Interchangeable parts are parts that are, for practical purposes, identical. They are made to specifications that ensure that they are so nearly identical that they will fit into any device 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 a ubiquitous element of modern manufacturing.
Interchangeability of parts was achieved by combining a number of innovations and improvements in machining operations and machine tools. These innovations included invention of new machine tools, 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 machines tools often have numerical control (NC) which evolved into CNC (computerized numeric control) when microprocessors became available.
Harder steels for cutting edges were developed which allowed steel rather than iron to be used for parts, eliminating the problem of warping and dimensional changes associated with heat treatment hardening of iron parts after machining. Modern cutting edges use materials such as tungsten carbide. Other innovations were drop forging and stamped steel parts, which reduced or eliminated the amount of machining.
The system for producing interchangeable parts is alternately called The American system of manufacturing because it was first most fully developed in the US, although contributions were made by other nations who soon implemented it.
Evidence of the use of interchangeable parts can be traced back as far as the Punic Wars (through both archaeological remains of boats now in Museo Archeologico Baglio Anselmi) and contemporary written accounts of boat building techniques, however it was not until relatively recently that the impact the adoption of this approach could have was more widely understood.
Before 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 to an expert gunsmith for custom repairs, or discarded and replaced by another weapon. During the 18th and early 19th centuries, the idea of replacing these methods with a system of interchangeable manufacture was gradually developed. The development took decades and involved many people.
In the late 18th century, French General Jean-Baptiste Vaquette de Gribeauval suggested that muskets could be manufactured faster and more economically if they were made from interchangeable parts. He provided patronage to Honoré Blanc, who attempted to implement the Système Gribeauval. By 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.
Numerous inventors began to try to implement the principle Blanc had described. The development of the machine tools and manufacturing practices required would be a great expense to the U.S. Ordinance Department, and for some years while trying to achieve interchangeabililty, the firearms produced cost more to manufacture. By 1854 there was evidence that interchangeable parts, then perfected by the Federal Armories, led to a savings. The Ordinance Department freely shared the techniques used with outside suppliers.
Eli Whitney and an early failed attempt
In the US, Eli Whitney saw the potential benefit of developing “interchangeable parts” for the firearms of the United States military. In July 1801 he built ten guns, all containing the same exact parts and mechanisms, then disassembled them before the United States Congress. He placed the parts in a 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 centered around the inability to consistently reproduce new parts for old equipment without the aid of significant hand finishing 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 catch was that the guns Whitney showed Congress were made by hand at great cost by skilled workmen.
Whitney was never able to design a manufacturing process capable of producing guns with interchangeable parts. Fitch (1882:4) 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 achieved interchangeable parts manufacturing. His family’s arms company, however, did so after his death.
Brunel’s sailing blocks
Mass production using interchangeable parts was first achieved in 1803 by Marc Isambard Brunel in cooperation with Henry Maudslay and Simon Goodrich, under the management of (and with contributions by) Brigadier-General Sir Samuel Bentham, the Inspector General of Naval Works at Portsmouth Block Mills, Portsmouth Dockyard, Hampshire, England. Blocks for sailing ships were made here for the Royal Navy, which was engaged in the Napoleonic Wars. By 1808, annual production had reached 130,000 blocks.         
Terry’s clocks: success in wood
The first mass production using interchangeable parts in America was, according to Diana Muir in Reflections in Bullough’s Pond, “The world’s first complex machine mass-produced from interchangeable parts”, which was Eli Terry‘s pillar-and-scroll clock, which came off the production line in 1814 at Plymouth, Connecticut. Terry’s clocks were made of wooden parts. Making a machine with moving parts mass-produced from metal would be much more difficult.
North and Hall: success in metal
The crucial step in that direction was taken by Simeon North, working only a few miles from Eli Terry. North created one of the world’s first true milling machines to do metal shaping that previously was done by hand with a file. Diana Muir believes that North’s milling machine was online around 1816. 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. 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.
