|Appeared in||1950s (first edition)|
|Designed by||Massachusetts Institute of Technology|
|Major implementations||many, mainly ISO 6983.|
|Usual filename extensions||.mpt, .mpf .nc and several others|
G-code is the common name for the most widely used computer-aided engineering. G-code is sometimes called G programming language.
In fundamental terms, G-code is a language in which people tell computerized cutting tool is moved according to these instructions, cutting away excess material to leave only the finished workpiece. Non-cutting tools, such as cold-forming tools, burnishing tools, or measuring probes, are also sometimes involved.
The first implementation of numerical control was developed at the MIT Servomechanisms Laboratory in the early 1950s. In the decades since, many implementations have been developed by many (commercial and noncommercial) organizations. G-code has often been used in these implementations. The main standardized version used in the United States was settled by the DIN 66025 or PN-73M-55256, PN-93/M-55251 in Poland.
Extensions and variations have been added independently by control manufacturers and machine tool manufacturers, and operators of a specific controller must be aware of differences of each manufacturer’s product.
One standardized version of G-code, known as BCL, is used only on very few machines.
During the 1970s through 1990s, many CNC machine tool builders attempted to overcome compatibility difficulties by standardizing on machine tool controllers built by Siemens was another market dominator in CNC controls, especially in Europe. In the 2010s, controller differences and incompatibility are not as troublesome because machining operations are developed with CAD/CAM applications that can output the appropriate G-code for a specific machine tool.
Some CNC machines use “conversational” programming, which is a wizard-like programming mode that either hides G-code or completely bypasses the use of G-code. Some popular examples are Southwestern Industries’ ProtoTRAK, Mazak’s Mazatrol, Hurco’s Ultimax, Haas’ Intuitive Programming System (IPS), and Mori Seiki’s CAPS conversational software.
G-code began as a limited type of language that lacked constructs such as loops, conditional operators, and programmer-declared variables with high-level programming language. The more a programmer can tell the machine what end result is desired, and leave the intermediate calculations to the machine, the more (s)he uses the machine’s computational power to full advantage.
 Specific codes
G-codes are also called preparatory codes, and are any word in a CNC program that begins with the letter “G”. Generally it is a code telling the machine tool what type of action to perform, such as:
- Rapid move (transport the tool through space to the place where it is needed for cutting; do this as quickly as possible)
- Controlled feed move in a straight line or arc
- Series of controlled feed moves that would result in a hole being bored, a workpiece cut (routed) to a specific dimension, or a profile (contour) shape added to the edge of a workpiece
- Set tool information such as offset
- Switch coordinate systems
There are other codes; the type codes can be thought of like registers in a computer.
 Letter addresses
Some letter addresses are used only in milling or only in turning; most are used in both. Bold below are the letters seen most frequently throughout a program.
|A||Absolute or incremental position of A axis (rotational axis around X axis)|
|B||Absolute or incremental position of B axis (rotational axis around Y axis)|
|C||Absolute or incremental position of C axis (rotational axis around Z axis)|
|D||Defines diameter or radial offset used for cutter compensation. D is used for depth of cut on lathes.|
|E||Precision feedrate for threading on lathes|
|F||Defines feed rate||Common units are distance per time for mills (inches per minute, IPM, or millimetres per minute, mm/min) and distance per revolution for lathes (inches per revolution, IPR, or millimetres per revolution, mm/rev)|
|G||Address for preparatory commands||G commands often tell the control what kind of motion is wanted (e.g., rapid positioning, linear feed, circular feed, fixed cycle) or what offset value to use.|
|H||Defines tool length offset;
Incremental axis corresponding to C axis (e.g., on a turn-mill)
|I||Defines arc center in X axis for G03 arc commands.
Also used as a parameter within some fixed cycles.
|J||Defines arc center in Y axis for G03 arc commands.
Also used as a parameter within some fixed cycles.
|K||Defines arc center in Z axis for G03 arc commands.
