|
G-code began as a limited language that lacked constructs such as loops, conditional operators, and programmer-declared variables with [[Natural language|natural]]-word-including names (or the expressions in which to use them). It was unable to encode logic but was just a way to "connect the dots" where the programmer figured out many of the dots' locations longhand. The latest implementations of G-code include macro language capabilities somewhat closer to a [[high-level programming language]]. Additionally, all primary manufacturers (e.g., Fanuc, Siemens, [[Heidenhain]]) provide access to [[programmable logic controller]] (PLC) data, such as axis positioning data and tool data,<ref>{{cite web |url-status=dead |archive-date=2014-05-03 |url=http://www.machinetoolhelp.com/Applications/macro/system_variables.html |title=Fanuc macro system variables |access-date=2014-06-30 |archive-url=https://web.archive.org/web/20140503030834/http://www.machinetoolhelp.com/Applications/macro/system_variables.html }}</ref> via variables used by NC programs. These constructs make it easier to develop automation applications.
== Specific codes ==
{{Overly detailed|section|details=the excessive listing of nearly 300 codes that would better served by linking to a reputable external site|date=March 2023}}
G-codes, also called preparatory codes, 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 movement (transport the tool as quickly as possible in between cuts)
* Controlled feed in a straight line or arc
* Series of controlled feed movements 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 [[processor register|registers]] in a computer.
It has been pointed out over the years that the term "G-code" is imprecise because "G" is only one of many letter addresses in the complete language. It comes from the literal sense of the term, referring to one letter address and to the specific codes that can be formed with it (for example, G00, G01, G28), but every letter of the English alphabet is [[#Letter addresses|used somewhere in the language]]. Nevertheless, "G-code" is [[metonymy|metonymically]] established as the common name of the language.
===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.
Sources: Smid 2008;<ref name="Smid2008">{{Harvnb|Smid|2008}}.</ref> Smid 2010;<ref name="Smid2010">{{Harvnb|Smid|2010}}.</ref> Green et al. 1996.<ref name="Greenetal1996">{{Harvnb|Machinery's Handbook|1996|pp=1162–1226}}.</ref>
{| class="wikitable"
|-
! Variable !! Description !! Corollary info
|-
| valign="top" | {{Visible anchor|A}} || Absolute or incremental position of A axis (rotational axis around X axis) ||Positive rotation is defined as a counterclockwise rotation looking from X positive towards X negative.
|-
| valign="top" | {{Visible anchor|B}} || Absolute or incremental position of B axis (rotational axis around Y axis) ||
|-
| valign="top" | {{Visible anchor|C}} || Absolute or incremental position of C axis (rotational axis around Z axis) ||
|-
| valign="top" | {{Visible anchor|D}} || Defines diameter or radial offset used for cutter compensation. D is used for depth of cut on lathes. It is used for aperture selection and commands on photoplotters. ||[[#G41|G41]]: left cutter compensation, [[#G42|G42]]: right cutter compensation
|-
| valign="top" | {{Visible anchor|E}} || Precision feedrate for threading on lathes ||
|-
| valign="top" | {{Visible anchor|F}} || Defines [[Speeds and feeds#Feed rate|feed rate]] || Common units are distance per time for mills (inches per minute, IPM, or millimeters per minute, mm/min) and distance per revolution for lathes (inches per revolution, IPR, or millimeters per revolution, mm/rev)
|-
| valign="top" | '''{{Visible anchor|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.
|-
| valign="top" | {{Visible anchor|H}} || Defines tool length offset; <br> Incremental axis corresponding to C axis (e.g., on a turn-mill) ||[[#G43|G43]]: Negative tool length compensation, [[#G44|G44]]: Positive tool length compensation
|-
| valign="top" | {{Visible anchor|I}} || Defines arc center in X axis for [[#G02|G02]] or [[#G03|G03]] arc commands. <br> Also used as a parameter within some fixed cycles. ||The arc center is the relative distance from the current position to the arc center, not the absolute distance from the work coordinate system (WCS).
|-
| valign="top" | {{Visible anchor|J}} || Defines arc center in Y axis for [[#G02|G02]] or [[#G03|G03]] arc commands. <br> Also used as a parameter within some fixed cycles. ||Same corollary info as I above.
|-
| valign="top" | {{Visible anchor|K}} || Defines arc center in Z axis for [[#G02|G02]] or [[#G03|G03]] arc commands. <br> Also used as a parameter within some fixed cycles, equal to [[#L|L]] address. ||Same corollary info as I above.
|-
| valign="top" | {{Visible anchor|L}} || Fixed cycle loop count; <br> Specification of what register to edit using [[#G10|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 [[#K|K]] address is used instead of L. With incremental positioning ([[#G91|G91]]), a series of equally spaced holes can be programmed as a loop rather than as individual positions. <br> ''[[#G10|G10]] use:'' Specification of what register to edit (work offsets, tool radius offsets, tool length offsets, etc.).
|-
| valign="top" | '''{{Visible anchor|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. such as M500 to save gcode in [[Additive Manufacturing]]
|-
| valign="top" | {{Visible anchor|N}} || Line (block) number in program; <br> System parameter number to change using [[#G10|G10]] || ''Line (block) numbers:'' Optional, so often omitted. Necessary for certain tasks, such as [[#M99|M99]] [[#P|P]] address (to tell the control which block of the program to return to if not the default) or [[goto|GoTo]] statements (if the control supports those). [[#N|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. <br> ''System parameter number:'' [[#G10|G10]] allows changing of system parameters under program control.<ref name="Smid2004">{{Harvnb|Smid|2004|p=61}}</ref>
|-
| valign="top" | {{Visible anchor|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.
|-
| valign="top" | {{Visible anchor|P}} || Serves as parameter address for various G and M codes ||
* With [[#G04|G04]], defines dwell time value.
