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G-code is the common name for the most widely used computer numerical control (CNC) programming language, which has many implementations. Used mainly in automation, it is part of computer-aided engineering. This general sense of the term, referring to the language overall (using the mass sense of "code"), is imprecise, because it comes metonymically from the literal sense of the term, referring to one letter address among many in the language (G address, for preparatory commands) and to the specific codes (count sense) that can be formed with it (for example, G00, G01, G28). In fact, every letter of the English alphabet is used somewhere in the language, although some letters' use is less common. Nevertheless, the general sense of the term is indelibly established as the common name of the language. G-code is sometimes called G programming language, but most people well versed in CNC work prefer the name G-code.

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 Electronic Industries Alliance in the early 1960s. A final revision was approved in February 1980 as RS274D. In Europe, the standard ISO 6983 is often used, although in varied states sometimes used other standards, example 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.

Some CNC machine manufacturers attempted to overcome compatibility difficulties by standardizing on machine tool controllers built by Fanuc. This semistandardization can be compared to other instances of market dominance, such as with IBM, Intel, or Microsoft. Pros and cons exist, and a wide variety of alternatives are available.

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 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 natural-word-including names (or the expressions in which to use them). It was thus unable to encode logic; it was essentially just a way to "connect the dots" where many of the dots' locations were figured out longhand by the programmer. The latest implementations of G-code include such constructs, creating a language somewhat closer to a 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
• 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 decorative profile shape added to the edge of a workpiece.
• set tool information such as offset.

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.

Sources: Smid; Green et al.

List of G-codes commonly found on Fanuc and similarly designed controls

Sources: Smid; Green et al.

GE Famuc Automation, Computer Numerical Control Products: "Operator's Manual", February 2000, publication number B-63004EN/02. (NO Copyright data or notice found.)

Explanation for Table:
One source (citation needed) has stated that the Haas brand name has the largest share of the marketplace in terms of machines sold both in the US and internationally. However, this is only in terms of machines (and controllers) sold from one manufacturer. G-codes for the Haas system are represented in the below table's 2nd column. The FANUC controller is the most common CNC controller used across all brand names of CNC machines in general. Within the GE-Fanuc series of controllers, the most common G-code system is "A" (or "standard"), as represented in the below table's 1st column. Some manufacturers also support "B" (or "Special") on some machines, these are shown in the below table's next-to-last column. Note that the G-code system is hard-wired into a machine, and is NOT generally selectable.