Also used as a parameter within some fixed cycles, equal to L address.
|L||Fixed cycle loop count;
Specification of what register to edit using G10
|Fixed cycle loop count: Defines number of repetitions (“loops”) of a fixed cycle at each position. Assumed to be 1 unless programmed with another integer. Sometimes the G91), a series of equally spaced holes can be programmed as a loop rather than as individual positions.
G10 use: Specification of what register to edit (work offsets, tool radius offsets, tool length offsets, etc.).
|M||Miscellaneous function||Action code, auxiliary command; descriptions vary. Many M-codes call for machine functions, which is why people often say that the “M” stands for “machine”, although it was not intended to.|
|N||Line (block) number in program;
System parameter number to be changed using G10
|Line (block) numbers: Optional, so often omitted. Necessary for certain tasks, such as N numbering need not increment by 1 (for example, it can increment by 10, 20, or 1000) and can be used on every block or only in certain spots throughout a program.
System parameter number: G10 allows changing of system parameters under program control.
|O||Program name||For example, O4501. For many years it was common for CNC control displays to use slashed zero glyphs to ensure effortless distinction of letter “O” from digit “0”. Today’s GUI controls often have a choice of fonts, like a PC does.|
|P||Serves as parameter address for various G and M codes|
|Q||Peck increment in canned cycles||For example, G83 (peck drilling cycles)|
|R||Defines size of arc radius or defines retract height in milling canned cycles|
|S||Defines speed, either spindle speed or surface speed depending on mode||Data type = integer. In Speeds and feeds. On multifunction (turn-mill or mill-turn) machines, which spindle gets the input (main spindle or subspindles) is determined by other M codes.|
|T||Tool selection||To understand how the T address works and how it interacts (or not) with M06.|
|U||Incremental axis corresponding to X axis (typically only lathe group A controls)
Also defines dwell time on some machines (instead of “X“).
|In these controls, X and U obviate G91, respectively. On these lathes, G90 is instead a fixed cycle address for roughing.|
|V||Incremental axis corresponding to Y axis||Until the 2000s, the V address was very rarely used, because most lathes that used U and W didn’t have a Y-axis, so they didn’t use V. (Green et al. 1996G18.|
|W||Incremental axis corresponding to Z axis (typically only lathe group A controls)||In these controls, Z and W obviate G91, respectively. On these lathes, G90 is instead a fixed cycle address for roughing.|
|X||Absolute or incremental position of X axis.
Also defines dwell time on some machines (instead of “U“).
|Y||Absolute or incremental position of Y axis|
|Z||Absolute or incremental position of Z axis||The main spindle’s axis of rotation often determines which axis of a machine tool is labeled as Z.|
 List of G-codes commonly found on Fanuc and similarly designed controls
( M )
( T )
|G00||Rapid positioning||M||T||On 2- or 3-axis moves, G00 (unlike G01) traditionally does not necessarily move in a single straight line between start point and end point. It moves each axis at its max speed until its vector is achieved. Shorter vector usually finishes first (given similar axis speeds). This matters because it may yield a dog-leg or hockey-stick motion, which the programmer needs to consider depending on what obstacles are nearby, to avoid a crash. Some machines offer interpolated rapids as a feature for ease of programming (safe to assume a straight line).|
|G01||Linear interpolation||M||T||The most common workhorse code for feeding during a cut. The program specs the start and end points, and the control automatically calculates (leadscrews via their servomotors or stepper motors. The computer performs thousands of calculations per second, and the motors react quickly to each input. Thus the actual toolpath of the machining takes place with the given feedrate on a path that is accurately linear to within very small limits.|
