Code  
Description
Milling
( M )
Turning
( T )
Corollary info
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 (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 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.
GO2 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 G02 or 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, counter clockwise M T Same corollary info as for G02.
G04 Dwell M T Takes an address for dwell period (may be XU, or P). The dwell period is specified in the controllers parameter, typically milliseconds.
G05P 10000 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, counter clockwise 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 G17 to G19 codes are used. This is now changing as the era begins in which live tooling, multitask/multifunction, and mill-turn/turn-mill gradually become the “new normal”. But the simpler, traditional form factor will probably not disappear—just move over to make room for the newer configurations. See also 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 G01 linear interpolation, except with automatic spindle synchronization for single-point threading.
G33 Constant-pitch threading M
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.
G34 Variable-pitch threading M
G40 Tool radius compensation off M T Cancels G41 or G42.
G41 Tool radius compensation left M T 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.)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 G43 or G44.
G50 Define the maximum spindle speed T 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.
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 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.
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 (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.
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 G73G83G88, 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.
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 Tapping cycle, righthand thread, 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, U and Ware the incremental addresses and X and 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, U and Ware the incremental addresses and X and 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 speeds and feeds. Takes an S address integer, which is interpreted as sfm in G20 mode or as m/min in 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.

From: Wikipedia | Sources: Smid; Green et al.