Numerical control (also computer numerical control, and commonly called CNC) is the automated control of machining tools (such as drills, lathes,

mills Mills is the plural form of mill, but may also refer to: As a name * Mills (surname), a common family name of English or Gaelic origin * Mills (given name) *Mills, a fictional British secret agent in a trilogy by writer Manning O'Brine Places Uni ...

grinders Grinder may refer to: Machinery *Various types of grinding machine, used in a machining operation to refine the surface of materials *Food grinders **Blade grinder, includes food processors, blenders, electric coffee and spice grinders, etc. **Co ...

, routers and

3D printer 3D printing or additive manufacturing is the construction of a three-dimensional object from a CAD model or a digital 3D model. It can be done in a variety of processes in which material is deposited, joined or solidified under computer co ...

s) by means of a computer. A CNC machine processes a piece of material (metal, plastic, wood, ceramic, or composite) to meet specifications by following coded programmed instructions and without a manual operator directly controlling the machining operation. 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 G-code (also RS-274) is the most widely used computer numerical control (CNC) programming language. It is used mainly in computer-aided manufacturing to control automated machine tools, and has many variants. G-code instructions are provided to ...

and M-code, and then executed. The program can be written by a person or, far more often, generated by graphical computer-aided design (CAD) or

computer-aided manufacturing Computer-aided manufacturing (CAM) also known as computer-aided modeling or computer-aided machining is the use of software to control machine tools in the manufacturing of work pieces. This is not the only definition for CAM, but it is the most ...

(CAM) software. In the case of 3D printers, the part to be printed is "sliced" before the instructions (or the program) are generated. 3D printers also use G-Code. CNC offers greatly increased productivity over non-computerized machining for repetitive production, where the machine must be manually controlled (e.g. using devices such as hand wheels or levers) or mechanically controlled by pre-fabricated pattern guides (see pantograph mill). However, these advantages come at significant cost in terms of both capital expenditure and job setup time. For some prototyping and small batch jobs, a good machine operator can have parts finished to a high standard whilst a cnc workflow is still in setup. In modern CNC systems, the design of a mechanical part and its manufacturing program are highly automated. The part's mechanical dimensions are defined using CAD software and then translated into manufacturing directives by

(CAM) software. The resulting directives are transformed (by "

post processor A Post Processor is a unique "driver" specific to a CNC machine, robot or mechanism; some machines start at different locations or require extra movement between each operation, the Post-Processor works with the CAM software or off-line programmi ...

" 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 several different tools – drills,

saw A saw is a tool consisting of a tough blade, wire, or chain with a hard toothed edge. It is used to cut through material, very often wood, though sometimes metal or stone. The cut is made by placing the toothed edge against the material and mov ...

s, etc. – modern machines often combine multiple tools into a single "cell". In other installations, several 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 drawing.

Description

Motion is controlling multiple axes, normally at least two (X and Y),Mike Lynch, "Key CNC Concept #1—The Fundamentals Of CNC", ''Modern Machine Shop'', 4 January 1997
Accessed 11 February 2015 and a tool spindle that moves in the Z (depth). The position of the tool is driven by direct-drive

stepper motors A stepper is a device used in the manufacture of integrated circuits (ICs) that is similar in operation to a slide projector or a photographic enlarger. ''Stepper'' is short for step-and-repeat camera. Steppers are an essential part of the comp ...

servo motor A servomotor (or servo motor) is a rotary actuator or linear actuator that allows for precise control of angular or linear position, velocity and acceleration. It consists of a suitable motor coupled to a sensor for position feedback. It also ...

s to provide highly accurate movements, or in older designs, motors through a series of step-down gears.