|G02||Circular interpolation, clockwise||M||T||Very similar in concept to G01. Again, the control interpolates intermediate points and commands the servo- or stepper motors to rotate the amount needed for the leadscrew to translate the motion to the correct tool tip positioning. This process repeated thousands of times per minute generates the desired toolpath. In the case of G02, the interpolation generates a circle rather than a line. As with G01, the actual toolpath of the machining takes place with the given feedrate on a path that accurately matches the ideal (in G02’s case, a circle) to within very small limits. In fact, the interpolation is so precise (when all conditions are correct) that milling an interpolated circle can obviate operations such as drilling, and often even fine boring. On most controls you cannot start G41 or G42 in G03 modes. You must already have compensated in an earlier G01 block. Often a short linear lead-in movement will be programmed, merely to allow cutter compensation before the main event, the circle-cutting, begins.|
|G03||Circular interpolation, counterclockwise||M||T||Same corollary info as for G02.|
|G04||Dwell||M||T||Takes an address for dwell period (may be P). The dwell period is specified in the controllers parameter, typically milliseconds.|
|G05 P10000||High-precision contour control (HPCC)||M||Uses a deep look-ahead buffer and simulation processing to provide better axis movement acceleration and deceleration during contour milling|
|G05.1 Q1.||AI Nano contour control||M||Uses a deep look-ahead buffer and simulation processing to provide better axis movement acceleration and deceleration during contour milling|
|G06.1||Non Uniform Rational B Spline Machining||M||Activates Non-Uniform Rational B Spline for complex curve and waveform machining (this code is confirmed in Mazatrol 640M ISO Programming)|
|G07||Imaginary axis designation||M|
|G09||Exact stop check||M||T|
|G10||Programmable data input||M||T|
|G11||Data write cancel||M||T|
|G12||Full-circle interpolation, clockwise||M||Fixed cycle for ease of programming 360° circular interpolation with blend-radius lead-in and lead-out. Not standard on Fanuc controls.|
|G13||Full-circle interpolation, counterclockwise||M||Fixed cycle for ease of programming 360° circular interpolation with blend-radius lead-in and lead-out. Not standard on Fanuc controls.|
|G17||XY plane selection||M|
|G18||ZX plane selection||M||T||On most CNC lathes (built 1960s to 2000s), ZX is the only available plane, so no V address.|
|G19||YZ plane selection||M|
|G20||Programming in inches||M||T||Somewhat uncommon except in USA and (to lesser extent) Canada and UK. However, in the global marketplace, competence with both G20 and G21 always stands some chance of being necessary at any time. The usual minimum increment in G20 is one ten-thousandth of an inch (0.0001″), which is a larger distance than the usual minimum increment in G21 (one thousandth of a millimeter, .001 mm, that is, one micrometre). This physical difference sometimes favors G21 programming.|
|G21||Programming in millimeters (mm)||M||T||Prevalent worldwide. However, in the global marketplace, competence with both G20 and G21 always stands some chance of being necessary at any time.|
|G28||Return to home position (machine zero, aka machine reference point)||M||T||Takes X Y Z addresses which define the intermediate point that the tool tip will pass through on its way home to machine zero. They are in terms of part zero (aka program zero), NOT machine zero.|
|G30||Return to secondary home position (machine zero, aka machine reference point)||M||T||Takes a P address specifying which machine zero point is desired, if the machine has several secondary points (P1 to P4). Takes X Y Z addresses which define the intermediate point that the tool tip will pass through on its way home to machine zero. They are in terms of part zero (aka program zero), NOT machine zero.|
|G31||Skip function (used for probes and tool length measurement systems)||M|
|G32||Single-point threading, longhand style (if not using a cycle, e.g., G76)||T||Similar to single-point threading.|
|G33||Single-point threading, longhand style (if not using a cycle, e.g., G76)||T||Some lathe controls assign this mode to G33 rather than G32.|
|G40||Tool radius compensation off||M||T||Cancels G41 or G42.|
|G41||Tool radius compensation left||M||T||Milling: Given righthand-helix cutter and H) that calls an offset register value for radius.
