Interchangeable parts
Encyclopedia
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 the invention of several machine tool
s, 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 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 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.
(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 was more widely understood.
Before the 18th century, devices such as guns were made one at a time by gunsmith
s, 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, promoted standardized weapons, which became known as the Système Gribeauval
after it was issued as a royal order in 1765. (Its focus at the time was artillery
more than musket
s or handgun
s.) One of the accomplishments of the system was solid cast cannons bored to precise tolerances which allowed the walls to be thinner than cannons poured with hollow cores, but because cores were often off center the wall thickness determined the size of the bore. Standardized boring allowed cannon to be shorter without sacrificing accuracy and range because of the tighter fit of the shells
. It also allowed standardization of the shells.
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 flint locks, although they were carefully made by craftsmen. Blanc demonstrated in front of a committee of scientists that his muskets could be fitted with flint locks picked at random from a pile of parts.
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 captivated and ordered a standard for all United States equipment. Interchangeable parts removed problems concerning the inability to consistently produce new parts for old equipment without significant hand finishing that had plagued the era of unique weapons and equipment. If one weapon part failed, another could be ordered, and the weapon wouldn't have to be discarded. The catch was that the Whitney's guns were costly and handmade 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.
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.
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.
, working only a few miles from Eli Terry
. North created one of the world's first true milling machine
s 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.
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.
in many industries. It was based on the use of templates and other jigs
and fixtures
, applied by semi-skilled labor using machine tool
s to augment (and later largely replace) the traditional hand tool
s. Throughout this century there was much development work to be done in creating gauges, measuring tools (such as caliper
s and micrometer
s), 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.
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 significant 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.
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.
, 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.
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 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.
An example of a product that might benefit from this approach could be a toolroom
-grade machine tool
, where not only is the accuracy highly important but also the fit and finish.
File (tool)
A file is a metalworking and woodworking tool used to cut fine amounts of material from a workpiece. It most commonly refers to the hand tool style, which takes the form of a steel bar with a case hardened surface and a series of sharp, parallel teeth. Most files have a narrow, pointed tang at one...
). 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.
Assembly line
An assembly line is a manufacturing process in which parts are added to a product in a sequential manner using optimally planned logistics to create a finished product much faster than with handcrafting-type methods...
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 the invention of several machine tool
Machine tool
A machine tool is a machine, typically powered other than by human muscle , used to make manufactured parts in various ways that include cutting or certain other kinds of deformation...
s, such as the slide rest lathe, screw-cutting lathe
Screw-cutting lathe
A screw-cutting lathe is a machine capable of cutting very accurate screw threads via single-point screw-cutting, which is the process of guiding the linear motion of the tool bit in a precisely known ratio to the rotating motion of the workpiece...
, turret lathe
Turret lathe
The turret lathe is a form of metalworking lathe that is used for repetitive production of duplicate parts, which by the nature of their cutting process are usually interchangeable...
, milling machine
Milling machine
A milling machine is a machine tool used to machine solid materials. Milling machines are often classed in two basic forms, horizontal and vertical, which refers to the orientation of the main spindle. Both types range in size from small, bench-mounted devices to room-sized machines...
and metal planer
Planer (metalworking)
A planer is a type of metalworking machine tool that uses linear relative motion between the workpiece and a single-point cutting tool to machine a linear toolpath. Its cut is analogous to that of a lathe, except that it is linear instead of helical...
. 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
Electrification
Electrification originally referred to the build out of the electrical generating and distribution systems which occurred in the United States, England and other countries from the mid 1880's until around 1940 and is in progress in developing countries. This also included the change over from line...
allowed individual machine tools to be powered by electric motors, eliminating line shaft
Line shaft
A line shaft is a power transmission system used extensively during the Industrial Revolution. Prior to the widespread use of electric motors small enough to be connected directly to each piece of machinery, line shafting was used to distribute power from a large central power source to machinery...
drives from steam engines or water power and allowing higher speeds, making modern large scale manufacturing possible. Modern machines tools often have numerical control
Numerical control
Numerical control refers to the automation of machine tools that are operated by abstractly programmed commands encoded on a storage medium, as opposed to controlled manually via handwheels or levers, or mechanically automated via cams alone...