G-Codes

A	H	Description	Group No.	Corollary info	Milling ( M )	Turning ( T )	B	C
${\displaystyle \bullet }$ G00	G00	Rapid motion positioning	00	On 2- or 3-axis moves, G00 (unlike G01) 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).	M	T	${\displaystyle \bullet }$ G00	${\displaystyle \bullet }$ G00
G01	G01	Linear interpolation	00	The most common workhorse code for feeding during a cut. The program specs the start and end points, and the control automatically calculates (interpolates) the intermediate points to pass through that will yield a straight line (hence "linear"). The control then calculates the angular velocities at which to turn the axis leadscrews. The computer performs thousands of calculations per second. Actual machining takes place with given feed on linear path.	M	T	G01	G01
G02	G02	Circular interpolation CW or Helical interpolation CW	00	Cannot start G41 or G42 in G02 or G03 modes. Must already be compensated in earlier G01 block.	M	T	G02	G02
G03	G03	Circular interpolation CCW or Helical interpolation CCW	00	Cannot start G41 or G42 in G02 or G03 modes. Must already be compensated in earlier G01 block.	M	T	G03	G03
G04	G04	Dwell	00	Takes an address for dwell period (may be X, U, or P)	M	T	G04	G04
G05 P10000	.	High-precision contour control (HPCC) or: High speed cycle cutting	00	Uses a deep look-ahead buffer and simulation processing to provide better axis movement acceleration and deceleration during contour milling	M		G05	G05
G05.1 Q1.	.	Ai Nano contour control	00	Uses a deep look-ahead buffer and simulation processing to provide better axis movement acceleration and deceleration during contour milling	M
G07	.	Imaginary axis designation	00	(or Hypothetical axis interpolation)	M		G07	G07
G07.1 (G107)	.	Cylindrical Interpolation	00		?	T	G07.1 (G107)	G07.1 (G107)
G08	.	___
G09	G09	Exact stop check	00		M	T
	G10	Set Offsets	00	___	?	T
${\displaystyle \bullet }$ G10	.	Programmable data input	00	___ ( G10 is not "single-shot"; see notes below.)	M	T	G10	G10
G10.6	.	Tool retract & recover	00				G10.6	G10.6
G11	.	Data write cancel	00	or Programmable data input CANCEL ( G11 is not "single-shot"; see notes below.)	M	T	G11	G11
G12	.	Full-circle interpolation, clockwise	?	Fixed cycle for ease of programming 360° circular interpolation with blend-radius lead-in and lead-out. Not standard on Fanuc controls.	M
G12.1 (G112)	.	Polar Coordinate Interpolation Mode	21		?	T	G12.1 (G112)	G12.1 (G112)
G13	.	Full-circle interpolation, counterclockwise	?	Fixed cycle for ease of programming 360° circular interpolation with blend-radius lead-in and lead-out. Not standard on Fanuc controls.	M
${\displaystyle \bullet }$ G13.1 (G113)	.	CANCEL Polar Coordinate Interpolation Mode	21		?	T	${\displaystyle \bullet }$ G13.1 (G113)	${\displaystyle \bullet }$ G13.1 (G113)
	G14	Secondary Spindle Swap	17 (Haas)		?	T
	G15	CANCEL Secondary Spindle Swap	17 (Haas)		?	T
G17	G17	XpYp plane selection	16		M		G17	G17
${\displaystyle \bullet }$ G18		ZpXp plane selection	16	On most lathes, ZX is the only available plane, so no G17 to G19 codes are used.	M	T	${\displaystyle \bullet }$ G18	${\displaystyle \bullet }$ G18
	G18	Plane selection	02 (Haas)	___	?	T
G19	G19	YpZp plane selection	16 (02 Haas)		M		G19	G19
G20	G20	Programming in inches	06	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.	M	T	G20	G70
G21	G21	Programming in millimeters (mm)	06	Prevalent worldwide. However, in the global marketplace, competence with both G20 and G21 always stands some chance of being necessary at any time.	M	T	G21	G71
${\displaystyle \bullet }$ G22	.	Stored stroke check function ON	09				${\displaystyle \bullet }$ G22	${\displaystyle \bullet }$ G22
G23	.	Stored stroke check function OFF	09				G23	G23
G24	.	___	?
${\displaystyle \bullet }$ G25	.	Spindle speed fluctuation detection OFF	08				${\displaystyle \bullet }$ G25	${\displaystyle \bullet }$ G25
G26	.	Spindle speed fluctuation detection ON	08				G26	G26
G27	.	Reference position return check	00				G27	G27
	G28	Return to Machine Zero, set optional G29 Reference point	00 (Haas)	___	?	T
G28		Return to home position (machine zero, aka machine reference point)	00	(or: Return to reference position) 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.	M	T	G28	G28
	G29	Return from Reference Point	00 (Haas)			T
G30	.	(or: 2nd, 3rd and 4th reference position return) Return to secondary home position (machine zero, aka machine reference point)	00	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.	M	T	G30	G30
G30.1	.	Floating Point Reference Return	00
G31	G31	Skip function	00	(used for probes and tool length measurement systems)	M		G31	G31