Turning: Often needs no D or H address on lathes, because whatever tool is active automatically calls its geometry offsets with it. (Each turret station is bound to its geometry offset register.)
G41 and G42 for milling has become less frequently used since CAM programming has become more common. CAM systems allow the user to program as if with a zero-diameter cutter. The fundamental concept of cutter radius compensation is still in play (i.e., that the surface produced will be distance R away from the cutter center), but the programming mindset is different; the human does not choreograph the toolpath with conscious, painstaking attention to G41, G42, and G40, because the CAM software takes care of it.
|G42||Tool radius compensation right||M||T||Similar corollary info as for G41. Given righthand-helix cutter and M03 spindle direction, G42 corresponds to conventional milling (up milling).
See also the comments for G41.
|G43||Tool height offset compensation negative||M||Takes an address, usually H, to call the tool length offset register value. The value is negative because it will be added to the gauge line position. G43 is the commonly used version (vs G44).|
|G44||Tool height offset compensation positive||M||Takes an address, usually H, to call the tool length offset register value. The value is positive because it will be subtracted from the gauge line position. G44 is the seldom-used version (vs G43).|
|G45||Axis offset single increase||M|
|G46||Axis offset single decrease||M|
|G47||Axis offset double increase||M|
|G48||Axis offset double decrease||M|
|G49||Tool length offset compensation cancel||M||Cancels G44.|
|G50||Define the maximum spindle speed||T||Takes an G96 mode (CSS) would rev the spindle to “wide open throttle” when closely approaching the axis of rotation.|
|G50||Scaling function cancel||M|
|G50||Position register (programming of vector from part zero to tool tip)||T||Position register is one of the original methods to relate the part (program) coordinate system to the tool position, which indirectly relates it to the machine coordinate system, the only position the control really “knows”. Not commonly programmed anymore because G92 for milling. Those G addresses also have alternate meanings (which see). Position register can still be useful for datum shift programming.|
|G52||Local coordinate system (LCS)||M||Temporarily shifts program zero to a new location. This simplifies programming in some cases.|
|G53||Machine coordinate system||M||T||Takes absolute coordinates (X,Y,Z,A,B,C) with reference to machine zero rather than program zero. Can be helpful for tool changes. Nonmodal and absolute only. Subsequent blocks are interpreted as “back to G54” even if it is not explicitly programmed.|
|G54 to G59||Work coordinate systems (WCSs)||M||T||Have largely replaced position register (G92). Each tuple of axis offsets relates program zero directly to machine zero. Standard is 6 tuples (G54 to G59), with optional extensibility to 48 more via G54.1 P1 to P48.|
|G54.1 P1 to P48||Extended work coordinate systems||M||T||Up to 48 more WCSs besides the 6 provided as standard by G54 to G59. Note floating-point extension of G-code data type (formerly all integers). Other examples have also evolved (e.g., G84.2). Modern controls have the hardware to handle it.|
|G70||Fixed cycle, multiple repetitive cycle, for finishing (including contours)||T|
|G71||Fixed cycle, multiple repetitive cycle, for roughing (Z-axis emphasis)||T|
|G72||Fixed cycle, multiple repetitive cycle, for roughing (X-axis emphasis)||T|
|G73||Fixed cycle, multiple repetitive cycle, for roughing, with pattern repetition||T|
|G73||Peck drilling cycle for milling – high-speed (NO full retraction from pecks)||M||Retracts only as far as a clearance increment (system parameter). For when chipbreaking is the main concern, but chip clogging of flutes is not.|
|G74||Peck drilling cycle for turning||T|
|G74||Tapping cycle for milling, lefthand thread, M04 spindle direction||M|
|G75||Peck grooving cycle for turning||T|
|G76||Fine boring cycle for milling||M|
|G76||Threading cycle for turning, multiple repetitive cycle||T|
|G80||Cancel canned cycle||M||T||Milling: Cancels all cycles such as G83, G88, etc. Z-axis returns either to Z-initial level or R-level, as programmed (G98 or G99, respectively).
