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Interchangeable parts are parts () that are identical for practical purposes. They are made to that ensure that they are so nearly identical that they will fit into any assembly of the same type. One such part can freely replace another, without any custom fitting, such as filing. This interchangeability allows easy assembly of new devices, and easier repair of existing devices, while minimizing both the time and skill required of the person doing the assembly or repair.
The concept of interchangeability was crucial to the introduction of the assembly line at the beginning of the 20th century, and has become an important element of some modern manufacturing but is missing from other important industries.
Interchangeability of parts was achieved by combining a number of innovations and improvements in machining operations and the invention of several , such as the slide rest lathe, screw-cutting lathe, turret lathe, milling machine and metal planer. Additional innovations included jigs for guiding the machine tools, fixtures for holding the workpiece in the proper position, and blocks and gauges to check the accuracy of the finished parts. Electrification allowed individual machine tools to be powered by electric motors, eliminating line shaft drives from steam engines or water power and allowing higher speeds, making modern large-scale manufacturing possible. Modern machine tools often have numerical control (NC) which evolved into CNC (computerized numeric control) when microprocessors became available.
Methods for industrial production of interchangeable parts in the United States were first developed in the nineteenth century. The term American system of manufacturing was sometimes applied to them at the time, in distinction from earlier methods. Within a few decades such methods were in use in various countries, so American system is now a term of historical reference rather than current industrial nomenclature.
Before the 18th century, devices such as guns were made one at a time by in a unique manner. If one single component of a firearm needed a replacement, the entire firearm either had to be sent to an expert gunsmith for custom repairs, or discarded and replaced by another firearm. During the 18th and early-19th centuries, the idea of replacing these methods with a system of interchangeable manufacture gradually developed., p. 4.. The development took decades and involved many people.
Gribeauval provided patronage to Honoré Blanc, who attempted to implement the Système Gribeauval at the musket level. By around 1778, Honoré Blanc began producing some of the first firearms with interchangeable flintlock mechanisms, although they were carefully made by craftsmen. Blanc demonstrated in front of a committee of scientists that his muskets could be fitted with flintlock mechanisms picked at random from a pile of parts.
In 1785 muskets with interchangeable locks caught the attention of the United States' Ambassador to France, Thomas Jefferson, through the efforts of Honoré Blanc. Jefferson tried unsuccessfully to persuade Blanc to move to America, then wrote to the American Secretary of War with the idea, and when he returned to the USA he worked to fund its development. President George Washington approved of the concept, and in 1798 Eli Whitney signed a contract to mass-produce 12,000 muskets built under the new system.James Burke, Connections (Little, Brown and Co.), 1978/1995 , p. 150
Between 4 July 1793 and 25 November 1795, the London gunsmith Henry Nock delivered 12,010 'screwless' or 'Duke's' locks to the British Board of Ordnance. These locks were intended to be interchangeable, being manufactured in large volumes in a steam-powered factory using gauges and lathes by a workforce combining salaried employees and Piece work. Subsequent experiments have suggested that the lock's components were interchangeable at a higher rate than those of the later British Brown Bess and the American M1816 musket.
Louis de Tousard, who fled the French Revolution, joined the U.S. Corp of Artillerists in 1795 and wrote an influential artillerist's manual that stressed the importance of standardization.
The Congress was captivated and ordered a standard for all United States equipment. The use of interchangeable parts removed the problems of earlier eras concerning the difficulty or impossibility of producing new parts for old equipment. If one firearm part failed, another could be ordered, and the firearm would not need to be discarded. The catch was that Whitney's guns were costly and handmade by skilled workmen.
Charles Fitch credited Whitney with successfully executing a firearms contract with interchangeable parts using the American System, but historians Merritt Roe Smith and Robert B. Gordon have since determined that Whitney never actually achieved interchangeable parts manufacturing. His family's arms company, however, did so after his death.
Marc Brunel, a pioneering engineer, and Maudslay, a founding father of machine tool technology who had developed the first industrially practical screw-cutting lathe in 1800 which standardized screw thread sizes for the first time,Quentin R. Skrabec, Jr. (2005). "The Metallurgic Age: The Victorian Flowering of Invention and Industrial Science". p. 169. McFarland collaborated on plans to manufacture block-making machinery; the proposal was submitted to the Admiralty who agreed to commission his services. By 1805, the dockyard had been fully updated with the revolutionary, purpose-built machinery at a time when products were still built individually with different components. A total of 45 machines were required to perform 22 processes on the blocks, which could be made in three different sizes. The machines were almost entirely made of metal, thus improving their accuracy and durability. The machines would make markings and indentations on the blocks to ensure alignment throughout the process. One of the many advantages of this new method was the increase in labour productivity due to the less labour-intensive requirements of managing the machinery. Richard Beamish, assistant to Brunel's son and engineer, Isambard Kingdom Brunel, wrote:
So that ten men, by the aid of this machinery, can accomplish with uniformity, celerity and ease, what formerly required the uncertain labour of one hundred and ten.
By 1808, annual production had reached 130,000 blocks and some of the equipment was still in operation as late as the mid-twentieth century....
Historians differ over the question of whether Hall or North made the crucial improvement. Merrit Roe Smith believes that it was done by Hall... Muir demonstrates the close personal ties and professional alliances between Simeon North and neighbouring 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.
as well as locomotive makers. Typewriters followed some years later. Then large scale production of bicycles in the 1880s began to use the interchangeable system.
During these decades, true interchangeability grew from a scarce and difficult achievement into an everyday capability throughout the manufacturing industries. In the 1950s and 1960s, historians of technology broadened the world's understanding of the history of the development. Few people outside that academic discipline knew much about the topic until as recently as the 1980s and 1990s, when the academic knowledge began finding wider audiences. As recently as the 1960s, when Alfred P. Sloan published his famous memoir and management treatise, My Years with General Motors, even the long-time president and chair of the largest manufacturing enterprise that had ever existed knew very little about the history of the development, other than to say that:
[Henry M. Leland was], I believe, one of those mainly responsible for bringing the technique of interchangeable parts into automobile manufacturing. … It has been called to my attention that Eli Whitney, long before, had started the development of interchangeable parts in connection with the manufacture of guns, a fact which suggests a line of descent from Whitney to Leland to the automobile industry..
One of the better-known books on the subject, which was first published in 1984 and has enjoyed a readership beyond academia, has been David A. Hounshell's From the American System to Mass Production, 1800–1932: The Development of Manufacturing Technology in the United States.
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