Late 19th and early 20th centuries: dissemination throughout manufacturing
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 longtime 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.
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 to augment (and later largely replace) the traditional hand tools. Throughout this century there was much development work to be done in creating gauges, measuring tools (such as calipers and micrometers), standards (such as those for screw threads), and processes (such as scientific management), 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.
Interchangeability relies on parts’ dimensions falling within the tolerance range. The most common mode of assembly is to design and manufacture such that, as long as each part that reaches assembly is within tolerance, the mating of parts can be totally random. This has value for all the reasons already discussed earlier.
There is another mode of assembly, called selective assembly, which gives up some of the randomness capability in trade-off for other value. There are two main areas of application that benefit economically from selective assembly: when tolerance ranges are so tight that they cannot quite be held reliably (making the total randomness unavailable); and when tolerance ranges can be reliably held, but the fit and finish of the final assembly is being maximized by voluntarily giving up some of the randomness (which makes it available but not ideally desirable). In either case the principle of selective assembly is the same: parts are selected for mating, rather than being mated at random. As the parts are inspected, they are graded out into separate bins based on what end of the range they fall in (or violate). Falling within the high or low end of a range is usually called being “light” or “heavy”; violating the high or low end of a range is usually called being oversize or undersize. Examples are given below.
One might ask, if parts must be selected for mating, then what makes selective assembly any different from the oldest craft methods? But there is in fact a big difference. Selective assembly merely grades the parts into several ranges; within each range, there is still random interchangeability. This is quite different from the older method of fitting by a craftsman, where each mated set of parts is specifically filed to fit each part with a specific, unique counterpart.
Random assembly not available: oversize and undersize parts
In contexts where the application requires extremely tight (narrow) tolerance ranges, the requirement may push slightly past the limit of the ability of the machining and other processes (stamping, rolling, bending, etc) to stay within the range. In such cases, selective assembly is used to compensate for a lack of total interchangeability among the parts. Thus, for a pin that must have a sliding fit in its hole (free but not sloppy), the dimension may be spec’d as 12.00 +0 -0.01 mm for the pin, and 12.00 +.01 -0 for the hole. Pins that came out oversize (say a pin at 12.003mm diameter) are not necessarily scrap, but they can only be mated with counterparts that also came out oversize (say a hole at 12.013mm). The same is then true for matching undersize parts with undersize counterparts. Inherent in this example is that for this product’s application, the 12 mm dimension does not require extreme accuracy, but the desired fit between the parts does require good precision (see the article on accuracy and precision). This allows the makers to “cheat a little” on total interchangeability in order to get more value out of the manufacturing effort by reducing the rejection rate (scrap rate). This is a sound engineering decision as long as the application and context support it. For example, for machines for which there is no intention for any future field service of a parts-replacing nature (but rather only simple replacement of the whole unit), this makes good economic sense. It lowers the unit cost of the products, and it doesn’t impede future service work.
An example of a product that might benefit from this approach could be a car transmission where there is no expectation that the field service person will repair the old transmission; instead, she will simply swap in a new one. Therefore, total interchangeability was not absolutely required for the assemblies inside the transmissions. It would have been specified anyway, simply on general principle, except for a certain shaft that required precision so high as to cause great annoyance and high scrap rates in the grinding area, but for which only decent accuracy was required, as long as the fit with its hole was good in every case. Money could be saved by saving many shafts from the scrap bin.