Turning: Usually not needed on lathes, because a new group-1 G address (G03) cancels whatever cycle was active.
|G81||Simple drilling cycle||M||No dwell built in|
|G82||Drilling cycle with dwell||M||Dwells at hole bottom (Z-depth) for the number of milliseconds specified by the P address. Good for when hole bottom finish matters.|
|G83||Peck drilling cycle (full retraction from pecks)||M||Returns to R-level after each peck. Good for clearing flutes of chips.|
|G84||M03 spindle direction||M|
|G84.2||Tapping cycle, righthand thread, M03 spindle direction, rigid toolholder||M|
|G90||Absolute programming||M||T (B)||Positioning defined with reference to part zero.
Milling: Always as above.
Turning: Sometimes as above (Fanuc group type B and similarly designed), but on most lathes (Fanuc group type A and similarly designed), G90/G91 are not used for absolute/incremental modes. Instead, Z are the absolute addresses. On these lathes, G90 is instead a fixed cycle address for roughing.
|G90||Fixed cycle, simple cycle, for roughing (Z-axis emphasis)||T (A)||When not serving for absolute programming (above)|
|G91||Incremental programming||M||T (B)||Positioning defined with reference to previous position.
Milling: Always as above.
Turning: Sometimes as above (Fanuc group type B and similarly designed), but on most lathes (Fanuc group type A and similarly designed), G90/G91 are not used for absolute/incremental modes. Instead, Z are the absolute addresses. On these lathes, G90 is a fixed cycle address for roughing.
|G92||Position register (programming of vector from part zero to tool tip)||M||T (B)||Same corollary info as at G50 position register.
Milling: Always as above.
Turning: Sometimes as above (Fanuc group type B and similarly designed), but on most lathes (Fanuc group type A and similarly designed), position register is G50.
|G92||Threading cycle, simple cycle||T (A)|
|G94||Feedrate per minute||M||T (B)||On group type A lathes, feedrate per minute is G98.|
|G94||Fixed cycle, simple cycle, for roughing (X-axis emphasis)||T (A)||When not serving for feedrate per minute (above)|
|G95||Feedrate per revolution||M||T (B)||On group type A lathes, feedrate per revolution is G99.|
|G96||Constant surface speed (CSS)||T||Varies spindle speed automatically to achieve a constant surface speed. See G21 mode.|
|G97||Constant spindle speed||M||T||Takes an S address integer, which is interpreted as rev/min (rpm). The default speed mode per system parameter if no mode is programmed.|
|G98||Return to initial Z level in canned cycle||M|
|G98||Feedrate per minute (group type A)||T (A)||Feedrate per minute is G94 on group type B.|
|G99||Return to R level in canned cycle||M|
|G99||Feedrate per revolution (group type A)||T (A)||Feedrate per revolution is G95 on group type B.|
 List of M-codes commonly found on Fanuc and similarly designed controls
( M )
( T )
|M00||Compulsory stop||M||T||Non-optional—machine will always stop upon reaching M00 in the program execution.|
|M01||Optional stop||M||T||Machine will only stop at M01 if operator has pushed the optional stop button.|
|M02||End of program||M||T||No return to program top; may or may not reset register values.|
|M03||Spindle on (clockwise rotation)||M||T||The speed of the spindle is determined by the address right-hand rule can be used to determine which direction is clockwise and which direction is counter-clockwise.
Right-hand-helix screws moving in the tightening direction (and right-hand-helix flutes spinning in the cutting direction) are defined as moving in the M03 direction, and are labeled “clockwise” by convention. The M03 direction is always M03 regardless of local vantage point and local CW/CCW distinction.
|M04||Spindle on (counterclockwise rotation)||M||T||See comment above at M03.|
|M06||Automatic tool change (ATC)||M||T (some-times)||Many lathes do not use M06 because the T address itself indexes the turret.