(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 parts after machining. Modern cutting edges use materials such as tungsten carbide
Tungsten carbide
Tungsten carbide is an inorganic chemical compound containing equal parts of tungsten and carbon atoms. Colloquially, tungsten carbide is often simply called carbide. In its most basic form, it is a fine gray powder, but it can be pressed and formed into shapes for use in industrial machinery,...
. 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
American system of manufacturing
The American system of manufacturing was a set of manufacturing methods that evolved in the 19th century. It involved semi-skilled labor using machine tools and jigs to make standardized, identical, interchangeable parts, manufactured to a tolerance, which could be assembled with a minimum of time...
because it was first most fully developed in the US, although contributions were made by other nations who soon implemented it.
Ideation
Evidence of the use of interchangeable parts can be traced back as far as the Punic WarsPunic Wars
The Punic Wars were a series of three wars fought between Rome and Carthage from 264 B.C.E. to 146 B.C.E. At the time, they were probably the largest wars that had ever taken place...
(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 was more widely understood.
Before the 18th century, devices such as guns were made one at a time by gunsmith
Gunsmith
A gunsmith is a person who repairs, modifies, designs, or builds firearms. This occupation is different from an armorer. The armorer primarily maintains weapons and limited repairs involving parts replacement and possibly work involving accurization...
s, 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, promoted standardized weapons, which became known as the Système Gribeauval
Gribeauval system
The Gribeauval system was an artillery system introduced by Lieutenant General Jean Baptiste Vaquette de Gribeauval during the 18th century. This system revolutionized French cannon, with a new production system that allowed lighter, more uniform guns without sacrificing range. The Gribeauval...
after it was issued as a royal order in 1765. (Its focus at the time was artillery
Artillery
Originally applied to any group of infantry primarily armed with projectile weapons, artillery has over time become limited in meaning to refer only to those engines of war that operate by projection of munitions far beyond the range of effect of personal weapons...
more than musket
Musket
A musket is a muzzle-loaded, smooth bore long gun, fired from the shoulder. Muskets were designed for use by infantry. A soldier armed with a musket had the designation musketman or musketeer....
s or handgun
Handgun
A handgun is a firearm designed to be held and operated by one hand. This characteristic differentiates handguns as a general class of firearms from long guns such as rifles and shotguns ....
s.) One of the accomplishments of the system was solid cast cannons bored to precise tolerances which allowed the walls to be thinner than cannons poured with hollow cores, but because cores were often off center the wall thickness determined the size of the bore. Standardized boring allowed cannon to be shorter without sacrificing accuracy and range because of the tighter fit of the shells
Shell (projectile)
A shell is a payload-carrying projectile, which, as opposed to shot, contains an explosive or other filling, though modern usage sometimes includes large solid projectiles properly termed shot . Solid shot may contain a pyrotechnic compound if a tracer or spotting charge is used...
. It also allowed standardization of the shells.
Gribeauval provided patronage to Honoré Blanc
Honoré Blanc
Honoré Blanc was a French gunsmith and a pioneer of the use of interchangeable parts.He was born in Avignon in 1736 and apprenticed to the gun-making trade at the age of twelve...
, who attempted to implement the Système Gribeauval
Gribeauval system
The Gribeauval system was an artillery system introduced by Lieutenant General Jean Baptiste Vaquette de Gribeauval during the 18th century. This system revolutionized French cannon, with a new production system that allowed lighter, more uniform guns without sacrificing range. The Gribeauval...
at the musket level. By around 1778, Honoré Blanc began producing some of the first firearms with interchangeable flint locks, although they were carefully made by craftsmen. Blanc demonstrated in front of a committee of scientists that his muskets could be fitted with flint locks picked at random from a pile of parts.
