City
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For other uses, see City (disambiguation).
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Tokyo skyline
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World's densest population
World's largest conurbations
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World megacities
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View from the Griboyedov Canal in Saint Petersburg, Russia

A satellite view of East Asia at night shows urbanization as illumination. Here the Taiheiyō Belt, which includes Tokyo, demonstrates how megalopolises can be identified by nighttime lighting.[1]

This 1908 map of Piraeus, the port of Athens, shows the city's grid plan, credited by Aristotle to Hippodamus of Miletus.[2][3]
A city is a large human settlement.[4][5] Cities generally have extensive systems for housing, transportation, sanitation, utilities, land use, and communication. Their density facilitates interaction between people, government organisations and businesses, sometimes benefiting different parties in the process.

Historically, city-dwellers have been a small proportion of humanity overall, but following two centuries of unprecedented and rapid urbanisation, roughly half of the world population now lives in cities, which has had profound consequences for global sustainability.[6] Present-day cities usually form the core of larger metropolitan areas and urban areas—creating numerous commuters traveling towards city centres for employment, entertainment, and edification. However, in a world of intensifying globalisation, all cities are in different degree also connected globally beyond these regions.

The most populated city proper is Chongqing[7] while the most populous metropolitan areas are the Greater Tokyo Area, the Shanghai area, and Jakarta metropolitan area.[8] The cities of Faiyum,[9] Damascus,[10] and Varanasi[11] are among those laying claim to longest continual inhabitation.


Contents
1 Meaning
2 Etymology
3 Geography
3.1 Site
3.2 Center
3.3 Public space
3.4 Internal structure
3.5 Urban areas
4 History
4.1 Ancient times
4.2 Middle Ages
4.3 Early modern
4.4 Industrial age
4.5 Post-industrial age
5 Urbanization
6 Government
6.1 Municipal services
6.2 Finance
6.3 Governance
6.4 Urban planning
7 Society
7.1 Social structure
7.2 Economics
7.3 Culture and communications
7.4 Warfare
8 Infrastructure
8.1 Utilities
8.2 Transportation
8.3 Housing
9 Ecology
10 World city system
10.1 Global city
10.2 Transnational activity
10.3 Global governance
10.4 United Nations System
11 Representation in culture
12 See also
13 Notes
14 References
15 External links
Meaning

Palitana represents the city's symbolic function in the extreme, devoted as it is to Jain temples.[12]
A city is distinguished[by whom?] from other human settlements by its relatively great size, but also by its functions and its special symbolic status, which may be conferred by a central authority. The term can also refer either to the physical streets and buildings of the city or to the collection of people who dwell there, and can be used in a general sense to mean urban rather than rural territory.[13][14]

National censuses use a variety of definitions - invoking factors such as population, population density, number of dwellings, economic function, and infrastructure - to classify populations as urban. Typical working definitions for small-city populations start at around 100,000 people.[15] Common population definitions for an urban area (city or town) range between 1,500 and 50,000 people, with most U.S states using a minimum between 1,500 and 5,000 inhabitants.[16][17] Some jurisdictions set no such minima.[18] In the United Kingdom, city status is awarded by the Crown and then remains permanently. (Historically, the qualifying factor was the presence of a cathedral, resulting in some very small cities such as Wells, with a population 12,000 as of 2018 and St Davids, with a population of 1,841 as of 2011.) According to the "functional definition" a city is not distinguished by size alone, but also by the role it plays within a larger political context. Cities serve as administrative, commercial, religious, and cultural hubs for their larger surrounding areas.[19][20] Examples of settlements called "city" which may not meet any of the traditional criteria to be named such include Broad Top City, Pennsylvania (population 452), and City Dulas, Anglesey, a hamlet.

The presence of a literate elite is sometimes included[by whom?] in the definition.[21] A typical city has professional administrators, regulations, and some form of taxation (food and other necessities or means to trade for them) to support the government workers. (This arrangement contrasts with the more typically horizontal relationships in a tribe or village accomplishing common goals through informal agreements between neighbors, or through leadership of a chief.) The governments may be based on heredity, religion, military power, work systems such as canal-building, food-distribution, land-ownership, agriculture, commerce, manufacturing, finance, or a combination of these. Societies that live in cities are often called civilizations.

Etymology
The word "city" and the related "civilization" come, via Old French, from the Latin root civitas, originally meaning citizenship or community member and eventually coming to correspond with urbs, meaning "city" in a more physical sense.[13] The Roman civitas was closely linked with the Greek polis—another common root appearing in English words such as metropolis.[22]

Geography

Hillside housing and graveyard in Kabul.