* Also serves as a parameter in some canned cycles, representing dwell times or other variables.
* Also used in the calling and termination of subprograms. (With [[#M98|M98]], it specifies which subprogram to call; with [[#M99|M99]], it specifies which block number of the main program to return to.)
|-
| valign="top" | {{Visible anchor|Q}} || Peck increment in canned cycles || For example, [[#G73_peck_drill|G73]], [[#G83|G83]] (peck drilling cycles)
|-
| valign="top" | {{Visible anchor|R}} || Defines size of arc radius, or defines retract height in milling canned cycles || For radii, not all controls support the R address for [[#G02|G02]] and [[#G03|G03]], in which case IJK vectors are used. For retract height, the "R level", as it's called, is returned to if [[#G99_return_to_R_level|G99]] is programmed.
|-
| valign="top" | {{Visible anchor|S}} || Defines [[Speeds and feeds|speed]], either spindle speed or surface speed depending on mode || Data type = integer. In [[#G97|G97]] mode (which is usually the default), an integer after S is interpreted as a number of [[Revolutions per minute|rev/min]] (rpm). In [[#G96|G96]] mode (Constant Surface Speed or CSS), an integer after S is interpreted as [[Surface feet per minute|surface speed]]—sfm ([[#G20|G20]]) or m/min ([[#G21|G21]]). See also [[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.
|-
| valign="top" | {{Visible anchor|T}} || Tool selection || To understand how the T address works and how it interacts (or not) with [[#M06|M06]], one must study the various methods, such as lathe turret programming, ATC (Automatic Tool Change, set by [[#M06|M06]]) fixed tool selection, ATC random memory tool selection, the concept of "next tool waiting", and empty tools.<ref name="Smid2008" /> Programming on any particular machine tool requires knowing which method that machine uses.<ref name="Smid2008" />
|-
| valign="top" | {{Visible anchor|U}} || Incremental axis corresponding to X axis (typically only lathe group A controls) <br> Also defines dwell time on some machines (instead of "[[#P|P]]" or "[[#X|X]]"). || In these controls, X and U obviate [[#G90_absolute|G90]] and [[#G91|G91]], respectively. On these lathes, G90 is instead [[#G90_roughing|a fixed cycle address for roughing]].
|-
| valign="top" | {{Visible anchor|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.'' 1996<ref name="Greenetal1996"/> did not even list V in their table of addresses.) That is still often the case, although the proliferation of live lathe tooling and turn-mill machining has made V address usage less rare than it used to be (Smid 2008<ref name="Smid2008"/> shows an example). See also [[#G18|G18]].
|-
| valign="top" | {{Visible anchor|W}} || Incremental axis corresponding to Z axis (typically only lathe group A controls) || In these controls, Z and W obviate [[#G90_absolute|G90]] and [[#G91|G91]], respectively. On these lathes, G90 is instead [[#G90_roughing|a fixed cycle address for roughing]].
|-
| valign="top" | '''{{Visible anchor|X}}''' || Absolute or incremental position of X axis. <br> Also defines dwell time on some machines (instead of "[[#P|P]]" or "[[#U|U]]"). ||
|-
| valign="top" | '''{{Visible anchor|Y}}''' || Absolute or incremental position of Y axis ||
|-
| valign="top" | '''{{Visible anchor|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.
|}
==={{anchor|List of G-codes}} List of G-codes commonly found on [[FANUC]] and similarly designed controls for milling and turning===
Sources: Smid 2008;<ref name="Smid2008"/> Smid 2010;<ref name="Smid2010"/> Green et al. 1996.<ref name="Greenetal1996"/>
::<small>'''Note''': '''''Modal''''' means a code stays in effect until replaced, or cancelled, by another permitted code. '''''Non-Modal''''' means it executes only once. See, for example, codes G09, G61 & G64 below.</small>
{| class="wikitable"
! style="width: 3em;" | Code
! style="width: 12em;" | Description
! style="width: 4em;" | Milling <br> ( M )
! style="width: 4em;" | Turning <br> ( T )
! Corollary info
|-
| valign="top" | {{Visible anchor|G00}} || Rapid positioning || M || T || On 2- or 3-axis moves, G00 (unlike [[#G01|G01]]) traditionally does not necessarily move in a single straight line between start point and endpoint. It moves each axis at its max speed until its vector quantity is achieved. A 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).
|-
| valign="top" | {{Visible anchor|G01}} || [[Linear interpolation]] || M || T || The most common workhorse code for feeding during a cut. The program specs the start and endpoints, and the control automatically calculates ([[Interpolation|interpolates]]) the intermediate points to pass through that yield a straight line (hence "[[linear]]"). The control then calculates the angular velocities at which to turn the axis [[leadscrew]]s 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 feed rate on a path that is accurately linear to within very small limits.
|-
| valign="top" | {{Visible anchor|G02}} || Circular interpolation, clockwise || M || T || Very similar in concept to G01. Again, the control [[interpolation|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 tooltip 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 feed rate on a path that accurately matches the ideal (in [[#G02|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 find boring. '''Addresses for radius or arc center:''' G02 and G03 take either an [[#R|R]] address (for the radius desired on the part) or [[#I|IJK]] addresses (for the component vectors that define the vector from the arc start point to the arc center point). '''Cutter comp:''' On most controls you cannot start [[#G41|G41]] or [[#G42|G42]] in [[#G02|G02]] or [[#G03|G03]] modes. You must already have compensated in an earlier [[#G01|G01]] block. Often, a short linear lead-in movement is programmed, merely to allow cutter compensation before the main action, the circle-cutting begins. '''Full circles:''' When the arc start point and the arc endpoint are identical, the tool cuts a 360° arc (a full circle). (Some older controls do not support this because arcs cannot cross between quadrants of the cartesian system. Instead, they require four quarter-circle arcs programmed back-to-back.)