Implementation
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 1853 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 WhitneyEli Whitney
Eli Whitney was an American inventor best known for inventing the cotton gin. This was one of the key inventions of the Industrial Revolution and shaped the economy of the Antebellum South...
saw the potential benefit of developing "interchangeable parts" for the firearms of the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
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 captivated and ordered a standard for all United States equipment. Interchangeable parts removed problems concerning the inability to consistently produce new parts for old equipment without significant hand finishing that had plagued the era of unique weapons and equipment. If one weapon part failed, another could be ordered, and the weapon wouldn't have to be discarded. The catch was that the Whitney's guns were costly and handmade 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
American system of manufacturing
The American system of manufacturing was a set of manufacturing methods that evolved in the 19th century. It involved semi-skilled labor using machine tools and jigs to make standardized, identical, interchangeable parts, manufactured to a tolerance, which could be assembled with a minimum of time...
, but historians Merritt Roe Smith
Merritt Roe Smith
Merritt Roe Smith is an American historian, and the Leverett and William Cutten Professor of the History of Technology, at MIT.-Life:Smith graduated from Georgetown University, and Pennsylvania State University with a Ph.D...
and Robert B. Gordon
Robert B. Gordon
Robert Bryarly Gordon was a U.S. Representative from Ohio.Born at St. Marys, Auglaize County, Ohio, Gordon attended the public schools. He served as Postmaster of St...
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
Marc Isambard Brunel
Sir Marc Isambard Brunel, FRS FRSE was a French-born engineer who settled in England. He preferred the name Isambard, but is generally known to history as Marc to avoid confusion with his more famous son Isambard Kingdom Brunel...
in cooperation with Henry Maudslay
Henry Maudslay
Henry Maudslay was a British machine tool innovator, tool and die maker, and inventor. He is considered a founding father of machine tool technology.-Early life:...
and Simon Goodrich, under the management of (and with contributions by) Brigadier-General Sir Samuel Bentham
Samuel Bentham
Sir Samuel Bentham was a noted English mechanical engineer and naval architect credited with numerous innovations, particularly related to naval architecture, including weapons...
, the Inspector General of Naval Works at Portsmouth Block Mills
Portsmouth Block Mills
The Portsmouth Block Mills form part of the Portsmouth Dockyard at Portsmouth, Hampshire, England, and were built during the Napoleonic Wars to supply the British Royal Navy with pulley blocks. They started the age of mass-production using all-metal machine tools and are regarded as one of the...
, Portsmouth Dockyard, Hampshire
Hampshire
Hampshire is a county on the southern coast of England in the United Kingdom. The county town of Hampshire is Winchester, a historic cathedral city that was once the capital of England. Hampshire is notable for housing the original birthplaces of the Royal Navy, British Army, and Royal Air Force...
, England. Blocks for sailing ships
Block (sailing)
In sailing, a block is a single or multiple pulley. One or a number of sheaves are enclosed in an assembly between cheeks or chocks. In use a block is fixed to the end of a line, to a spar or to a surface...
were made here for the Royal Navy
Royal Navy
The Royal Navy is the naval warfare service branch of the British Armed Forces. Founded in the 16th century, it is the oldest service branch and is known as the Senior Service...
, which was engaged in the Napoleonic Wars
Napoleonic Wars
The Napoleonic Wars were a series of wars declared against Napoleon's French Empire by opposing coalitions that ran from 1803 to 1815. As a continuation of the wars sparked by the French Revolution of 1789, they revolutionised European armies and played out on an unprecedented scale, mainly due to...
. 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 MuirDiana Muir
Diana Muir, also known as Diana Muir Appelbaum, is a Newton, Massachusetts writer and historian. Muir is best known for her 2000 book, Reflections in Bullough's Pond, a history of the impact of human activity on the New England ecosystem....
in Reflections in Bullough's Pond
Reflections in Bullough's Pond
Reflections in Bullough's Pond: Economy and Ecosystem in New England is a book by Diana Muir. The Providence Journal called Bullough’s Pond "a masterpiece," and Publishers Weekly called it "lyrical"...