Panoramic view of Tirana, Albania from Mount Dajt in 2004.

Downtown Pittsburgh sits at the confluence of the Monongahela and Allegheny rivers, which become the Ohio.

The L'Enfant Plan for Washington, D.C., inspired by the design of Versailles, combines a utilitarian grid pattern with diagonal avenues and a symbolic focus on monumental architecture.[23]

This aerial view of the Gush Dan metropolitan area in Israel shows the geometrically planned[24] city of Tel Aviv proper (upper left) as well as Givatayim to the east and some of Bat Yam to the south. Tel Aviv's population is 433,000; the total population of its metropolitan area is 3,785,000.[25]
Urban geography deals both with cities in their larger context and with their internal structure.[26]

Site
Town siting has varied through history according to natural, technological, economic, and military contexts. Access to water has long been a major factor in city placement and growth, and despite exceptions enabled by the advent of rail transport in the nineteenth century, through the present most of the world's urban population lives near the coast or on a river.[27]

Urban areas as a rule cannot produce their own food and therefore must develop some relationship with a hinterland which sustains them.[28] Only in special cases such as mining towns which play a vital role in long-distance trade, are cities disconnected from the countryside which feeds them.[29] Thus, centrality within a productive region influences siting, as economic forces would in theory favor the creation of market places in optimal mutually reachable locations.[30]

Center
Main article: City centre
The vast majority of cities have a central area containing buildings with special economic, political, and religious significance. Archaeologists refer to this area by the Greek term temenos or if fortified as a citadel.[31] These spaces historically reflect and amplify the city's centrality and importance to its wider sphere of influence.[30] Today cities have a city center or downtown, sometimes coincident with a central business district.


Public space
Cities typically have public spaces where anyone can go. These include privately owned spaces open to the public as well as forms of public land such as public domain and the commons. Western philosophy since the time of the Greek agora has considered physical public space as the substrate of the symbolic public sphere.[32][33] Public art adorns (or disfigures) public spaces. Parks and other natural sites within cities provide residents with relief from the hardness and regularity of typical built environments.

Internal structure
Urban structure generally follows one or more basic patterns: geomorphic, radial, concentric, rectilinear, and curvilinear. Physical environment generally constrains the form in which a city is built. If located on a mountainside, urban structure may rely on terraces and winding roads. It may be adapted to its means of subsistence (e.g. agriculture or fishing). And it may be set up for optimal defense given the surrounding landscape.[34] Beyond these "geomorphic" features, cities can develop internal patterns, due to natural growth or to city planning.

In a radial structure, main roads converge on a central point. This form could evolve from successive growth over a long time, with concentric traces of town walls and citadels marking older city boundaries. In more recent history, such forms were supplemented by ring roads moving traffic around the outskirts of a town. Dutch cities such as Amsterdam and Haarlem are structured as a central square surrounded by concentric canals marking every expansion. In cities such as and also Moscow, this pattern is still clearly visible.

A system of rectilinear city streets and land plots, known as the grid plan, has been used for millennia in Asia, Europe, and the Americas. The Indus Valley Civilisation built Mohenjo-Daro, Harappa and other cities on a grid pattern, using ancient principles described by Kautilya, and aligned with the compass points.[35][19][36][37] The ancient Greek city of Priene exemplifies a grid plan with specialized districts used across the Hellenistic Mediterranean.

Urban areas
Urban-type settlement extends far beyond the traditional boundaries of the city proper[38] in a form of development sometimes described critically as urban sprawl.[39] Decentralization and dispersal of city functions (commercial, industrial, residential, cultural, political) has transformed the very meaning of the term and has challenged geographers seeking to classify territories according to an urban-rural binary.[17]

Metropolitan areas include suburbs and exurbs organized around the needs of commuters, and sometimes edge cities characterized by a degree of economic and political independence. (In the US these are grouped into metropolitan statistical areas for purposes of demography and marketing.) Some cities are now part of a continuous urban landscape called urban agglomeration, conurbation, or megalopolis (exemplified by the BosWash corridor of the Northeastern United States.)[40]

History
Main article: History of the city
Further information: Urban history, Historical urban community sizes, and List of largest cities throughout history

An arch from the ancient Sumerian city Ur, which flourished in the third millennium BC, can be seen at present-day Tell el-Mukayyar in Iraq

Mohenjo-daro, a city of the Indus Valley Civilization in Pakistan, which was rebuilt six or more times, using bricks of standard size, and adhering to the same grid layout—also in the third millennium BC.