Economic and commercial realities
Examples like the one above are not as common in real commerce as they conceivably could be, mostly because of separation of concerns, where each part of a complex system is expected to give performance that does not make any limiting assumptions about other parts of the system. In the car transmission example, the separation of concerns is that individual firms and customers accept no lack of freedom or options from others in the supply chain. For example, in the car buyer’s view, the car manufacturer is “not within its rights” to assume that no field-service mechanic will ever repair the old transmission instead of replacing it. The customer expects that that decision will be preserved for him to make later, at the repair shop, based on which option is less expensive for him at that time (figuring that replacing one shaft is cheaper than replacing a whole transmission). This logic is not always valid in reality; it might have been better for the customer’s total ownership cost to pay a lower initial price for the car (especially if the transmission service is covered under the standard warranty for 10 years, and the buyer intends to replace the car before then anyway) than to pay a higher initial price for the car but preserve the option of total interchangeability of every last nut, bolt, and shaft throughout the car (when it is not going to be taken advantage of anyway). But commerce is generally too chaotically multivariate for this logic to prevail, so total interchangeability ends up being specified and achieved even when it adds expense that was “needless” from a holistic view of the commercial system. But this may be avoided to the extent that customers experience the overall value (which their minds can detect and appreciate) without having to understand its logical analysis. Thus buyers of an amazingly affordable car (surprisingly low initial price) will probably never complain that the transmission was not field-serviceable as long as they themselves never had to pay for transmission service in the lifespan of their ownership. This analysis can be important for the manufacturer to understand (even if it is lost on the customer), because he can carve for himself a competitive advantage in the marketplace if he can accurately predict where to “cut corners” in ways that the customer will never have to pay for. Thus he could give himself lower transmission unit cost. However, he must be sure when he does so that the transmissions he’s using are reliable, because their replacement, being covered under a long warranty, will be at his expense.
Special exceptions in wartime
The World War II materiel effort is an interesting historical case for studying a special context in which separation of concerns was lowered. The command-and-control nature of entire supply chains allowed a degree of concern transfer that the normal peacetime commercial environment would never accept, from an agency like the War Production Board, through civilian businesses being told what to make (and to some extent how to make it), to the military logistics chain, out to the “point of the spear”, the frontline combat units. Thus, as one small example, if a factory did not have the specified alloy to make certain gears, a decision could be made to press ahead without it, knowlingly putting a slightly defective product into the supply chain. This decision could be more than justified by the serving of larger ends, ignoring separation of concerns. Thus a tank transmission could be prone to premature wearing out; but merely getting a good-enough tank into the field in 30 days was a much more important concern than getting a perfect, long-lasting tank into the field in 50 days. Many variables of the environment supported this rationale:
- The end user had no channel for rejecting the product; he was a soldier under orders, not a customer being placated or accommodated.
- The tank’s planned lifespan was only a few years anyway.
- Getting the tank into a certain region immediately was a much better service to the infantry soldiers anyway, as opposed to having them in that area under fire while waiting for delayed tanks to show up.
- The opportunity for field service was restricted anyway. Swapping in a new transmission was fine. No one in the field was expected to “get down inside” the transmission for complicated component repairs anyway.
The management calculus above was done to serve the ends of least harm and greatest common good. It was quite different from mere fraudulence in the materiel effort, such as an episode at Curtiss-Wright, where the difference is that pushing production quotas at the expense of quality was done in a way that caused harm without preventing any.
Random assembly available but not ideally desirable: “light” and “heavy” parts
The other main area of application for selective assembly is in contexts where total interchangeability is in fact achieved, but the “fit and finish” of the final products can be enhanced by minimizing the dimensional mismatch between mating parts. Consider another application similar to the one above with the 12 mm pin. But say that in this example, not only is the precision important (to produce the desired fit), but the accuracy is also important (because the 12 mm pin must interact with something else that will have to be accurately sized at 12 mm). Some of the implications of this example are that the rejection rate cannot be lowered; all parts must fall within tolerance range or be scrapped. So there are no savings to be had from salvaging oversize or undersize parts from scrap, then. However, there is still one bit of value to be had from selective assembly: having all the mated pairs have as close to identical sliding fit as possible (as opposed to some tighter fits and some looser fits—all sliding, but with varying resistance).
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This information originally retrieved from http://en.wikipedia.org/wiki/Interchangeable_parts
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