G32	G32	Single-point threading, longhand style (if not using a cycle, e.g., G76)	01	Similar to G01 linear interpolation, except with automatic spindle synchronization for single-point threading.		T	G33	G33
G33	.	Constant-pitch threading	_		M
G33	.	Single-point threading, longhand style (if not using a cycle, e.g., G76)	_	Some lathe controls assign this mode to G33 rather than G32.		T
G34	.	Variable-pitch threading	01		M		G34	G34
G35	.	Circular threading (clockwise)	01				G35	G35
G36	.	Circular threading (counterclockwise)	01				G36	G36
G36	.	Automatic tool compensation X	00				G36	G36
G37	.	Automatic tool compensation Z	00				G37	G37
G38	.
G39	.	Corner circular interpolation	00				G39	G39
${\displaystyle \bullet }$ G40	G40	Tool radius compensation off	07	Kills G41 or G42.	M	T	${\displaystyle \bullet }$ G40	${\displaystyle \bullet }$ G40
G41	G41	Tool nose radius compensation left	07	Milling: Given righthand-helix cutter and M03 spindle direction, G41 corresponds to climb milling (down milling). Takes an address (D or 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.)	M	T	G41	G41
G42	G42	Tool nose radius compensation right	07	Similar corollary info as for G41. Given righthand-helix cutter and M03 spindle direction, G42 corresponds to conventional milling (up milling).	M	T	G42	G42
G43	.	Tool height offset compensation negative	_	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).	M
G44	.	Tool height offset compensation positive	_	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).	M
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	_	Kills G43 or G44.	M
G50	G50	Define the maximum spindle speed	00	Takes an S address integer which is interpreted as rpm. Without this feature, G96 mode (CSS) would rev the spindle to "wide open throttle" when closely approaching the axis of rotation.		T	G92	G92
G50		Scaling function cancel	_		M
G50		Position register (programming of vector from part zero to tool tip)	_	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 (WCSs) are a better, newer method. Called via G50 for turning, G92 for milling. Those G addresses also have alternate meanings (which see). Position register can still be useful for datum shift programming.		T	G92	G92
G50.3		Workpiece coordinate system preset	00				G92.1	G92.1
${\displaystyle \bullet }$ G50.2 (G250)	G__	Polygonal turning CANCEL	20				${\displaystyle \bullet }$ G50.2 (G250)	${\displaystyle \bullet }$ G50.2 (G250)
G51.2 (G251)	G__	Polygonal turning	20				G51.2 (G251)	G51.2 (G251)
G52	G__	Local coordinate system (LCS)	00	Temporarily shifts program zero to a new location. This simplifies programming in some cases.	M		G52	G52
G53	G__	Machine coordinate system	00	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.	M	T	G53	G53
${\displaystyle \bullet }$ G54	G54	Work coordinate system 1 (WCSs)	14	Have largely replaced position register (G50 and 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.	M	T	${\displaystyle \bullet }$ G54	${\displaystyle \bullet }$ G54
G55, G56, G57, G58, G59	G55, G56, G57, G58, G59	Work coordinate systems 2 to 6 (WCSs)	_	Have largely replaced position register (G50 and 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.	M	T	G55, G56, G57, G58, G59	G55, G56, G57, G58, G59
G54.1 P1 to P48		Extended work coordinate systems	_	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.	M	T
G65	.	Macro calling	00				G65	G65
G66	.	Macro modal call	12				G66	G66
${\displaystyle \bullet }$ G67	.	Macro modal call CANCEL	12				${\displaystyle \bullet }$ G67	${\displaystyle \bullet }$ G67
G68	.	Mirror image for double turrets ON or balance cut mode	04				G68	G68
${\displaystyle \bullet }$ G69	.	Mirror image for double turrets OFF or balance cut mode CANCEL	04				${\displaystyle \bullet }$ G69	${\displaystyle \bullet }$ G69
G70	G70	Fixed cycle, multiple repetitive cycle, for finishing (including contours)	00	Finishing Cycle		T	G70	G72
G71	G71	Fixed cycle, multiple repetitive cycle, for roughing (Z-axis emphasis)	00	O.D./I.D. Stock Removal Cycle		T	G71	G73
G72	G__	Fixed cycle, multiple repetitive cycle, for roughing (X-axis emphasis)	_			T	G72	G74
G73	G__	Fixed cycle, multiple repetitive cycle, for roughing, with pattern repetition	_			T	G73	G75
G73	G__	Peck drilling cycle for milling - high-speed (NO full retraction from pecks)	__	Retracts only as far as a clearance increment (system parameter). For when chipbreaking is the main concern, but chip clogging of flutes is not.	M		G73	G75
G74	G__	Peck drilling cycle for turning	__			T	G74	G75
G74	G__	Tapping cycle for milling, lefthand thread, M04 spindle direction	_		M		G74	G75
G75	G__	Peck grooving cycle for turning	_			T	G75	G77
G76	G__	Fine boring cycle for milling	_		M		G__	G__