|-
| valign="top" | {{Visible anchor|G03}} || Circular interpolation, counterclockwise || M || T || Same corollary info as for G02.
|-
| valign="top" | {{Visible anchor|G04}} || Dwell || M || T || Takes an address for dwell period (may be [[#X|X]], [[#U|U]], or [[#P|P]]). The dwell period is specified by a control parameter, typically set to [[millisecond]]s. Some machines can accept either X1.0 ([[second|s]]) or P1000 ([[millisecond|ms]]), which are equivalent. '''{{Visible anchor|Choosing dwell duration}}''': Often the dwell needs only to last one or two full spindle rotations. This is typically much less than one second. Be aware when choosing a duration value that a long dwell is a waste of cycle time. In some situations, it won't matter, but for high-volume repetitive production (over thousands of cycles), it is worth calculating that perhaps you only need 100 [[millisecond|ms]], and you can call it 200 to be safe, but 1000 is just a waste (too long).
|-
| valign="top" | {{Visible anchor|G05}} P10000 || High-precision contour control (HPCC) || M || || Uses a deep look-ahead [[data buffer|buffer]] and simulation processing to provide better axis movement acceleration and deceleration during contour milling
|-
| valign="top" | {{Visible anchor|G05.1 Q1.}} || AI Advanced Preview Control || M || || Uses a deep look-ahead [[data buffer|buffer]] and simulation processing to provide better axis movement acceleration and deceleration during contour milling
|-
| valign="top" | {{Visible anchor|G06.1 }} || [[Non-uniform rational B-spline]] (NURBS) Machining || M || || Activates Non-Uniform Rational B Spline for complex curve and waveform machining (this code is confirmed in Mazatrol 640M ISO Programming)
|-
| valign="top" | {{Visible anchor|G07}} || Imaginary axis designation || M || ||
|-
| valign="top" | {{Visible anchor|G08}} || Lookahead || M || || On older versions, look-ahead. G08P1 = on, G08P0 = off. Newer systems use G05
|-
| valign="top" | {{Visible anchor|G09}} || Exact stop check, non-modal || M || T || The modal version is [[#G61|G61]].
|-
| valign="top" | {{Visible anchor|G10}} || Programmable data input || M || T || Modifies the value of work coordinate and tool offsets<ref>{{cite web|url=http://atyourservice.haascnc.com/faqs/clearing-all-offsets/|title=FAQ's - At Your Service|website=atyourservice.haascnc.com|access-date=5 April 2018|archive-url=https://web.archive.org/web/20150101054250/http://atyourservice.haascnc.com/faqs/clearing-all-offsets/|archive-date=1 January 2015|url-status=dead}}</ref><ref name="Smid2004"/>
|-
| valign="top" | {{Visible anchor|G11}} || Data write cancel || M || T ||
|-
| valign="top" | {{Visible anchor|G17}} || XY plane selection || M || ||
|-
| valign="top" | {{Visible anchor|G18}} || ZX plane selection || M || T ||
|-
| valign="top" | {{Visible anchor|G19}} || YZ plane selection || M || ||
|-
| valign="top" | {{Visible anchor|G20}} || Programming in [[inch]]es || 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.
|-
| valign="top" | {{Visible anchor|G21}} || Programming in [[millimeter]]s (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.
|-
| valign="top" | {{Visible anchor|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.
|-
| valign="top" | {{Visible anchor|G30}} || Return to secondary home position (machine zero, aka machine reference point) || M || T || Takes a P address specifying ''which'' machine zero point to use ''if'' the machine has several secondary points (P1 to P4). Takes X Y Z addresses that define the intermediate point that the tooltip passes through on its way home to machine zero. These are expressed in terms of part zero (aka program zero), NOT machine zero.
|-
| valign="top" | {{Visible anchor|G31}} || Feed until skip function || M || || Used for probes and tool length measurement systems.
|-
| valign="top" | {{Visible anchor|G32}} || Single-point threading, longhand style (if not using a cycle, e.g., [[#G76_thread_on_lathe|G76]]) || || T || Similar to [[#G01|G01]] linear interpolation, except with automatic spindle synchronization for [[Threading (manufacturing)#Single-point threading|single-point threading]].
|-
| valign="top" | {{Visible anchor|G33}} || Constant-[[Screw thread#Lead, pitch, and starts|pitch]] threading || M || ||
|-
| valign="top" | {{Visible anchor|G33}} || Single-point threading, longhand style (if not using a cycle, e.g., [[#G76_thread_on_lathe|G76]]) || || T || Some lathe controls assign this mode to G33 rather than G32.
|-
| valign="top" | {{Visible anchor|G34}} || Variable-pitch threading || M || ||
|-
| valign="top" | {{Visible anchor|G40}} || Tool radius compensation off || M || T || Turn off [[cutter ___location|cutter radius compensation (CRC)]]. Cancels G41 or G42.