This aerial view of what was once downtown Teotihuacan shows the Pyramid of the Sun, Pyramid of the Moon, and the processional avenue serving as the spine of the city's street system.
Cities, characterized by population density, symbolic function, and urban planning, have existed for thousands of years. In the conventional view, civilization and the city both followed from the development of agriculture, which enabled production of surplus food, and thus a social division of labour (with concomitant social stratification) and trade.[41][42] Early cities often featured granaries, sometimes within a temple.[43] A minority viewpoint considers that cities may have arisen without agriculture, due to alternative means of subsistence (fishing),[44] to use as communal seasonal shelters,[45] to their value as bases for defensive and offensive military organization,[46][47] or to their inherent economic function.[48][49][50] Cities played a crucial role in the establishment of political power over an area, and ancient leaders such as Alexander the Great founded and created them with zeal.[51]

Ancient times
Further information: Cities of the Ancient Near East, Polis, City-state, and Late Antiquity § Cities
Jericho and Çatalhöyük, dated to the eighth millennium BC, are among the earliest proto-cities known to archaeologists.[45][52]

In the fourth and third millennium BC, complex civilizations flourished in the river valleys of Mesopotamia, India, China, and Egypt. Excavations in these areas have found the ruins of cities geared variously towards trade, politics, or religion. Some had large, dense populations, but others carried out urban activities in the realms of politics or religion without having large associated populations. Among the early Old World cities, Mohenjo-daro of the Indus Valley Civilization in present-day Pakistan, existing from about 2600 BC, was one of the largest, with a population of 50,000 or more and a sophisticated sanitation system.[53] China's planned cities were constructed according to sacred principles to act as celestial microcosms.[54] The Ancient Egyptian cities known physically by archaeologists are not extensive.[19] They include (known by their Arab names) El Lahun, a workers' town associated with the pyramid of Senusret II, and the religious city Amarna built by Akhenaten and abandoned. These sites appear planned in a highly regimented and stratified fashion, with a minimalistic grid of rooms for the workers and increasingly more elaborate housing available for higher classes.[55]

In Mesopotamia, the civilization of Sumer, followed by Assyria and Babylon, gave rise to numerous cities, governed by kings and fostering multiple languages written in cuneiform.[56] The Phoenician trading empire, flourishing around the turn of the first millennium BC, encompassed numerous cities extending from Tyre, Cydon, and Byblos to Carthage and Cádiz.

In the following centuries, independent city-states of Greece developed the polis, an association of male landowning citizens who collectively constituted the city.[57] The agora, meaning "gathering place" or "assembly", was the center of athletic, artistic, spiritual and political life of the polis.[58] Rome's rise to power brought its population to one million. Under the authority of its empire, Rome transformed and founded many cities (coloniae), and with them brought its principles of urban architecture, design, and society.[59]

In the ancient Americas, early urban traditions developed in the Andes and Mesoamerica. In the Andes, the first urban centers developed in the Norte Chico civilization, Chavin and Moche cultures, followed by major cities in the Huari, Chimu and Inca cultures. The Norte Chico civilization included as many as 30 major population centers in what is now the Norte Chico region of north-central coastal Peru. It is the oldest known civilization in the Americas, flourishing between the 30th century BC and the 18th century BC.[60] Mesoamerica saw the rise of early urbanism in several cultural regions, beginning with the Olmec and spreading to the Preclassic Maya, the Zapotec of Oaxaca, and Teotihuacan in central Mexico. Later cultures such as the Aztec drew on these earlier urban traditions.

Jenné-Jeno, located in present-day Mali and dating to the third century BC, lacked monumental architecture and a distinctive elite social class—but nevertheless had specialized production and relations with a hinterland.[61] Pre-Arabic trade contacts probably existed between Jenné-Jeno and North Africa.[62] Other early urban centers in sub-Saharan Africa, dated to around 500 AD, include Awdaghust, Kumbi-Saleh the ancient capital of Ghana, and Maranda a center located on a trade route between Egypt and Gao.[63]

In the first millennium AD, Angkor in the Khmer Empire grew into one of the most extensive cities in the world[64][65] and may have supported up to one million people.[66]

Middle Ages

Imperial Free Cities in the Holy Roman Empire 1648

This map of Haarlem, the Netherlands, created around 1550, shows the city completely surrounded by a city wall and defensive canal, with its square shape inspired by Jerusalem.
In the remnants of the Roman Empire, cities of late antiquity gained independence but soon lost population and importance. The locus of power in the West shifted to Constantinople and to the ascendant Islamic civilization with its major cities Baghdad, Cairo, and Córdoba.[67] From the 9th through the end of the 12th century, Constantinople, capital of the Eastern Roman Empire, was the largest and wealthiest city in Europe, with a population approaching 1 million.[68][69] The Ottoman Empire gradually gained control over many cities in the Mediterranean area, including Constantinople in 1453.