Programming on any particular machine tool requires knowing which method that machine uses. To understand how the T address works and how it interacts (or not) with M06, one must study the various methods, such as lathe turret programming, ATC fixed tool selection, ATC random memory tool selection, the concept of “next tool waiting”, and empty tools. These concepts are taught in textbooks such as Smid, and online multimedia (videos, simulators, etc.); all of these teaching resources are usually paywalled to pay back the costs of their development. They are used in training classes for operators, both on-site and remotely (e.g., Tooling University).
|M07||Coolant on (mist)||M||T|
|M08||Coolant on (flood)||M||T|
|M10||Pallet clamp on||M||For machining centers with pallet changers|
|M11||Pallet clamp off||M||For machining centers with pallet changers|
|M13||Spindle on (clockwise rotation) and coolant on (flood)||M||This one M-code does the work of both M08. It is not unusual for specific machine models to have such combined commands, which make for shorter, more quickly written programs.|
|M19||Spindle orientation||M||T||Spindle orientation is more often called within cycles (automatically) or during setup (manually), but it is also available under program control via M19. The abbreviation OSS (oriented spindle stop) may be seen in reference to an oriented stop within cycles.
The relevance of spindle orientation has increased as technology has advanced. Although 4- and 5-axis contour milling and CNC indexing head setups; in a sense, indexing heads were invented as separate pieces of equipment, to be used in separate operations, which could provide precise spindle orientation in a world where it otherwise mostly didn’t exist (and didn’t need to). But as CAD/CAM and multiaxis CNC machining with multiple rotary-cutter axes becomes the norm, even for “regular” (non-“special”) applications, machinists now frequently care about stepping just about any spindle through its 360° with precision.
|M23||Thread gradual pullout ON||T|
|M24||Thread gradual pullout OFF||T|
|M30||End of program with return to program top||M||T|
|M41||Gear select – gear 1||T|
|M42||Gear select – gear 2||T|
|M43||Gear select – gear 3||T|
|M44||Gear select – gear 4||T|
|M48||Feedrate override allowed||M||T|
|M49||Feedrate override NOT allowed||M||T||This rule is also called (automatically) within tapping cycles or single-point threading cycles, where feed is precisely correlated to speed. Same with spindle speed override and feed hold button.|
|M52||Unload Last tool from spindle||M||T||Also empty spindle.|
|M60||Automatic pallet change (APC)||M||For machining centers with pallet changers|
|M98||Subprogram call||M||T||Takes an address P to specify which subprogram to call, for example, “M98 P8979″ calls subprogram O8979.|
|M99||Subprogram end||M||T||Usually placed at end of subprogram, where it returns execution control to the main program. The default is that control returns to the block following the M98 call in the main program. Return to a different block number can be specified by a P address. M99 can also be used in main program with block skip for endless loop of main program on bar work on lathes (until operator toggles block skip).|
 Example program
This is a generic program that demonstrates the use of G-Code to turn a 1″ diameter X 1″ long part. Assume that a bar of material is in the machine and that the bar is slightly oversized in length and diameter and that the bar protrudes by more than 1″ from the face of the chuck. (Caution: This is generic, it might not work on any real machine! Pay particular attention to point 5 below.)