|-
| valign="top" | {{Visible anchor|G41}} || Tool radius compensation left || M || T || Turn on [[cutter ___location|cutter radius compensation (CRC)]], left, for climb milling. <br> '''Milling:''' Given righthand-helix cutter and [[#M03|M03]] spindle direction, G41 corresponds to [[Milling cutter#Conventional milling versus climb milling|climb milling (down milling)]]. Takes an address ([[#D|D]] or [[#H|H]]) that calls an offset register value for radius. <br> '''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 have been partially automated and obviated (although not completely) since [[computer-aided manufacturing|CAM]] programming has become more common. CAM systems let the user program as if using 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 that. The software has various CRC mode selections, such as ''computer, control, wear, reverse wear, off'', some of which do not use G41/G42 at all (good for roughing, or wide finish tolerances), and others that use it so that the wear offset can still be tweaked at the machine (better for tight finish tolerances).
|-
| valign="top" | {{Visible anchor|G42}} || Tool radius compensation right || M || T || Turn on [[cutter ___location|cutter radius compensation (CRC)]], right, for conventional milling. Similar corollary info as for [[#G41|G41]]. Given righthand-helix cutter and M03 spindle direction, G42 corresponds to [[Milling cutter#Conventional milling versus climb milling|conventional milling (up milling)]].
|-
| valign="top" | {{Visible anchor|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).
|-
| valign="top" | {{Visible anchor|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).
|-
| valign="top" | {{Visible anchor|G45}} || Axis offset single increase || M || ||
|-
| valign="top" | {{Visible anchor|G46}} || Axis offset single decrease || M || ||
|-
| valign="top" | {{Visible anchor|G47}} || Axis offset double increase || M || ||
|-
| valign="top" | {{Visible anchor|G48}} || Axis offset double decrease || M || ||
|-
| valign="top" | {{Visible anchor|G49}} || Tool length offset compensation cancel || M || || Cancels [[#G43|G43]] or [[#G44|G44]].
|-
| valign="top" | {{Anchor|G50_RPM_clamp}} G50 || Define the maximum spindle speed || || T || Takes an [[#S|S]] address integer, which is interpreted as rpm. Without this feature, [[#G96|G96]] mode (CSS) would rev the spindle to "wide open throttle" when closely approaching the axis of rotation.
|-
| valign="top" | {{Anchor|G50_scaling_off}} G50 || Scaling function cancel || M || ||
|-
| valign="top" | {{Anchor|G50_position_register}} G50 || Position register (programming of the vector from part zero to tooltip) || || 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 [[#G54 to G59|G54 to G59]] (WCSs) are a better, newer method. Called via G50 for turning, [[#G92_position_register|G92]] for milling. Those G addresses also have alternate meanings (''which see''). Position register can still be useful for datum shift programming. The "manual absolute" switch, which has very few useful applications in WCS contexts, was more useful in position register contexts because it allowed the operator to move the tool to a certain distance from the part (for example, by touching off a 2.0000" gage) and then declare to the control what the distance-to-go shall be (2.0000).
|-
| valign="top" | {{Visible anchor|G52}} || Local coordinate system (LCS) || M || || Temporarily shifts program zero to a new ___location. It is simply "an offset from an offset", that is, an additional offset added onto the [[#G54 to G59|WCS]] offset. This simplifies programming in some cases. A typical example is moving from part to part in a multipart setup. With '''G54''' active, {{Code|G52 X140.0 Y170.0|gcode}} shifts program zero 140 mm over in X and 170 mm over in Y. When the part "over there" is done, {{Code|G52 X0 Y0|gcode}} returns program zero to normal G54 (by reducing G52 offset to nothing). The same result can also be achieved (1) using multiple WCS origins, G54/G55/G56/G57/G58/G59; (2) on newer controls, G54.1 P1/P2/P3/etc. (all the way up to P48); or (3) using [[#G10|G10]] for programmable data input, in which the program can write new offset values to the offset registers.<ref name="Smid2004"/> The method to use depends on the shop-specific application.
|-
| valign="top" | {{Visible anchor|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 from the previously selected Work Coordinate System, [[G-code#G54 to G59|G54 to G59]], even if it is not explicitly programmed.
|-
| valign="top" | {{Visible anchor|G54 to G59}} || Work coordinate systems (WCSs) || M || T || Have largely replaced position register ([[#G50_position_register|G50]] and [[#G92_position_register|G92]]). Each tuple of axis offsets relates program zero directly to machine zero. The Standard is 6 tuples (G54 to G59), with optional extensibility to 48 more via G54.1 P1 to P48.
|-
| valign="top" | {{Visible anchor|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 the floating-point extension of the G-code data type (formerly all integers). Other examples have also evolved (e.g., [[#G84.2|G84.2]]). Modern controls have the [[computer hardware|hardware]] to handle it.
|-
| valign="top" | {{Visible anchor|G61}} || Exact stop check, modal || M || T || Can be canceled with [[#G64|G64]]. The non-modal version is [[#G09|G09]].
|-
| valign="top" | {{Visible anchor|G62}} || Automatic corner override || M || T ||
|-
| valign="top" | {{Visible anchor|G64}} || Default cutting mode (cancel exact stop check mode) || M || T || Cancels [[#G61|G61]].
|-
| valign="top" | {{Visible anchor|G68}} || Rotate coordinate system || M || || Rotates coordinate system in the current plane given with [[#G17|G17]], [[#G18|G18]], or [[#G19|G19]]. Center of rotation is given with two parameters, which vary with each vendor's implementation. Rotate with the angle given with argument R. This can be used, for instance, to align the coordinate system with a misaligned part. It can also be used to repeat movement sequences around a center. Not all vendors support coordinate system rotation.
|-
| valign="top" | {{Visible anchor|G69}} || Turn off coordinate system rotation || M || || Cancels [[#G68|G68]].