In the Holy Roman Empire, beginning in the 12th. century, free imperial cities such as Nuremberg, Strasbourg, Frankfurt, Zurich, Nijmegen became a privileged elite among towns having won self-governance from their local lay or secular lord or having been granted self-governanace by the emperor and being placed under his immediate protection. By 1480, these cities, as far as still part of the empire, became part of the Imperial Estates governing the empire with the emperor through the Imperial Diet.[70]

By the thirteenth and fourteenth centuries, some cities become powerful states, taking surrounding areas under their control or establishing extensive maritime empires. In Italy medieval communes developed into city-states including the Republic of Venice and the Republic of Genoa. In Northern Europe, cities including Lübeck and Bruges formed the Hanseatic League for collective defense and commerce. Their power was later challenged and eclipsed by the Dutch commercial cities of Ghent, Ypres, and Amsterdam.[71] Similar phenomena existed elsewhere, as in the case of Sakai, which enjoyed a considerable autonomy in late medieval Japan.

Early modern
In the West, nation-states became the dominant unit of political organization following the Peace of Westphalia in the seventeenth century.[72][73] Western Europe's larger capitals (London and Paris) benefited from the growth of commerce following the emergence of an Atlantic trade. However, most towns remained small.

During the Spanish colonization of the Americas the old Roman city concept was extensively used. Cities were founded in the middle of the newly conquered territories, and were bound to several laws regarding administration, finances and urbanism.

Industrial age
The growth of modern industry from the late 18th century onward led to massive urbanization and the rise of new great cities, first in Europe and then in other regions, as new opportunities brought huge numbers of migrants from rural communities into urban areas.

Numerical control
From Wikipedia, the free encyclopedia
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"CNC" redirects here. For other uses, see CNC (disambiguation).
"Numerics" redirects here. For the field of computer science, see Numerical analysis.

A CNC machine that operates on wood
Numerical control (also computer numerical control, and commonly called CNC) is the automated control of machining tools (drills, boring tools, lathes) and 3D printers by means of a computer. A CNC machine processes a piece of material (metal, plastic, wood, ceramic, or composite) to meet specifications by following a coded programmed instruction and without a manual operator.

A CNC machine is a motorized maneuverable tool and often a motorized maneuverable platform, which are both controlled by a computer, according to specific input instructions. Instructions are delivered to a CNC machine in the form of a sequential program of machine control instructions such as G-code and then executed. The program can be written by a person or, far more often this century, generated by graphical computer-aided design (CAD) software. In the case of 3D Printers, the part to be printed is "sliced", before the instructions (or the program) is generated. 3D printers also use G-Code.

CNC is a vast improvement over non-computerized machining that must be manually controlled (e.g., using devices such as hand wheels or levers) or mechanically controlled by pre-fabricated pattern guides (cams). In modern CNC systems, the design of a mechanical part and its manufacturing program is highly automated. The part's mechanical dimensions are defined using CAD software, and then translated into manufacturing directives by computer-aided manufacturing (CAM) software. The resulting directives are transformed (by "post processor" software) into the specific commands necessary for a particular machine to produce the component, and then are loaded into the CNC machine.

Since any particular component might require the use of a number of different tools – drills, saws, etc. – modern machines often combine multiple tools into a single "cell". In other installations, a number of different machines are used with an external controller and human or robotic operators that move the component from machine to machine. In either case, the series of steps needed to produce any part is highly automated and produces a part that closely matches the original CAD.


Contents
1 History
2 Description
3 Parts Description
4 Examples of CNC machines
4.1 Other CNC tools
5 Tool / machine crashing
6 Numerical precision and equipment backlash
7 Positioning control system
8 M-codes
9 G-codes
10 Coding
11 See also
12 References
13 Further reading
14 External links
History
Main article: History of numerical control
The first NC machines were built in the 1940s and 1950s, based on existing tools that were modified with motors that moved the tool or part to follow points fed into the system on punched tape. These early servomechanisms were rapidly augmented with analog and digital computers, creating the modern CNC machine tools that have revolutionized machining processes.