, "The world's first complex machine mass-produced from interchangeable parts", which was Eli Terry
Eli Terry
Eli Terry Sr. was an inventor and clockmaker in Connecticut. He received a United States patent for a shelf clock mechanism. He introduced mass production to the art of clockmaking, which made clocks affordable for the average American citizen...
's pillar-and-scroll clock, which came off the production line in 1814 at Plymouth, Connecticut
Plymouth, Connecticut
Plymouth is a town in Litchfield County, Connecticut, United States. It is named after Plymouth, Devon, England. The population was 11,634 at the 2000 census. The Town of Plymouth includes the villages of Terryville and Pequabuck.-Geography:...
. 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 NorthSimeon North
Simeon North was a Middletown, Connecticut, gun manufacturer, who developed one of America's first milling machines in 1818 and played an important role in the development of interchangeable parts manufacturing.North was born in Berlin, Connecticut, into a prosperous family able to provide all...
, working only a few miles from Eli Terry
Eli Terry
Eli Terry Sr. was an inventor and clockmaker in Connecticut. He received a United States patent for a shelf clock mechanism. He introduced mass production to the art of clockmaking, which made clocks affordable for the average American citizen...
. North created one of the world's first true milling machine
Milling machine
A milling machine is a machine tool used to machine solid materials. Milling machines are often classed in two basic forms, horizontal and vertical, which refers to the orientation of the main spindle. Both types range in size from small, bench-mounted devices to room-sized machines...
s 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
Robert B. Gordon
Robert Bryarly Gordon was a U.S. Representative from Ohio.Born at St. Marys, Auglaize County, Ohio, Gordon attended the public schools. He served as Postmaster of St...
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 technologyHistory of technology
The history of technology is the history of the invention of tools and techniques, and is similar in many ways to the history of humanity. Background knowledge has enabled people to create new things, and conversely, many scientific endeavors have become possible through technologies which assist...
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
Alfred P. Sloan
Alfred Pritchard Sloan, Jr. was an American business executive in the automotive industry. He was a long-time president, chairman, and CEO of General Motors Corporation...
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
Henry Martyn Leland was a machinist, inventor, engineer and automotive entrepreneur who founded the two premier American luxury marques, Cadillac and Lincoln. Retrieved December 30, 2008....
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
David A. Hounshell
David Allen Hounshell is the David M. Roderick Professor of Technology and Social Change in the Department of Social and Decision Sciences, Department of History, and the Department of Engineering and Public Policy at Carnegie Mellon University....
's From the American System to Mass Production, 1800-1932: The Development of Manufacturing Technology in the United States.
Socioeconomic context
The principle of interchangeable parts flourished and developed throughout the 19th century, and led to mass productionMass production
Mass production is the production of large amounts of standardized products, including and especially on assembly lines...
in many industries. It was based on the use of templates and other jigs
Jig (tool)
In metalworking and woodworking, a jig is a type of tool used to control the location and/or motion of another tool. A jig's primary purpose is to provide repeatability, accuracy, and interchangeability in the manufacturing of products. A jig is often confused with a fixture; a fixture holds the...
and fixtures
Fixture (tool)
A fixture is a work-holding or support device used in the manufacturing industry. What makes a fixture unique is that each one is built to fit a particular part or shape. The main purpose of a fixture is to locate and in some cases hold a workpiece during either a machining operation or some...
, applied by semi-skilled labor using machine tool
Machine tool
A machine tool is a machine, typically powered other than by human muscle , used to make manufactured parts in various ways that include cutting or certain other kinds of deformation...
s to augment (and later largely replace) the traditional hand tool
Hand tool
A hand tool is a device for performing work on a material or a physical system using only hands. The hand tools can be manually used employing force, or electrically powered, using electrical current...
s. Throughout this century there was much development work to be done in creating gauges, measuring tools (such as caliper
Caliper
A caliper is a device used to measure the distance between two opposing sides of an object. A caliper can be as simple as a compass with inward or outward-facing points...
s and micrometer
Micrometer
A micrometer , sometimes known as a micrometer screw gauge, is a device incorporating a calibrated screw used widely for precise measurement of small distances in mechanical engineering and machining as well as most mechanical trades, along with other metrological instruments such as dial, vernier,...
s), standards (such as those for screw threads), and processes (such as scientific management
Scientific management
Scientific management, also called Taylorism, was a theory of management that analyzed and synthesized workflows. Its main objective was improving economic efficiency, especially labor productivity. It was one of the earliest attempts to apply science to the engineering of processes and to management...