|-
| valign="top" | {{Visible anchor|G70}} || Fixed cycle, multiple repetitive cycle, for finishing (including contours) || || T ||
|-
| valign="top" | {{Visible anchor|G71}} || Fixed cycle, multiple repetitive cycles, for roughing (Z-axis emphasis) || || T ||
|-
| valign="top" | {{Visible anchor|G72}} || Fixed cycle, multiple repetitive cycles, for roughing (X-axis emphasis) || || T ||
|-
| valign="top" | {{Anchor|G73_rough_turn_pattern_repeat}} G73 || Fixed cycle, multiple repetitive cycle, for roughing, with pattern repetition || || T ||
|-
| valign="top" | {{Anchor|G73_peck_drill}} 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 chip breaking is the main concern, but chip clogging of flutes is not. Compare [[#G83|G83]].
|-
| valign="top" | {{Anchor|G74_pecking}} G74 || Peck drilling cycle for turning || || T ||
|-
| valign="top" | {{Anchor|G74_tapping}} G74 || Tapping cycle for milling, [[Screw thread#Handedness|lefthand thread]], [[#M04|M04 spindle direction]] || M || || See notes at [[#G84|G84]].
|-
| valign="top" | {{Visible anchor|G75}} || Peck grooving cycle for turning || || T ||
|-
| valign="top" | {{Anchor|G76_bore_on_mill}} G76 || Fine boring cycle for milling || M || || Includes OSS and shift (oriented spindle stop and shift tool off centerline for retraction)
|-
| valign="top" | {{Anchor|G76_thread_on_lathe}} G76 || Threading cycle for turning, multiple repetitive cycle || || T ||
|-
| valign="top" | {{Visible anchor|G80}} || Cancel [[canned cycle]] || M || T || '''Milling:''' Cancels all cycles such as [[#G73_peck_drill|G73]], [[#G81|G81]], [[#G83|G83]], etc. Z-axis returns either to Z-initial level or R level, as programmed ([[#G98_return_to_initial|G98]] or [[#G99_return_to_R_level|G99]], respectively). <br> '''Turning:''' Usually not needed on lathes, because a new group-1 G address ([[#G00|G00]] to [[#G03|G03]]) cancels whatever cycle was active.
|-
| valign="top" | {{Visible anchor|G81}} || Simple drilling cycle || M || || No dwell built in
|-
| valign="top" | {{Visible anchor|G82}} || Drilling cycle with dwell || M || || Dwells at hole bottom (Z-depth) for the number of [[millisecond]]s specified by the [[#P|P]] address. Good for when hole bottom finish matters. Good for spot drilling because the divot is certain to clean up evenly. Consider the "[[#Choosing dwell duration|choosing dwell duration]]" note at [[#G04|G04]].
|-
| valign="top" | {{Visible anchor|G83}} || Peck drilling cycle (full retraction from pecks) || M || || Returns to R-level after each peck. Good for clearing flutes of [[swarf|chips]]. Compare [[#G73_peck_drill|G73]].
|-
| valign="top" | {{Visible anchor|G84}} || [[Tap and die|Tapping]] cycle, [[screw thread#Handedness|righthand thread]], [[#M03|M03]] spindle direction || M || || [[#G74_tapping|G74]] and G84 are the righthand and lefthand "pair" for old-school tapping with a non-rigid toolholder ("tapping head" style). Compare the rigid tapping "pair", [[#G84.2|G84.2]] and [[#G84.3|G84.3]].
|-
| valign="top" | {{Visible anchor|G84.2}} || Tapping cycle, [[screw thread#Handedness|righthand thread]], [[#M03|M03]] spindle direction, rigid toolholder || M || || See notes at [[#G84|G84]]. Rigid tapping synchronizes speed and feeds according to the desired thread helix. That is, it synchronizes degrees of spindle rotation with microns of axial travel. Therefore, it can use a rigid tool holder to hold the tap. This feature is not available on old machines or newer low-end machines, which must use "tapping head" motion ([[#G74_tapping|G74]]/[[#G84|G84]]).
|-
| valign="top" | {{Visible anchor|G84.3}} || Tapping cycle, [[screw thread#Handedness|lefthand thread]], [[#M04|M04]] spindle direction, rigid toolholder || M || || See notes at [[#G84|G84]] and [[#G84.2|G84.2]].
|-
| valign="top" | {{Visible anchor|G85}} || boring cycle, feed in/feed out || M || ||
* Good cycle for a reamer.
* In some cases good for single-point boring tool, although in other cases the lack of depth of cut on the way back out is bad for surface finish, in which case, [[#G76_bore_on_mill|G76]] (OSS/shift) can be used instead.
* If need dwell at hole bottom, see [[#G89|G89]].
|-
| valign="top" | {{Visible anchor|G86}} || boring cycle, feed in/spindle stop/rapid out || M || || Boring tool leaves a slight score mark on the way back out. Appropriate cycle for some applications; for others, [[#G76_bore_on_mill|G76]] (OSS/shift) can be used instead.
|-
| valign="top" | {{Visible anchor|G87}} || boring cycle, backboring || M || || For [[boring (manufacturing)|backboring]]. Returns to initial level only ([[#G98_return_to_initial|G98]]); this cycle cannot use [[#G99_return_to_R_level|G99]] because its [[#R|R level]] is on the far side of the part, away from the spindle headstock.
|-
| valign="top" | {{Visible anchor|G88}} || boring cycle, feed in/spindle stop/manual operation || M || ||
|-
| valign="top" | {{Visible anchor|G89}} || boring cycle, feed in/dwell/feed out || M || || G89 is like [[#G85|G85]] but with dwell added at bottom of hole.