G76	G76	Threading cycle for turning, multiple repetitive cycle	00			T	G76	G78
${\displaystyle \bullet }$ G80	G__	CANCEL canned cycle	10	Milling: Kills all cycles such as G73, 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 (G00 to G03) cancels whatever cycle was active.	M	T	${\displaystyle \bullet }$ G80	${\displaystyle \bullet }$ G80
G81	G__	Simple drilling cycle	(09 Haas)	No dwell built in (or: "Drill Canned Cycle")	M
G82	G__	Drilling cycle with dwell	_	Dwells at hole bottom (Z-depth) for the number of milliseconds specified by the P address. Good for when hole bottom finish matters.	M		G__	G__
G83	G__	Peck drilling cycle (full retraction from pecks)	10	Returns to R-level after each peck. Good for clearing flutes of chips.	M		G83	G83
G84	G__	Tapping cycle, righthand thread, M03 spindle direction	10		M		G84	G84
G84.2	G__	Tapping cycle, righthand thread, M03 spindle direction, rigid toolholder	_		M		G__	G__
G85	G__	Cycle for face boring	10	No dwell built in	M		G85	G85
G87	G__	Cycle for side drilling	10	Dwells at hole bottom (Z-depth) for the number of milliseconds specified by the P address. Good for when hole bottom finish matters.	M		G87	G87
G88	G__	Cycle for side tapping	10	Returns to R-level after each peck. Good for clearing flutes of chips.	M		G88	G88
G89	G__	Cycle for side boring	10		M		G89	G89
G90	G__	Absolute programming	_	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, U and W are the incremental addresses and X and Z are the absolute addresses. On these lathes, G90 is instead a fixed cycle address for roughing.	M	T (B)	G__	G__
G90	G__	Fixed cycle, simple cycle, for roughing (Z-axis emphasis)	01	When not serving for absolute programming (above)		T (A)	G__	G__
G91	G__	Incremental programming	_	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, U and W are the incremental addresses and X and Z are the absolute addresses. On these lathes, G90 is a fixed cycle address for roughing.	M	T (B)	G__	G__
G92		Position register (programming of vector from part zero to tool tip)	01	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.	M	T (B)	G__	G__
G92	G92	Threading cycle, simple cycle	01			T (A)	G__	G__
G94		Feedrate per minute	01	On group type A lathes, feedrate per minute is G98.	M	T (B)	G__	G__
G94	G94	Fixed cycle, simple cycle, for roughing (X-axis emphasis)	01	When not serving for feedrate per minute (above)		T (A)	G__	G__
	G95	Live Tooling Rigid Tap (Face)	09 (Haas)	_________
G95	G__	Feedrate per revolution	_	On group type A lathes, feedrate per revolution is G99.	M	T (B)	G__	G__
G96	G96	Constant surface speed (CSS)	02 (13 Haas)	Varies spindle speed automatically to achieve a constant surface speed. See speeds and feeds. Takes an S address integer, which is interpreted as sfm in G20 mode or as m/min in G21 mode.		T	G96	G96
${\displaystyle \bullet }$ G97	G97	Constant spindle speed	02 (13 Haas)	Takes an S address integer, which is interpreted as rev/min (rpm). The default speed mode per system parameter if no mode is programmed.	M	T	G97	G97
G98		Return to initial Z level in canned cycle	05		M
G98	G98	Feedrate per minute (group type A)	05 (10 Haas)	Feedrate per minute is G94 on group type B.		T (A)	G94	G94
${\displaystyle \bullet }$ G99		Return to R level in canned cycle	05 (10 Haas)		M		${\displaystyle \bullet }$ G95	${\displaystyle \bullet }$ G95
G99	G99	Feedrate per revolution (group type A)	05	Feedrate per revolution is G95 on group type B.		T (A)	G95	G95
	G100	Disable Mirror Image	00 (Haas)	___		T
	G101	Enable Mirro Image	00 (Haas)	___		T
	G102	Programmable Output to RS-232	00 (Haas)	___		T
	G103	Limit Block Lookahead	00 (Haas)	___		T
	G105	Servo Bar Command	__ (Haas)	___		T
G107		See G07.1
	G110	Coordinate System	12 (Haas)	___		T
	G111	Coordinate System	12 (Haas)	___		T
G112		See G12.1
G113		See G13.1
	G114 - G129	Coordinate System	12 (Haas)	___		T
	G112	XY to XZ interpretation	04 (Haas)	___		T
	G113	G112 Cancel	04 (Haas)	___		T
	G154	Select Work Coordinates P1-99	12 (Haas)	___		T
	G159	Background Pickup / Part Return	__ (Haas)	___		T
	G160	APL Axis Command Mode On	__ (Haas)	___		T
	G161	APL Axis Command Mode Off	__ (Haas)	___		T
	G184	Reverse Tapping Canned Cycle for Left Hand Threads	09 (Haas)	___		T
	G186	Reverse Live Tool Rig Tap (For Left Hand Threads)	09 (Haas)	___		T
	G187	Accuracy Control	00 (Haas)	___		T
	G195	Live Tool Radial Tapping (Diameter)	00 (Haas)	___		T
	G196	Reverse Live Tool Vector Tapping (Diameter)	00 (Haas)	___		T
	G198	Disengage Synchronous Spindle Control	00 (Haas)	___		T
	G199	Engage Synchronous Spindle Control	00 (Haas)	___		T
	G200	Index on the Fly	00 (Haas)	___		T
	G211	Manual Tool Setting	__ (Haas)	___		T
	G212	Auto Tool Setting	__ (Haas)	___		T
G250		See G50.2
G251		See G51.2