), but the principle of interchangeability remained constant. With the introduction of the assembly line
Assembly line
An assembly line is a manufacturing process in which parts are added to a product in a sequential manner using optimally planned logistics to create a finished product much faster than with handcrafting-type methods...
at the beginning of the 20th century, interchangeable parts became ubiquitous elements of manufacturing.
Selective assembly
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
Value (economics)
An economic value is the worth of a good or service as determined by the market.The economic value of a good or service has puzzled economists since the beginning of the discipline. First, economists tried to estimate the value of a good to an individual alone, and extend that definition to goods...
. 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
Inspection
An inspection is, most generally, an organized examination or formal evaluation exercise. In engineering activities inspection involves the measurements, tests, and gauges applied to certain characteristics in regard to an object or activity...
, 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 significant 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 requirementRequirement
In engineering, a requirement is a singular documented physical and functional need that a particular product or service must be or perform. It is most commonly used in a formal sense in systems engineering, software engineering, or enterprise engineering...
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
Scrap
Scrap is a term used to describe recyclable and other materials left over from every manner of product consumption, such as parts of vehicles, building supplies, and surplus materials. Unlike waste, scrap has significant monetary value...
, 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
Accuracy and precision
In the fields of science, engineering, industry and statistics, the accuracy of a measurement system is the degree of closeness of measurements of a quantity to that quantity's actual value. The precision of a measurement system, also called reproducibility or repeatability, is the degree to which...
). 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
Unit cost
The unit cost of a product is the cost per standard unit supplied, which may be a single sample or a container of a given number. When purchasing more than a single unit, the total cost will increase with the number of units, but it is common for the unit cost to decrease as quantity is increased...
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 concernsSeparation of concerns
In computer science, separation of concerns is the process of separating a computer program into distinct features that overlap in functionality as little as possible. A concern is any piece of interest or focus in a program. Typically, concerns are synonymous with features or behaviors...
, 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 materielMateriel
Materiel is a term used in English to refer to the equipment and supplies in military and commercial supply chain management....
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
War Production Board
The War Production Board was established as a government agency on January 16, 1942 by executive order of Franklin D. Roosevelt.The purpose of the board was to regulate the production and allocation of materials and fuel during World War II in the United States...
, 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
Tank
A tank is a tracked, armoured fighting vehicle designed for front-line combat which combines operational mobility, tactical offensive, and defensive capabilities...
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).An example of a product that might benefit from this approach could be a toolroom
Toolroom
The term toolroom can refer to three related concepts. The concepts have evolved over the past two centuries as technology itself has evolved.- Storing tools :...
-grade machine tool
Machine tool
A machine tool is a machine, typically powered other than by human muscle , used to make manufactured parts in various ways that include cutting or certain other kinds of deformation...
, where not only is the accuracy highly important but also the fit and finish.
See also
- Allowance (engineering)Allowance (engineering)In engineering and machining, an allowance is a planned deviation between an actual dimension and a nominal or theoretical dimension, or between an intermediate-stage dimension and an intended final dimension. The unifying abstract concept is that a certain amount of difference allows for some...
- Engineering fitEngineering fitFit refers to the mating of two mechanical components. Manufactured parts are very frequently required to mate with one another. They may be designed to slide freely against one another or they may be designed to bind together to form a single unit...
- Engineering tolerance
- Just-in-time (business)
- Louis de TousardLouis de TousardLouis de Tousard was a French artillerist who served in the American Continental Army under La Fayette, and later was given a US commission...
- Preferred numberPreferred numberIn industrial design, preferred numbers are standard guidelines for choosing exact product dimensions within a given set of constraints....
s