Description
Motion is controlling multiple axes, normally at least two (X and Y),[1] and a tool spindle that moves in the Z (depth). The position of the tool is driven by direct-drive stepper motors or servo motors in order to provide highly accurate movements, or in older designs, motors through a series of step-down gears. Open-loop control works as long as the forces are kept small enough and speeds are not too great. On commercial metalworking machines, closed loop controls are standard and required in order to provide the accuracy, speed, and repeatability demanded.

Parts Description
As the controller hardware evolved, the mills themselves also evolved. One change has been to enclose the entire mechanism in a large box as a safety measure, often with additional safety interlocks to ensure the operator is far enough from the working piece for safe operation. Most new CNC systems built today are 100% electronically controlled.

CNC-like systems are used for any process that can be described as movements and operations. These include laser cutting, welding, friction stir welding, ultrasonic welding, flame and plasma cutting, bending, spinning, hole-punching, pinning, gluing, fabric cutting, sewing, tape and fiber placement, routing, picking and placing, and sawing.

Examples of CNC machines
CNC Machine Description Image
Mill Translates programs consisting of specific numbers and letters to move the spindle (or workpiece) to various locations and depths. Many use G-code. Functions include: face milling, shoulder milling, tapping, drilling and some even offer turning. Today, CNC mills can have 3 to 6 axes. Most CNC mills require placing your workpiece on or in them and must be at least as big as your workpiece, but new 3-axis machines are being produced that you can put on your workpiece, and can be much smaller.[2]
Lathe Cuts workpieces while they are rotated. Makes fast, precision cuts, generally using indexable tools and drills. Effective for complicated programs designed to make parts that would be infeasible to make on manual lathes. Similar control specifications to CNC mills and can often read G-code. Generally have two axes (X and Z), but newer models have more axes, allowing for more advanced jobs to be machined.
Plasma cutter Involves cutting a material using a plasma torch. Commonly used to cut steel and other metals, but can be used on a variety of materials. In this process, gas (such as compressed air) is blown at high speed out of a nozzle; at the same time, an electrical arc is formed through that gas from the nozzle to the surface being cut, turning some of that gas to plasma. The plasma is sufficiently hot to melt the material being cut and moves sufficiently fast to blow molten metal away from the cut.
File:CNC Plasma Cutting.ogv
CNC plasma cutting
Electric discharge machining (EDM), also known as spark machining, spark eroding, burning, die sinking, or wire erosion, is a manufacturing process in which a desired shape is obtained using electrical discharges (sparks). Material is removed from the workpiece by a series of rapidly recurring current discharges between two electrodes, separated by a dielectric fluid and subject to an electric voltage. One of the electrodes is called the tool electrode, or simply the "tool" or "electrode," while the other is called the workpiece electrode, or "workpiece."

Master at top, badge die workpiece at bottom, oil jets at left (oil has been drained). Initial flat stamping will be "dapped" to give a curved surface.
Multi spindle machine Type of screw machine used in mass production. Considered to be highly efficient by increasing productivity through automation. Can efficiently cut materials into small pieces while simultaneously utilizing a diversified set of tooling. Multi-spindle machines have multiple spindles on a drum that rotates on a horizontal or vertical axis. The drum contains a drill head which consists of a number of spindles that are mounted on ball bearings and driven by gears. There are two types of attachments for these drill heads, fixed or adjustable, depending on whether the centre distance of the drilling spindle needs to be varied.[3]
Wire EDM Also known as wire cutting EDM, wire burning EDM, or traveling wire EDM, this process uses spark erosion to machine or remove material from any electrically conductive material, using a traveling wire electrode. The wire electrode usually consists of brass- or zinc-coated brass material. Wire EDM allows for near 90 degree corners and applies very little pressure on the material.[4] Since the wire is eroded in this process, a wire EDM machine feeds fresh wire from a spool while chopping up the used wire and leaving it in a bin for recycling.[5]
Sinker EDM Also called cavity type EDM or volume EDM, a sinker EDM consists of an electrode and workpiece submerged in oil or another dielectric fluid. The electrode and workpiece are connected to a suitable power supply, which generates an electrical potential between the two parts. As the electrode approaches the workpiece, dielectric breakdown occurs in the fluid forming a plasma channel and small spark jumps. Production dies and moulds are often made with sinker EDM. Some materials, such as soft ferrite materials and epoxy-rich bonded magnetic materials are not compatible with sinker EDM as they are not electrically conductive.[6]
Water jet cutter Also known as a "waterjet", is a tool capable of slicing into metal or other materials (such as granite) by using a jet of water at high velocity and pressure, or a mixture of water and an abrasive substance, such as sand. It is often used during fabrication or manufacture of parts for machinery and other devices. Waterjet is the preferred method when the materials being cut are sensitive to the high temperatures generated by other methods. It has found applications in a diverse number of industries from mining to aerospace where it is used for operations such as cutting, shaping, carving, and reaming.
Other CNC tools
Many other tools have CNC variants, including:

Drills
EDMs
Embroidery machines
Lathes
Milling machine
Canned cycle
Wood routers
Sheet metal works (Turret punch)
Tube, pipe and wire bending machines
Hot-wire foam cutters
Plasma cutters
Water jet cutters
Laser cutting
Oxy-fuel
Surface grinder
Cylindrical grinders
3D printing
Induction hardening machines
Submerged arc welding
Glass cutting
CNC router
Tool / machine crashing
In CNC, a "crash" occurs when the machine moves in such a way that is harmful to the machine, tools, or parts being machined, sometimes resulting in bending or breakage of cutting tools, accessory clamps, vises, and fixtures, or causing damage to the machine itself by bending guide rails, breaking drive screws, or causing structural components to crack or deform under strain. A mild crash may not damage the machine or tools, but may damage the part being machined so that it must be scrapped.

Many CNC tools have no inherent sense of the absolute position of the table or tools when turned on. They must be manually "homed" or "zeroed" to have any reference to work from, and these limits are just for figuring out the location of the part to work with it, and aren't really any sort of hard motion limit on the mechanism. It is often possible to drive the machine outside the physical bounds of its drive mechanism, resulting in a collision with itself or damage to the drive mechanism. Many machines implement control parameters limiting axis motion past a certain limit in addition to physical limit switches. However, these parameters can often be changed by the operator.

Many CNC tools also don't know anything about their working environment. Machines may have load sensing systems on spindle and axis drives, but some do not. They blindly follow the machining code provided and it is up to an operator to detect if a crash is either occurring or about to occur, and for the operator to manually abort the active process. Machines equipped with load sensors can stop axis or spindle movement in response to an overload condition, but this does not prevent a crash from occurring. It may only limit the damage resulting from the crash. Some crashes may not ever overload any axis or spindle drives.

If the drive system is weaker than the machine structural integrity, then the drive system simply pushes against the obstruction and the drive motors "slip in place". The machine tool may not detect the collision or the slipping, so for example the tool should now be at 210 mm on the X axis, but is, in fact, at 32mm where it hit the obstruction and kept slipping. All of the next tool motions will be off by −178mm on the X axis, and all future motions are now invalid, which may result in further collisions with clamps, vises, or the machine itself. This is common in open loop stepper systems, but is not possible in closed loop systems unless mechanical slippage between the motor and drive mechanism has occurred. Instead, in a closed loop system, the machine will continue to attempt to move against the load until either the drive motor goes into an overload condition or a servo motor fails to get to the desired position.

Collision detection and avoidance is possible, through the use of absolute position sensors (optical encoder strips or disks) to verify that motion occurred, or torque sensors or power-draw sensors on the drive system to detect abnormal strain when the machine should just be moving and not cutting, but these are not a common component of most hobby CNC tools.

Instead, most hobby CNC tools simply rely on the assumed accuracy of stepper motors that rotate a specific number of degrees in response to magnetic field changes. It is often assumed the stepper is perfectly accurate and never missteps, so tool position monitoring simply involves counting the number of pulses sent to the stepper over time. An alternate means of stepper position monitoring is usually not available, so crash or slip detection is not possible.

Commercial CNC metalworking machines use closed loop feedback controls for axis movement. In a closed loop system, the controller monitors the actual position of each axis with an absolute or incremental encoder. With proper control programming, this will reduce the possibility of a crash, but it is still up to the operator and programmer to ensure that the machine is operated in a safe manner. However, during the 2000s and 2010s, the software for machining simulation has been maturing rapidly, and it is no longer uncommon for the entire machine tool envelope (including all axes, spindles, chucks, turrets, tool holders, tailstocks, fixtures, clamps, and stock) to be modeled accurately with 3D solid models, which allows the simulation software to predict fairly accurately whether a cycle will involve a crash. Although such simulation is not new, its accuracy and market penetration are changing considerably because of computing advancements.[7]