Open-loop control In control theory, an open-loop controller, also called a non-feedback controller, is a control system in which the control action is independent of the "process output", which is the process variable that is being controlled."Feedback and contr ...

works as long as the forces are kept small enough and speeds are not too great. On commercial

metalworking Metalworking is the process of shaping and reshaping metals to create useful objects, parts, assemblies, and large scale structures. As a term it covers a wide and diverse range of processes, skills, and tools for producing objects on every scal ...

machines, closed-loop controls are standard and required 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 (with safety glass in the doors to permit the operator to monitor the machine's function), 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 Welding is a fabrication process that joins materials, usually metals or thermoplastics, by using high heat to melt the parts together and allowing them to cool, causing fusion. Welding is distinct from lower temperature techniques such as bra ...

friction stir welding Friction stir welding (FSW) is a solid-state joining process that uses a non-consumable tool to join two facing workpieces without melting the workpiece material. Heat is generated by friction between the rotating tool and the workpiece material ...

ultrasonic welding Ultrasonic welding is an industrial process whereby high-frequency ultrasonic acoustic vibrations are locally applied to work pieces being held together under pressure to create a solid-state weld. It is commonly used for plastics and metals, an ...

, flame and

plasma cutting Plasma cutting is a process that cuts through electrically conductive materials by means of an accelerated jet of hot plasma. Typical materials cut with a plasma torch include steel, stainless steel, aluminum, brass and copper, although other c ...

bending In applied mechanics, bending (also known as flexure) characterizes the behavior of a slender structural element subjected to an external load applied perpendicularly to a longitudinal axis of the element. The structural element is assumed to ...

, spinning, hole-punching, pinning, gluing, fabric cutting, sewing, tape and fiber placement, routing, picking and placing, and sawing.

History

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 Five- and eight-hole punched paper tape Paper tape reader on the Harwell computer with a small piece of five-hole tape connected in a circle – creating a physical program loop Punched tape or perforated paper tape is a form of data storage ...

. These early servomechanisms were rapidly augmented with analog and digital computers, creating the modern CNC machine tools that have revolutionized machining processes.

Examples of CNC machines

Other CNC tools

Many other tools have CNC variants, including: *

3D printing 3D printing or additive manufacturing is the construction of a three-dimensional object from a CAD model or a digital 3D model. It can be done in a variety of processes in which material is deposited, joined or solidified under computer co ...

* CNC router * Canned cycle *

Cylindrical grinder The cylindrical grinder is a type of grinding machine used to shape the outside of an object. The cylindrical grinder can work on a variety of shapes, however the object must have a central axis of rotation. This includes but is not limited to such ...

s * Drills *

Embroidery machines Machine embroidery is an embroidery process whereby a sewing machine or embroidery machine is used to create patterns on textiles. It is used commercially in product branding, corporate advertising, and uniform adornment. It is also used in th ...

Glass cutting A glass cutter is a tool used to make a shallow score in one surface of a piece of glass (normally a flat one) that is to be broken in two pieces, for example to fit a window. The scoring makes a split in the surface of the glass which encourages ...

Hot-wire foam cutter Hotwire or hot wire may refer to: Technology * Hot-wiring, a method of starting a car with no key * Hot-wire foam cutter, a tool used to cut foam and polystyrene * "Hot" wire, a wire conductor with non-zero potential in electric power distributi ...

s *

Induction hardening Induction hardening is a type of surface hardening in which a metal part is induction-heated and then quenched. The quenched metal undergoes a martensitic transformation, increasing the hardness and brittleness of the part. Induction hardening is ...

machines * Laser cutting * Lathes * Leather cutter *

Milling machine Milling is the process of machining using rotary cutters to remove material by advancing a cutter into a workpiece. This may be done by varying direction on one or several axes, cutter head speed, and pressure. Milling covers a wide variety of d ...

Oxy-fuel Oxy-fuel combustion is the process of burning a fuel using pure oxygen, or a mixture of oxygen and recirculated flue gas, instead of air. Since the nitrogen component of air is not heated, fuel consumption is reduced, and higher flame temperatures ...

* Plasma cutters * Sheet metal works (

Turret punch A turret punch or turret press is a type of punch press used for metal forming by punching. Punching, and press work in general, is a process well suited to mass production. However the initial tooling costs, of both the machine and the job- ...

) *

Submerged arc welding Submerged arc welding (SAW) is a common arc welding process. The first SAW patent was taken out in 1935. The process requires a continuously fed consumable solid or tubular (metal cored) electrode. The molten weld and the arc zone are protec ...