|-
| valign="top" | {{Anchor|G90_absolute}} G90 || Absolute programming || M || T (B) || Positioning defined with reference to part zero. <br> '''Milling:''' Always as above. <br> '''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, [[#U|U]] and [[#W|W]] are the incremental addresses and [[#X|X]] and [[#Z|Z]] are the absolute addresses. On these lathes, G90 is instead a fixed cycle address for roughing.
|-
| valign="top" | {{Anchor|G90_roughing}} G90 || Fixed cycle, simple cycle, for roughing (Z-axis emphasis) || || T (A) || When not serving for absolute programming (above)
|-
| valign="top" | {{Visible anchor|G90.1}} || Absolute arc programming || M || || I, J, K positioning defined with reference to part zero.
|-
| valign="top" | {{Visible anchor|G91}} || Incremental programming || M || T (B) || Positioning defined with reference to previous position. <br> '''Milling:''' Always as above. <br> '''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, [[#U|U]] and [[#W|W]] are the incremental addresses and [[#X|X]] and [[#Z|Z]] are the absolute addresses. On these lathes, G90 is a fixed cycle address for roughing.
|-
| valign="top" | {{Visible anchor|G91.1}} || Incremental arc programming || M || || I, J, K positioning defined with reference to previous position.
|-
| valign="top" | {{Anchor|G92_position_register}} G92 || Position register (programming of vector from part zero to tool tip) || M || T (B) || Same corollary info as at [[#G50_position_register|G50 position register]]. <br> '''Milling:''' Always as above. <br> '''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_position_register|G50]].
|-
| valign="top" | {{Anchor|G92_threading}} G92 || Threading cycle, simple cycle || || T (A) ||
|-
| valign="top" | {{Anchor|G93_inverse_time}} G93 || Inverse feed time || M (with radial axes) || || Means the move should be completed in [1/F number] minutes. An F number is required on all lines with G1, G2, G3 movements when in G93 mode.<ref>{{cite web | url=https://tormach.com/feed-rate-mode-g93-g94-and-g95 | title=Feed Rate Mode (G93, G94, and G95) }}</ref> Used to translate distance-per-time to rotary axes.
|-
| valign="top" | {{Anchor|G94_feed_per_minute}} G94 || Feedrate per minute || M || T (B) || On group type A lathes, feedrate per minute is [[#G98_feed_per_minute|G98]].
|-
| valign="top" | {{Anchor|G94_roughing}} G94 || Fixed cycle, simple cycle, for roughing ([[#X|X]]-axis emphasis) || || T (A) || When not serving for feedrate per minute (above)
|-
| valign="top" | {{Anchor|G95_feed_per_rev}} G95 || Feedrate per revolution || M || T (B) || On group type A lathes, feedrate per revolution is [[#G99_feed_per_rev|G99]].
|-
| valign="top" | {{Visible anchor|G96}} || Constant surface speed (CSS) || || T || Varies spindle speed automatically to achieve a constant surface speed. See [[speeds and feeds]]. Takes an [[#S|S]] address integer, which is interpreted as [[Surface feet per minute|sfm]] in [[#G20|G20]] mode or as m/min in [[#G21|G21]] mode.
|-
| valign="top" | {{Visible anchor|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.
|-
| valign="top" | {{Anchor|G98_return_to_initial}} G98 || Return to initial Z level in canned cycle || M || ||
|-
| valign="top" | {{Anchor|G98_feed_per_minute}} G98 || Feedrate per minute (group type A) || || T (A) || Feedrate per minute is [[#G94_feed_per_minute|G94]] on group type B.
|-
| valign="top" | {{Anchor|G99_return_to_R_level}} G99 || Return to [[#R|R level]] in canned cycle || M || ||
|-
| valign="top" | {{Anchor|G99_feed_per_rev}} G99 || Feedrate per revolution (group type A) || || T (A) || Feedrate per revolution is [[#G95_feed_per_rev|G95]] on group type B.
|-
| valign="top" | {{Anchor|G100_Tool_Length_Measurement}} G100 || Tool length measurement || M || ||
|-
|}
==={{anchor|List of M-codes}} List of M-codes commonly found on FANUC and similarly designed controls for milling and turning===
Sources: Smid 2008;<ref name="Smid2008"/> Smid 2010;<ref name="Smid2010"/> Green et al. 1996.<ref name="Greenetal1996"/>
Some older controls require M codes to be in separate blocks (that is, not on the same line).
{| class="wikitable"
! Code !! Description !! Milling <br> ( M ) !! Turning <br> ( T ) !! Corollary info
|-
| valign="top" | {{Visible anchor|M00}} || Compulsory stop || M || T || Non-optional—machine always stops on reaching M00 in the program execution.
|-
| valign="top" | {{Visible anchor|M01}} || Optional stop || M || T || Machine only stops at M01 if operator pushes the optional stop button.
|-
| valign="top" | {{Visible anchor|M02}} || End of program || M || T || Program ends; execution may or may not return to program top (depending on the control); may or may not reset register values. M02 was the original program-end code, now considered obsolete, but still supported for backward compatibility.<ref name="Smid2010pp29-30">{{Harvnb|Smid|2010|pp=29–30}}.</ref> Many modern controls treat M02 as equivalent to [[#M30|M30]].<ref name="Smid2010pp29-30"/> See [[#M30|M30]] for additional discussion of control status upon executing M02 or M30.
|-
| valign="top" | {{Visible anchor|M03}} || Spindle on (clockwise rotation) || M || T || The speed of the spindle is determined by the address [[#S|S]], in either [[revolutions per minute]] ([[#G97|G97]] mode; default) or [[surface feet per minute]] or [surface] meters per minute ([[#G96|G96]] mode [CSS] under either [[#G20|G20]] or [[#G21|G21]]). The [[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 the local vantage point and local CW/CCW distinction.
