch was that the guns Whitney showed congress were made by hand at great cost by extremely skilled workmen. Whitney, however, was never able to design a manufacturing process capable of producing guns with interchangeable parts. Historians Merritt Roe Smith and Robert B. Gordon have demonstrated conclusively that Whitney never achieved interchangeable parts manufacturing.
The men who did achieve mass production using interchangeable parts were, however, Americans. According to Diana Muir writing in Reflections in Bullough's Pond, "The world's first complex machine mass-produced from interchangeable parts" was Eli Terry's pillar-and-scroll clock, which rolled off the production line in 1814 at Plymouth, Connecticut. Terry's clocks, however, were made of wooden parts. Making a machine with moving parts mass-produced from metal would be much more difficult.
The crucial step in that direction was taken by Simeon North, working only a few miles from Eli Terry. North is known to have created the world's first machine capable of shaping metal (work that previously, as under Eli Whitney, had to be done by hand with a file.) Muir believes that North's milling machine was online around 1816. Diana Muir Merritt Roe Smith and Robert B. Gordon all agree that before 1832 both Simeon North and John Hall were able to mass-produce complex machines with moving parts (guns) using a system that entailed the use of rough-forged parts, with a milling machine that milled the parts to near-correct size, and that were then "filed to gage by hand with the aid of filing jigs."
Historians differ over the question of whether Hall or North made the crucial improvement. Merrit Roe Smith believes that it was done by Hall. Diana Muir demonstrates the close personal ties and professional alliances between Simeon North and neighboring mechanics mass-producing wooden clocks to argue that the process for manufacturing guns with interchangeable parts was most probably devised by North in emulation of the successful methods used in mass-producing clocks. It may not be possible to resolve the question with absolute certainty unless documents now unknown should surface in the future.
The principle of interchangeable parts flourished and developed throughout the 19th century, and led to mass production in many industries. It was based on the use of templates and other jigs and fixtures, applied by semi-skilled labor using machine tools instead of the traditional hand tools. Throughout this century there was a lot of development work to be done in creating gauges, measuring tools (such as calipers and micrometers), standards (such as those for screw threads), and processes (from scientific management to lean manufacturing), but the principle of interchangeability remained constant. With the introduction of the assembly line at the beginning of the 20th century, interchangeable parts became ubiquitous elements of manufacturing.
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