Numerical precision and equipment backlash
Within the numerical systems of CNC programming it is possible for the code generator to assume that the controlled mechanism is always perfectly accurate, or that precision tolerances are identical for all cutting or movement directions. This is not always a true condition of CNC tools. CNC tools with a large amount of mechanical backlash can still be highly precise if the drive or cutting mechanism is only driven so as to apply cutting force from one direction, and all driving systems are pressed tightly together in that one cutting direction. However a CNC device with high backlash and a dull cutting tool can lead to cutter chatter and possible workpiece gouging. Backlash also affects precision of some operations involving axis movement reversals during cutting, such as the milling of a circle, where axis motion is sinusoidal. However, this can be compensated for if the amount of backlash is precisely known by linear encoders or manual measurement.

The high backlash mechanism itself is not necessarily relied on to be repeatedly precise for the cutting process, but some other reference object or precision surface may be used to zero the mechanism, by tightly applying pressure against the reference and setting that as the zero reference for all following CNC-encoded motions. This is similar to the manual machine tool method of clamping a micrometer onto a reference beam and adjusting the Vernier dial to zero using that object as the reference.[citation needed]

Positioning control system
In numerical control systems, the position of the tool is defined by a set of instructions called the part program.

Positioning control is handled by means of either an open loop or a closed loop system. In an open loop system, communication takes place in one direction only: from the controller to the motor. In a closed loop system, feedback is provided to the controller so that it can correct for errors in position, velocity, and acceleration, which can arise due to variations in load or temperature. Open loop systems are generally cheaper but less accurate. Stepper motors can be used in both types of systems, while servo motors can only be used in closed systems.

Cartesian Coordinates

The G & M code positions are all based on a three dimensional Cartesian coordinate system. This system is a typical plane often seen in mathematics when graphing. This system is required to map out the machine tool paths and any other kind of actions that need to happen in a specific coordinate. Absolute coordinates are what is generally used more commonly for machines and represent the (0,0,0) point on the plane. This point is set on the stock material in order to give a starting point or "home position" before starting the actual machining.

M-codes
[Code Miscellaneous Functions (M-Code)][citation needed]. M-codes are miscellaneous machine commands that do not command axis motion. The format for an M-code is the letter M followed by two to three digits; for example:

[M02 End of Program]
[M03 Start Spindle - Clockwise]
[M04 Start Spindle - Counter Clockwise]
[M05 Stop Spindle]
[M06 Tool Change]
[M07 Coolant on mist coolant]
[M08 Flood coolant on]
[M09 Coolant off]
[M10 Chuck open]
[M11 Chuck close]
[M13 BOTH M03&M08 Spindle clockwise rotation & flood coolant]
[M14 BOTH M04&M08 Spindle counter clockwise rotation & flood coolant]
[M16 Special tool call]
[M19 Spindle orientate]
[M29 DNC mode ]
[M30 Program reset & rewind]
[M38 Door open]
[M39 Door close]
[M40 Spindle gear at middle]
[M41 Low gear select]
[M42 High gear select]
[M53 Retract Spindle] (raises tool spindle above current position to allow operator to do whatever they would need to do)
[M68 Hydraulic chuck close]
[M69 Hydraulic chuck open]
[M78 Tailstock advancing]
[M79 Tailstock reversing]
G-codes
G-codes are used to command specific movements of the machine, such as machine moves or drilling functions. The format for a G-code is the letter G followed by two to three digits; for example G01. G-codes differ slightly between a mill and lathe application, for example:

[G00 Rapid Motion Positioning]
[G01 Linear Interpolation Motion]
[G02 Circular Interpolation Motion-Clockwise]
[G03 Circular Interpolation Motion-Counter Clockwise]
[G04 Dwell (Group 00) Mill]
[G10 Set offsets (Group 00) Mill]
[G12 Circular Pocketing-Clockwise]
[G13 Circular Pocketing-Counter Clockwise]
Coding
Example:

O0001
G20 G40 G80 G90 G94 G54(Inch, Cutter Comp. Cancel, Deactivate all canned cycles, moves axes to machine coordinate, feed per min., origin coordinate system)
M06 T01 (Tool change to tool 1)
G43 H01 (Tool length comp. in positive direction, length compensation for tool)
M03 S1200 (Spindle turns CW at 1200RPM)
G00 X0. Y0. (Rapid Traverse to X=0. Y=0.)
G00 Z.5 (Rapid Traverse to z=.5)
G00 X1. Y-.75 (Rapid traverse to X1. Y-.75)
G01 Z-.1 F10 (Plunge into part at Z-.25 at 10in per min.)
G03 X.875 Y-.5 I.1875 J-.75 (CCW arc cut to X.875 Y-.5 with radius origin at I.625 J-.75)
G03 X.5 Y-.75 I0.0 J0.0 (CCW arc cut to X.5 Y-.75 with radius origin at I0.0 J0.0)
G03 X.75 Y-.9375 I0.0 J0.0(CCW arc cut to X.75 Y-.9375 with radius origin at I0.0 J0.0)
G02 X1. Y-1.25 I.75 J-1.25 (CW arc cut to X1. Y-1.25 with radius origin at I.75 J-1.25)
G02 X.75 Y-1.5625 I0.0 J0.0 (CW arc cut to X.75 Y-1.5625 with same radius origin as previous arc)
G02 X.5 Y-1.25 I0.0 J0.0 (CW arc cut to X.5 Y-1.25 with same radius origin as previous arc)
G00 Z.5 (Rapid traverse to z.5)
M05 (spindle stops)
G00 X0.0 Y0.0 (Mill returns to origin)
M30 (Program End)
Having the correct speeds and feeds in the program provides for a more efficient and smoother product run. Incorrect speeds and feeds will cause damage to the tool, machine spindle and even the product. The quickest and simplest way to find these numbers would be to use a calculator that can be found online. A formula can also be used to calculate the proper speeds and feeds for a material. This values can be found online or in Machinery's Handbook.

See also
Automatic Tool Changer
Binary Cutter Location
Computer-aided technologies
Computer-aided engineering (CAE)
Coordinate-measuring machine (CMM)
Design for Manufacturability for CNC machining
Direct numerical control (DNC)
EIA RS-274
EIA RS-494
G-code
Gerber format
Home automation
Maslow CNC
Multiaxis machining
Part program
Robotics
Wireless DNC
References
Mike Lynch, "Key CNC Concept #1—The Fundamentals Of CNC", Modern Machine Shop, 4 January 1997. Accessed 11 February 2015
Grace-flood, Liam (2017-11-10). "Goliath Represents a New Breed of CNC Machine". Wevolver. Retrieved 2018-01-20.
"Multi Spindle Machines - An In Depth Overview". Davenport Machine. Retrieved 2017-08-25.
"Machining Types - Parts Badger". Parts Badger. Retrieved 2017-07-07.
"How it Works – Wire EDM | Today's Machining World". todaysmachiningworld.com. Retrieved 2017-08-25.
"Sinker EDM - Electrical Discharge Machining". www.qualityedm.com. Retrieved 2017-08-25.
Zelinski, Peter (2014-03-14), "New users are adopting simulation software", Modern Machine Shop.
Further reading
Brittain, James (1992), Alexanderson: Pioneer in American Electrical Engineering, Johns Hopkins University Press, ISBN 0-8018-4228-X.
Holland, Max (1989), When the Machine Stopped: A Cautionary Tale from Industrial America, Boston: Harvard Business School Press, ISBN 978-0-87584-208-0, OCLC 246343673.
Noble, David F. (1984), Forces of Production: A Social History of Industrial Automation, New York, New York, USA: Knopf, ISBN 978-0-394-51262-4, LCCN 83048867.
Reintjes, J. Francis (1991), Numerical Control: Making a New Technology, Oxford University Press, ISBN 978-0-19-506772-9.
Weisberg, David, The Engineering Design Revolution, archived from the original (PDF) on 9 March 2010.
Wildes, Karl L.; Lindgren, Nilo A. (1985), A Century of Electrical Engineering and Computer Science at MIT, MIT Press, ISBN 0-262-23119-0.
Herrin, Golden E. "Industry Honors The Inventor Of NC", Modern Machine Shop, 12 January 1998.
Siegel, Arnold. "Automatic Programming of Numerically Controlled Machine Tools", Control Engineering, Volume 3 Issue 10 (October 1956), pp. 65–70.
Smid, Peter (2008), CNC Programming Handbook (3rd ed.), New York: Industrial Press, ISBN 9780831133474, LCCN 2007045901.
Christopher jun Pagarigan (Vini) Edmnton Alberta Canada. CNC Infomatic, Automotive Design & Production.
The Evolution of CNC Machines (2018). Retrieved October 15, 2018, from Engineering Technology Group
Fitzpatrick, Michael (2019), "Machining and CNC Technology".

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