|-
| valign="top" | {{Visible anchor|M04}} || Spindle on (counterclockwise rotation) || M || T || See comment above at M03.
|-
| valign="top" | {{Visible anchor|M05}} || Spindle stop || M || T ||
|-
| valign="top" | {{Visible anchor|M06}} || Automatic tool change (ATC) || M || T (some-times) || Many lathes do not use M06 because the [[#T|T]] address itself indexes the turret. <br> 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.<ref name="Smid2008" />
|-
| valign="top" | {{Visible anchor|M07}} || [[Cutting fluid|Coolant]] on (mist) || M || T ||
|-
| valign="top" | {{Visible anchor|M08}} || Coolant on (flood) || M || T ||
|-
| valign="top" | {{Visible anchor|M09}} || Coolant off || M || T ||
|-
| valign="top" | {{Visible anchor|M10}} || Pallet clamp on || M || || For machining centers with pallet changers
|-
| valign="top" | {{Visible anchor|M11}} || Pallet clamp off || M || || For machining centers with pallet changers
|-
| valign="top" | {{Visible anchor|M13}} || Spindle on (clockwise rotation) and coolant on (flood) || M || || This one M-code does the work of both [[#M03|M03]] and [[#M08|M08]]. It is not unusual for specific machine models to have such combined commands, which make for shorter, more quickly written programs.
|-
| valign="top" | {{Visible anchor|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|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 [[Threading (manufacturing)#Single-point threading|single-pointing]] have depended on spindle position encoders for decades, before the advent of widespread live tooling and mill-turn/turn-mill systems, it was not as often relevant in "regular" (non-"special") machining for the operator (as opposed to the machine) to know the angular orientation of a spindle as it is today, except in certain contexts (such as [[#T|tool change]], or [[#G76_bore_on_mill|G76]] fine boring cycles with choreographed tool retraction). Most milling of features indexed around a turned workpiece was accomplished with separate operations on [[indexing head]] setups; in a sense, indexing heads were originally 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.
|-
| valign="top" | {{Visible anchor|M21}} || Mirror, [[#X|X]]-axis || M || ||
|-
| M21 || Tailstock forward || || T ||
|-
| valign="top" | {{Visible anchor|M22}} || Mirror, [[#Y|Y]]-axis || M || ||
|-
| M22 || Tailstock backward || || T ||
|-
| valign="top" | {{Visible anchor|M23}} || Mirror OFF || M || ||
|-
| M23 || Thread gradual pullout ON || || T ||
|-
| valign="top" | {{Visible anchor|M24}} || Thread gradual pullout OFF || || T ||
|-
| valign="top" | {{Visible anchor|M30}} || End of program, with return to program top || M || T || Today, M30 is considered the standard program-end code, and returns execution to the top of the program. Most controls also still support the original program-end code, [[#M02|M02]], usually by treating it as equivalent to M30. '''Additional info:''' Compare [[#M02|M02]] with M30. First, M02 was created, in the days when the [[punched tape]] was expected to be short enough to splice into a continuous loop (which is why on old controls, M02 triggered no tape rewinding).<ref name="Smid2010pp29-30"/> The other program-end code, M30, was added later to accommodate longer punched tapes, which were wound on a [[reel]] and thus needed rewinding before another cycle could start.<ref name="Smid2010pp29-30"/> On many newer controls, there is no longer a difference in how the codes are executed—both act like M30.
|-
| valign="top" | {{Visible anchor|M41}} || Gear select – gear 1 || || T ||
|-
| valign="top" | {{Visible anchor|M42}} || Gear select – gear 2 || || T ||
|-
| valign="top" | {{Visible anchor|M43}} || Gear select – gear 3 || || T ||
|-
| valign="top" | {{Visible anchor|M44}} || Gear select – gear 4 || || T ||
|-
| valign="top" | {{Visible anchor|M48}} || Feedrate override allowed || M || T || [[#MFO|MFO]] (manual feedrate override)
|-
| valign="top" | {{Visible anchor|M49}} || Feedrate override NOT allowed || M || T || Prevent [[#MFO|MFO]] (manual feedrate override). This rule is also usually called (automatically) within tapping cycles or single-point threading cycles, where feed is precisely correlated to speed. Same with [[#SSO|SSO]] (spindle speed override) and feed hold button. Some controls are capable of providing [[#Arbitrary-speed_threading|SSO and MFO during threading]].
|-
| valign="top" | {{Visible anchor|M52}} || Unload Last tool from spindle || M || T || Also empty spindle.
|-
| valign="top" | {{Visible anchor|M60}} || Automatic pallet change (APC) || M || || For machining centers with pallet changers
|-
| valign="top" | {{Visible anchor|M88}} || Internal cooling on || M || || (also known as high pressure cooling on)
|-
| valign="top" | {{Visible anchor|M89}} || Internal cooling off || M || || (also known as high pressure cooling off)
|-
| valign="top" | {{Visible anchor|M98}} || Subprogram call || M || T || Takes an address [[#P|P]] to specify which subprogram to call, for example, "M98 P8979" calls subprogram O8979.
|-
| valign="top" | {{Visible anchor|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).
|-
|M100
|Clean Nozzle
|
|
|Some 3d printers have a predefined routine for wiping the extruder nozzle in the X and Y direction often against a flexible scraper mounted to the dump area.
|-
|}
==Example program==
This is a generic program that demonstrates the use of G-Code to turn a part that is 1" diameter by 1" long. 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.)