Surface grinder Surface grinding is done on flat surfaces to produce a smooth finish. It is a widely used abrasive machining process in which a spinning wheel covered in rough particles (grinding wheel) cuts chips of metallic or nonmetallic substance from a work ...

* Tube, pipe and wire bending machines *

Vinyl cutter A vinyl cutter is an entry level machine for making signs. Computer designed vector files with patterns and letters are directly cut on the roll of vinyl which is mounted and fed into the vinyl cutter through USB or serial cable. Vinyl cutters ar ...

* Water jet cutters * Wood routers

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 are no 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 switch In electrical engineering, a limit switch is a switch operated by the motion of a machine part or the presence of an object. A limit switch can be used for controlling machinery as part of a control system, as a safety interlock, or as a counter en ...

es. However, these parameters can often be changed by the operator. Many CNC tools also do not 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's 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 210mm 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 are 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

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 An incremental encoder is a linear or rotary electromechanical device that has two output signals, ''A'' and ''B'', which issue pulses when the device is moved. Together, the ''A'' and ''B'' signals indicate both the occurrence of and direction ...

. Proper control programming will reduce the possibility of a crash, but it is still up to the operator and programmer to ensure that the machine is operated safely. 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.

Numerical precision and equipment backlash

Within the numerical systems of CNC programming, the code generator can 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 Backlash may refer to: Literature * '' Backlash: The Undeclared War Against American Women'', a 1991 book by Susan Faludi * ''Backlash'' (Star Wars novel), a 2010 novel by Aaron Allston * Backlash (Marc Slayton), comic book character * ''Backla ...

can still be highly precise if the drive or cutting mechanism is only driven 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. The backlash also affects the 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 references for all following CNC-encoded motions. This is similar to the manual machine tool method of clamping a

micrometer Micrometer can mean: * Micrometer (device), used for accurate measurements by means of a calibrated screw * American spelling of micrometre The micrometre ( international spelling as used by the International Bureau of Weights and Measures; ...

onto a reference beam and adjusting the Vernier dial to zero using that object as the reference.

Positioning control system

In numerical control systems, the position of the tool is defined by a set of instructions called the

part program The part program is a sequence of instructions, which describe the work, which has to be done on a part, in the form required by a computer under the control of computer numerical control (CNC) software. It is the task of preparing a program sheet ...

. Positioning control is handled using 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 are generally used more commonly for machines and represent the (0,0,0) point on the plane. This point is set on the stock material to give a starting point or "home position" before starting the actual machining.

Coding

G-codes

s are used to command specific movements of the machine, such as machine moves or drilling functions. The majority of G-Code programs start with a percent (%) symbol on the first line, then followed by an "O" with a numerical name for the program (i.e. "O0001") on the second line, then another percent (%) symbol on the last line of the program. 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: : 00 Rapid Motion Positioning:

01 Linear Interpolation Motion 1 (one, unit, unity) is a number representing a single or the only entity. 1 is also a numerical digit and represents a single unit of counting or measurement. For example, a line segment of ''unit length'' is a line segment of length 1 ...

: 02 Circular Interpolation Motion-Clockwise:

03 Circular Interpolation Motion-Counter Clockwise 3 (three) is a number, numeral and digit. It is the natural number following 2 and preceding 4, and is the smallest odd prime number and the only prime preceding a square number. It has religious or cultural significance in many societie ...

04 Dwell (Group 00) Mill 4 (four) is a number, numeral (linguistics), numeral and numerical digit, digit. It is the natural number following 3 and preceding 5. It is the smallest semiprime and composite number, and is tetraphobia, considered unlucky in many East Asian c ...

10 Set offsets (Group 00) Mill 1 (one, unit, unity) is a number representing a single or the only entity. 1 is also a numerical digit and represents a single unit of counting or measurement. For example, a line segment of ''unit length'' is a line segment of length 1 ...

12 Circular Pocketing-Clockwise 1 (one, unit, unity) is a number representing a single or the only entity. 1 is also a numerical digit and represents a single unit of counting or measurement. For example, a line segment of ''unit length'' is a line segment of length 1 ...