Notes:
1. If the CNC enters the clear state when the power it turned on or the CNC is reset, the modal G codes change as follows.

(1) G codes marked with a "${\displaystyle \bullet }$" (bullet) in above table are enabled. (This includes G00, G10, G13.1/G112, G18, G22, G25, G40, G50.2/G250, G54, G67, G69, G80, G97 and G99.)

(2) When the system is cleared due to power-on or reset, whichever is specified, either G20 or G21, remains effective.

2. G codes of group 00 except G10 and G11 are single shot G codes.
4. G codes of different groups can be specified in the same block. If G codes of the same group are specified in the same block, the G code specified last is valid.

References

1. ^ Smid 2008.
2. ^ Green 1996, pp. 1162–1226 harvnb error: no target: CITEREFGreen1996 (help).
3. Doosan is an example of one company that supports both Systems "A" and "B".
4. If a G-code appears in the "A" column, but there is no matching information in that row's columns "H", "B" or "C", then that data likely came from the Smid or Green et al. sources.
5. When a G-code appears in the "H" column, the source is one of the two Haas Operator's Manuals.
6. When a G-code appears in the "B" or "C" column, the source is the GE-Fanuc manual.
7. The last column, for GE-Fanuc's G-code system C is included for the sake of 'completeness'. No information has been found stating how much or how little System-C is used. Information derived from the same GE-Fanuc manual referred to earlier.

List of M-codes commonly found on Fanuc and similarly designed controls

Sources: Smid; Green et al.

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.)