|%||(Demarcates the start and end of a program. Originally indicated the start and end of tape feed on NC machines, generally but not always required to be present on newer machines.)|
|O4968||(Sample face and turn program)|
|N01||M216||(Turn on load monitor)|
|N02||G20 G90 G54 D200 G40||(Inch units. Absolute mode. Call work offset values. Moving coordinate system to the location specified in the register D200. Cancel any existing tool radius offset.)|
|N03||G50 S2000||(Set maximum spindle speed rev/min – preparing for G96 CSS coming soon)|
|N05||T0300||(Index turret to tool 3. Clear wear offset (00).)|
|N06||G96 S854 M42 M03 M08||(Constant surface speed [automatically varies the spindle speed], 854 sfm, select spindle gear, start spindle CW rotation, turn on the coolant flood)|
|N07||G41 G00 X1.1 Z1.1 T0303||(Call tool radius offset. Call tool wear offset. Rapid feed to a point about 0.100″ from the end of the bar [not counting 0.005" or 0.006" that the bar-pull-and-stop sequence is set up to leave as a stock allowance for facing off] and 0.050″ from the side)|
|N08||G01 Z1.0 F.05||(Feed in horizontally until the tool is standing 1″ from the datum i.e. program Z-zero)|
|N09||X-0.002||(Feed down until the tool is slightly past center, thus facing the end of the bar)|
|N10||G00 Z1.1||(Rapid feed 0.1″ away from the end of the bar – clear the part)|
|N11||X1.0||(Rapid feed up until the tool is standing at the finished OD)|
|N12||G01 Z0.0 F.05||(Feed in horizontally cutting the bar to 1″ diameter all the way to the datum, feeding at 0.050″ per revolution)|
|N13||G00 X1.1 M05 M09||(Clear the part, stop the spindle, turn off the coolant)|
|N14||G91 G28 X0||(Home X axis – return to machine X-zero passing through no intermediate X point [incremental X0])|
|N15||G91 G28 Z0||(Home Z axis – return to machine Z-zero passing through no intermediate Z point [incremental Z0])|
|N16||G90 M215||(Return to absolute mode. Turn off load monitor)|
|N17||M30||(Program stop, rewind to beginning of program)|
Several points to note:
- There is room for some programming style, even in this short program. The grouping of codes in line N06 could have been put on multiple lines. Doing so may have made it easier to follow program execution.
- Many codes are “modal”, meaning that they stay in effect until they are cancelled or replaced by a contradictory code. For example, once variable speed cutting (CSS) had been selected (G96), it stayed in effect until the end of the program. In operation, the spindle speed would increase as the tool neared the center of the work in order to maintain a constant surface speed. Similarly, once rapid feed was selected (G00), all tool movements would be rapid until a feed rate code (G01, G02, G03) was selected.
- It is common practice to use a load monitor with CNC machinery. The load monitor will stop the machine if the spindle or feed loads exceed a preset value that is set during the set-up operation. The jobs of the load monitor are various:
- Prevent machine damage in the event of tool breakage or a programming mistake.
- This is especially important because it allows safe “lights-out machining”, in which the operators set up the job and start it running during the day, then go home for the night, leaving the machines running and cutting parts during the night. Because no human is around to hear, see, or smell a problem such as a broken tool, the load monitor serves an important sentry duty. When it senses overload condition, which semantically suggests a dull or broken tool, it commands a stop to the machining. Technology is available nowadays to send an alert to someone remotely (e.g., the sleeping owner, operator, or owner-operator) if desired, which can allow them to come intercede and get production going again, then leave once more. This can be the difference between profitability or loss on some jobs, because lights-out machining reduces labor hours per part.
- Warn of a tool that is becoming dull and needs to be replaced or sharpened. Thus an operator who is busy tending multiple machines will be told by a machine, essentially, “Hey, pause what you’re doing over there, and come attend to a need over here.”
- Prevent machine damage in the event of tool breakage or a programming mistake.
- It is common practice to bring the tool in rapidly to a “safe” point that is close to the part – in this case 0.1″ away – and then start feeding the tool. How close that “safe” distance is, depends on the skill of the programmer and maximum material condition for the raw stock.
- If the program is wrong, there is a high probability that the machine will crash, or ram the tool into the part under high power. This can be costly, especially in newer machining centers. It is possible to intersperse the program with optional stops (M01 code) which allow the program to be run piecemeal for testing purposes. The optional stops remain in the program but they are skipped during the normal running of the machine. Fortunately, most CAD/CAM software ships with CNC simulators that will display the movement of the tool as the program executes. Many modern CNC machines also allow programmers to execute the program in a simulation mode and observe the operating parameters of the machine at a particular execution point. This enables programmers to discover semantic errors (as opposed to syntax errors) before losing material or tools to an incorrect program. Depending on the size of the part, wax blocks may be used for testing purposes as well.