13 Circular Pocketing-Counter Clockwise Thirteen or 13 may refer to: * 13 (number), the natural number following 12 and preceding 14 * One of the years 13 BC, AD 13, 1913, 2013 Music * 13AD (band), an Indian classic and hard rock band Albums * ''13'' (Black Sabbath album), 2013 * ...

M-codes

ode Miscellaneous Functions (M-Code) An ode (from grc, ᾠδή, ōdḗ) is a type of lyric poetry. Odes are elaborately structured poems praising or glorifying an event or individual, describing nature intellectually as well as emotionally. A classic ode is structured in three majo ...

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: : 02 End of Program:

03 Start Spindle - Clockwise 3 (three) is a number, numeral and digit. It is the natural number following 2 and preceding 4, and is the smallest odd prime number and the only prime preceding a square number. It has religious or cultural significance in many societie ...

04 Start Spindle - Counter Clockwise 4 (four) is a number, numeral and digit. It is the natural number following 3 and preceding 5. It is the smallest semiprime and composite number, and is considered unlucky in many East Asian cultures. In mathematics Four is the smallest c ...

05 Stop Spindle 5 (five) is a number, numeral and digit. It is the natural number, and cardinal number, following 4 and preceding 6, and is a prime number. It has attained significance throughout history in part because typical humans have five digits on eac ...

06 Tool Change 6 (six) is the natural number following 5 and preceding 7. It is a composite number and the smallest perfect number. In mathematics Six is the smallest positive integer which is neither a square number nor a prime number; it is the second smalles ...

07 Coolant on mist coolant 7 (seven) is the natural number following 6 and preceding 8. It is the only prime number preceding a cube. As an early prime number in the series of positive integers, the number seven has greatly symbolic associations in religion, mythology, ...

: 08 Flood coolant on: 09 Coolant off:

10 Chuck open 1 (one, unit, unity) is a number representing a single or the only entity. 1 is also a numerical digit and represents a single unit of counting or measurement. For example, a line segment of ''unit length'' is a line segment of length 1. I ...

11 Chuck close Eleven or 11 may refer to: *11 (number), the natural number following 10 and preceding 12 * one of the years 11 BC, AD 11, 1911, 2011, or any year ending in 11 Literature * ''Eleven'' (novel), a 2006 novel by British author David Llewellyn *'' ...

: 12 Spindle up: 13 BOTH M03&M08 Spindle clockwise rotation & flood coolant: 14 BOTH M04&M08 Spindle counter clockwise rotation & flood coolant: 16 Special tool call: 19 Spindle orientate: 29 DNC mode : 30 Program reset & rewind: 38 Door open:

39 Door close 39 may refer to: * 39 (number), the natural number following 38 and preceding 40 * one of the years: ** 39 BC ** AD 39 ** 1939 ** 2039 * ''39'' (album), a 2000 studio album by Mikuni Shimokawa * " '39", a 1975 song by Queen * "Thirty Nine", a so ...

40 Spindle gear at middle 4 (four) is a number, numeral and digit. It is the natural number following 3 and preceding 5. It is the smallest semiprime and composite number, and is considered unlucky in many East Asian cultures. In mathematics Four is the smallest c ...

41 Low gear select 41 may refer to: * 41 (number) * one of the years 41 BC, AD 41, 1941, 2041 Art and entertainment * ''41'' (film), a 2007 documentary about Nicholas O'Neill, the youngest victim of the Station nightclub fire * ''41'', a 2012 film by Glenn Tri ...

42 High gear select 4 (four) is a number, numeral and digit. It is the natural number following 3 and preceding 5. It is the smallest semiprime and composite number, and is considered unlucky in many East Asian cultures. In mathematics Four is the smallest c ...

: 53 Retract Spindle(raises tool spindle above current position to allow operator to do whatever they would need to do) : 68 Hydraulic chuck close: 69 Hydraulic chuck open: 78 Tailstock advancing: 79 Tailstock reversing

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 a positive direction, length compensation for the 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 the 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 the 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. These values can be found online or in Machinery's Handbook.

References

External links

* {{DEFAULTSORT:Numerical Control Articles containing video clips