Several points to note:

1. 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.
2. 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.
3. 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 job of the load monitor is to prevent machine damage in the event of tool breakage or a programming mistake. On small or hobby machines, it can warn of a tool that is becoming dull and needs to be replaced or sharpened.
4. 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.
5. 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. Thankfully, 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.
6. 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).
7. 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 stack computer-aided design (CAD), computer-aided manufacturing (CAM), and richly featured G-code editors. (G-code editors are analogous to XML editors, using colors and indents semantically to aid the user in ways that basic text editors can't. CAM packages are analogous to 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 CAM software systems that generate G-code automatically via postprocessors (analogous to the development of visual techniques in general programming), and (2) abandoning hardcoded constructs for parametric ones (analogous to the difference in general programming between hardcoding a constant into an equation versus declaring it a variable and assigning new values to it at will). Macro (parametric) CNC programming uses human-friendly variable names, relational operators, and loop structures much as general programming does, to capture information and logic with machine-readable semantics. Whereas older manual CNC programming could only describe particular instances of parts in numeric form, parametric CAM programming describes abstractions which can be flowed with ease into a wide variety of instances. The difference is analogous to creating text as bitmaps versus using character encoding and glyphs, or to the way that HTML passed through a phase of using content markup for presentation purposes, then matured toward the CSS model. In all of these cases, a higher layer of abstraction was introduced in order to pursue what was missing semantically.

STEP-NC reflects the same theme, which can be viewed as yet another step along a path that started with the development of machine tools, jigs and fixtures, and numerical control, which all sought to "build the skill into the tool". Recent developments of G-code and STEP-NC aim to build the information and semantics into the tool. The idea itself is not new; from the beginning of numerical control, the concept of an end-to-end CAD/CAM environment was the goal of such early technologies as DAC-1 and APT. Those efforts were fine for huge corporations like GM and Boeing. However, for 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.

MTConnect aims to connect machine tools to each other and to other systems in the factory with a much higher level of interaction and capability than has previously existed. Although direct numerical control (DNC) has been networking CNC machine tools to the rest of the enterprise for years, the ability of the various kinds of machines "to talk to each other" has been rather limited in practice (more often than not), compared to the theoretical possibilities. DNC has a lot more potential than just "sending a program to a machine tool over a wire instead of on a tape or disk." But unlocking that potential has been a slow process so far. By creating open-source industry standards (e.g., APIs, XML schemas), MTConnect hopes to spur greater interaction between proprietary systems and a wider developer community. MT Connect might be for manufacturing-segment IT what the Web and app stores have been for other IT domains (commerce, personal, telecoms): a way to bridge the gap between the traditional corporate development environment and the hacker universe. Just as hackers may bring novel uses to smartphones, tablet computers, or Kinects, perhaps they will soon be able to innovate similarly in manufacturing. The enthusiasm of the additive manufacturing community shows how much interest hackers and inventors have in such endeavors.

See also

• STEP-NC
• Cutter location
• Direct Numerical Control
• Gerber file

References

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Bibliography

• Oberg, Erik; Jones, Franklin D.; Horton, Holbrook L.; Ryffel, Henry H. (1996), Green, Robert E.; McCauley, Christopher J. (eds.), Machinery's Handbook (25th ed.), New York: Industrial Press, ISBN 978-0-8311-2575-2, OCLC 473691581.
• Smid, Peter (2008), CNC Programming Handbook (3rd ed.), New York: Industrial Press, ISBN 9780831133474, LCCN 2007045901.
• GE Famuc Automation, Computer Numerical Control Products: "Operator's Manual", February 2000, publication number B-63004EN/02. (NO Copyright data or notice found.)
• Haas Automation, "Mill Operator's Manual, 96-8000 Rev AH March 2011", pages 151, 152, and 223+. File name: "98-8000-2.pdf", retrieved from: THIS WEB PAGE, Feb. 26th 2011.
• Haas Automation, "Lathe Operator's Manual, 96-8700 Rev AH March 2011", pages 183, 184, and 249. File name: "98-8700.pdf", retrieved from: THIS WEB PAGE, Feb. 26th 2011.

External links

• Code descriptions with graphics and example code files (examples can be downloaded).
• CNC G-Code and M-Code Programming
• Tutorial for G-code
• The NIST RS274NGC Standard - Version 3 Aug 2000 also available as a PDF

• Computer-aided engineering
• Domain-specific programming languages
• Encodings