- For pedagogical purposes, line numbers have been included in the program above. They are usually not necessary for operation of a machine, so they are seldom used in industry. However, if branching or looping statements are used in the code, then line numbers may well be included as the target of those statements (e.g. GOTO N99).
- Some machines do not allow multiple M codes in the same line.
 Programming environments
G-code’s programming environments have evolved in parallel with those of general programming—from the earliest environments (e.g., writing a program with a pencil, typing it into a tape puncher) to the latest environments that IDEs in general programming.)
Two high-level paradigm shifts have been (1) abandoning “manual programming” (with nothing but a pencil or text editor and a human mind) for CSS model. In all of these cases, a higher layer of abstraction was introduced in order to pursue what was missing semantically.
small and medium enterprises, there had to be an era in which the simpler implementations of NC, with relatively primitive “connect-the-dots” G-code and manual programming, ruled the day until CAD/CAM could improve and disseminate throughout the economy.
Any machine tool with a great number of axes, spindles, and tool stations is difficult to program well manually. It has been done over the years, but not easily. This challenge has existed for decades in CNC screw machine and rotary transfer programming, and it now also arises with today’s newer machining centers called “turn-mills”, “mill-turns”, “multitasking machines”, and “multifunction machines”. Now that  At the same time, however, programmers still must thoroughly understand the principles of manual programming and must think critically and second-guess some aspects of the software’s decisions.
Since about the mid-2000s, the era has finally arrived when “the death of manual programming” (that is, of writing lines of G-code without CAM assistance) sometimes seems to be approaching. However, it is currently only in some contexts that manual programming is obsolete. Although it is true that plenty of CAM programming can and does take place nowadays among people who are rusty on, or incapable of, manual programming, it is not true that all CNC programming can be done, or done as well or as efficiently, without being able to speak the language of G-code. Tailoring and refining the CNC program at the machine is an area of practice where it can be easier or more efficient to edit the G-code directly rather than editing the CAM toolpaths and re-post-processing the program.
 See also
 Extended developments
 Similar concepts
 Concerns during application
- MMS editorial staff (2010-12-20), “CAM system simplifies Swiss-type lathe programming”, Modern Machine Shop 83 (8 [2011 Jan]): 100–105, http://www.mmsonline.com/articles/cam-system-simplifies-swiss-type-lathe-programming. Online ahead of print.
- Lynch, Mike (2010-01-18), “When programmers should know G code”, Modern Machine Shop, http://www.mmsonline.com/columns/when-programmers-should-know-g-code.
- Lynch, Mike (2011-10-19), “Five CNC myths and misconceptions [CNC Tech Talk column, Editor's Commentary"], Modern Machine Shop, http://www.mmsonline.com/columns/five-cnc-myths-and-misconceptions.
- Green, Robert E. et al. (eds) (1996), Machinery’s Handbook (25 ed.), New York, NY, USA: Industrial Press, ISBN 978-0-8311-2575-2, http://www.worldcat.org/title/machinerys-handbook/oclc/473691581.
- Smid, Peter (2008), CNC Programming Handbook (3rd ed.), New York: Industrial Press, 2007045901.
- Code descriptions with graphics and example code files (examples can be downloaded).
- CNC G-Code and M-Code Programming
- Tutorial for G-code
- Kramer, T. R.; Proctor, F. M.; Messina, E. R. (1 Aug 2000), The NIST RS274NGC Interpreter – Version 3, http://www.nist.gov/manuscript-publication-search.cfm?pub_id=823374
- http://museum.mit.edu/150/86 Has several links (including history of MIT Servo Lab)