{| class="wikitable"
!Block / Code
!Description
|-
| {{codett|%|gcode}} || Signals start of data during file transfer. Originally used to stop tape rewind, not necessarily start of the program. For some controls (FANUC) the first LF (EOB) is the start of the program. ISO uses {{mono|%}}, EIA uses ER ({{mono|0x0B}}).
|-
| {{nowrap|{{mono| }}{{codett|O4968 (OPTIONAL PROGRAM DESCRIPTION OR COMMENT)|gcode}}}} || Sample face and turn program. Comments are enclosed in parentheses.
|-
| {{codett|N10 M216 |gcode}} || Turn on load monitor
|-
| {{codett|N20 G20 G90 G54 D200 G40|gcode}} || Inch units. Absolute mode. Activate work offset. Activate tool offset. Deactivate tool nose radius compensation. <br/> ''Significance:'' This block is often called the '''safe block''' or safety block. Its commands can vary but are usually similar to the ones shown here. The idea is that a safety block should always be given near the top of any program, as a general default, unless some very specific/concrete reason exists to omit it. The safety block is like a [[sanity check]] or a [[preflight checklist]]: it explicitly ensures conditions that otherwise would be implicit, left merely to assumption. The safety block reduces risk of crashes, and it can also helpfully refocus the thinking of the humans who write or read the program under hurried conditions.
|-
| {{codett|N30 G50 S2000 |gcode}} || Set maximum spindle speed in rev/min — This setting affects Constant Surface Speed mode
|-
| {{codett|N40 T0300 |gcode}} || Index turret to tool 3. Clear wear offset (00).
|-
| {{codett|N50 G96 S854 M03 |gcode}} || Constant surface speed [automatically varies the spindle speed], 854 [[Surface feet per minute|sfm]], start spindle CW rotation
|-
| {{codett|N60 G41 G00 X1.1 Z1.1 T0303 M08 |gcode}} || Enable cutter radius compensation mode, rapid position to 0.55" above axial centerline (1.1" in diameter) and {{val|1.1 |u=inches}} positive from the work offset in Z, activate flood coolant
|-
| {{codett|N70 G01 Z1.0 F.05 |gcode}} || Feed in horizontally at rate of 0.050" per revolution of the spindle until the tool is positioned 1" positive from the work offset
|-
| {{codett|N80 X-0.016 |gcode}} || Feed the tool slightly past center—the tool must travel by at least its nose radius past the center of the part to prevent a leftover scallop of material.
|-
| {{codett|N90 G00 Z1.1|gcode}} || Rapid positioning; retract to start position
|-
| {{codett|N100 X1.0 |gcode}} || Rapid positioning; next pass
|-
| {{codett|N110 G01 Z0.0 F.05 |gcode}} || Feed-in horizontally cutting the bar to 1" diameter all the way to the datum, [[Revolutions per minute|in/rev]]
|-
| {{codett|N120 G00 X1.1 M05 M09 |gcode}} || Clear the part, stop the spindle, turn off the coolant
|-
| {{codett|N130 G91 G28 X0 |gcode}} || Home X axis — return the machine's home position for the X axis
|-
| {{codett|N140 G91 G28 Z0 |gcode}} || Home Z axis — return to machine's home position for the Z axis
|-
| {{codett|N150 G90 |gcode}} || Return to absolute mode. Turn off load monitor
|-
| {{codett|N160 M30 |gcode}} || Program stop, rewind to the top of the program, wait for cycle start
|-
| {{codett|%|gcode}} || Signal end of data during file transfer. Originally used to mark the end of the tape, not necessarily the end of the program. ISO uses {{mono|%}}, EIA uses ER ({{mono|0x0B}}).
|}
[[File:ToolPath.svg|thumb|right|200px|Tool Path for program]]
Several points to note:
# There is room for some programming style, even in this short program. The grouping of codes in line {{mono|N60}} could have been put on multiple lines. Doing so may have made it easier to follow program execution.
# Many codes are ''modal'', meaning they remain in effect until cancelled or replaced by a contradictory code. For example, once variable speed cutting (CSS) had been selected ({{mono|{{pslink|G96}}}}), it stays in effect until the end of the program. In operation, the spindle speed increases as the tool near the center of the work to maintain constant surface speed. Similarly, once rapid feed is selected ({{mono|{{pslink|G00}}}}), all tool movements are rapid until a feed rate code ({{mono|G01, G02, G03}}) is selected.
# It is common practice to use a load monitor with CNC machinery. The load monitor stops 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 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 to 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 must be replaced or sharpened. Thus, an operator tending multiple machines is told by a machine, essentially, "Pause what you're doing over there, and come attend to something over here."
# 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 preference of the programmer and/or operator and the 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, vice, or machine under high power. This can be costly, especially in newer machining centers. It is possible to intersperse the program with optional stops ({{mono|{{pslink|M01}}}} code) that let the program run piecemeal for testing purposes. The optional stops remain in the program but are skipped during normal running. Fortunately, most CAD/CAM software ships with CNC simulators that display the movement of the tool as the program executes. Nowadays the surrounding objects (chuck, clamps, fixture, tailstock, and more) are included in the [[3D model]]s, and the simulation is much like an entire video game or virtual reality environment, making unexpected crashes much less likely.
##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. Additionally, many machines support operator overrides for both rapid and feed rate that can be used to reduce the speed of the machine, allowing operators to stop program execution before a crash occurs.
# The line numbers that have been included in the program above (i.e. {{mono|N0}} ... {{mono|N16}}) are usually not necessary for the operation of a machine and increase file sizes, so they are seldom used in the 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. {{mono|GOTO N99}}).
# Some machines do not allow multiple M codes in the same line.
==Programming environments==
| 