G Code vs M Code: CNC Programming Basics for Engineers and Buyers

Every CNC machined part starts life as a CAD model, gets turned into toolpaths in CAM software, and arrives at the machine as a text file of G code and M code commands. The control reads that file one line (called a block) at a time and executes exactly what it says — no more, no less. Understanding how CNC machining works starts with understanding this language, because it is the literal bridge between the drawing and the cut.

G codes are the geometry and motion commands. The G stands for preparatory — they prepare the control for a type of operation. G00 says move fast and do not cut, G01 says cut in a straight line at feed rate F, G02 and G03 say cut arcs. Other G codes do not move anything at all: they set the context in which motion happens — inch or metric units (G20/G21), the plane arcs live in (G17–G19), where part zero is (G54–G59), and whether coordinates are absolute or incremental (G90/G91).

M codes are the machine-function commands. The M stands for miscellaneous. They switch the machine’s auxiliary hardware: spin the spindle up clockwise (M03) or counterclockwise (M04) at the speed given by the S word, stop it (M05), swap tools (M06), flood the cut with coolant (M08), shut coolant off (M09), and control program flow with stops and program-end commands. In short: G codes cut the part, M codes run the machine. A typical block mixes both — a line might rapid to position while the next line starts the spindle and coolant together.

The core vocabulary is standardized, so the commands below behave the same on the Fanuc-style controls that run most of the world’s mills and lathes. Machine builders do add proprietary codes for probes, pallet changers, and special cycles, so a program written for one control still needs a post-processor check before it runs on another.

M codes are fewer and simpler — most programs use fewer than a dozen — but forgetting one has immediate consequences: a tool change without M05 spins a live tool into the changer, and a long program without M30 leaves the control sitting at the end instead of rewound for the next part. Here is the working set.

Note the M02/M30 distinction, because it confuses people: both end the program, but M30 also rewinds the control back to the first line and resets it for the next cycle, which is what production running needs. M02 is the older convention and still appears in legacy programs. Beyond this list, machine builders define their own M codes — M60-series pallet changes, probe enable codes, chip-conveyor commands — so the edges of the M-code map are always control-specific.

Most G codes and many M codes are modal: once issued, they stay in force until another code from the same group replaces them. G01 is modal — after one G01 line, every subsequent motion line keeps feeding in a straight line until a G00, G02, or G03 appears, so programmers write only the coordinates on following lines. The modal groups are organized so that codes within a group are mutually exclusive: G00/G01/G02/G03 form the motion group, G90/G91 the positioning group, G20/G21 the units group, G54–G59 the offset group. One code per group is always active, whether the program mentions it or not.

Non-modal codes act only on the line where they appear. G04 dwell — pause for a programmed time — is the textbook example: it delays that one block and is then gone. Some machine-specific commands behave this way too.

The practical consequence is that a CNC control has a hidden state, and that state survives between programs. If a previous job left the machine in incremental mode (G91) or with an unusual work offset active, and the next program assumes absolute mode, the first motion line drives the tool somewhere dangerous. This is why every well-written program opens with a safety line — the G21 G17 G90 G94 block in the example above — that forces all critical modal groups to known values before anything moves. When you see a programmer insist on this line, they are not being fussy; they have paid for the lesson already.