Basic Knowledge of CNC Milling Code

A milling program is written in blocks. One block is one line of code, and each block is made of words. A word is a letter address plus a number: G01, X50.0, F800, S6000, T05, M08. The letter says what kind of information follows; the number gives the value. Word order inside a block is flexible on most controls, but the conventional order (N, G, X/Y/Z, F, S, T, M) makes programs easier to read and easier to debug.

Letter addresses do not overlap. G is a preparatory function that sets a mode, such as linear interpolation or a drilling cycle. M is a miscellaneous function that switches machine behavior on or off, such as spindle rotation or coolant. X, Y, and Z are axis positions. I, J, and K define arc centers or cycle parameters. F is feed rate, S is spindle speed, and T calls a tool.

Block numbers (N) are optional on modern controls but useful. They let an operator jump to a line and they appear in alarm messages, so keeping them in the posted output saves time when something stops mid-cycle.

One more habit worth keeping: group related words so each block does one thing. A block that changes the tool, sets the speed, turns on coolant, and moves the axis is legal, but when it alarms you have four suspects instead of one. Programs from our CAM posts separate these into short blocks for that reason.

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  WordLetter address plus value, e.g. G01 or F800
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  BlockOne line of code, executed as a unit
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  ModalA G-code that stays active until cancelled
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  Non-modalA code that applies to one block only, e.g. G04

Most of a milling program is motion. G00 moves the tool at rapid traverse to a position, usually above the part. G01 feeds in a straight line at the programmed feed rate and is the code that actually cuts. G02 and G03 cut clockwise and counterclockwise arcs; the arc is defined by the end point plus either the center offset (I, J, K) or the radius (R).

G17, G18, and G19 select the plane for arcs and cutter compensation. On a vertical mill, G17 (XY plane) is nearly always active. G20 and G21 set the unit system to inches or millimeters, and getting this wrong is one of the fastest ways to scrap a part or crash a machine. Check the units line before you run anything.

Fixed cycles handle repeating operations. G81 is a simple drill cycle, G83 is peck drilling, G73 is high-speed peck, and G84 is tapping. Each cycle has parameters for retract height, depth, and peck increment, usually set with G98 or G99 for the return plane. Once a cycle is active it repeats at each new position until G80 cancels it.

Cutter compensation is where a lot of confusion starts. G41 offsets the tool to the left of the path, G42 to the right, and G40 cancels it. On a finish pass the operator may dial a wear offset of a few thousandths instead of editing the program. That is normal practice, and it is why the coordinate values in the program are not always the values the tool actually cuts to.

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  G00 / G01Rapid move and linear feed cut
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  G02 / G03Clockwise and counterclockwise arc
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  G81 / G83Drill cycle and peck drill cycle
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  G41 / G42Cutter compensation left and right

M-codes control the machine rather than the path. M03 starts the spindle clockwise, M04 counterclockwise, and M05 stops it. M08 turns coolant on and M09 turns it off. M06 changes the tool, usually together with a T word that selects the pocket. M30 ends the program and rewinds it; M00 is a planned stop for the operator.

Tool length and diameter live in the offset table, not in the program. The program calls T05, then H05 applies the length and D05 applies the diameter when compensation is active. If a tool is replaced, the operator measures it and updates H05. The program itself does not change. This separation is what lets the same file run after a tool swap.

Work offsets define where the part sits in the machine. G54 through G59 are the standard set, and G54 is the default on most jobs. The operator touches off a corner or a datum and stores the position in the work offset page. A program that runs on one vise with G54 may run on a second vise with G55 if the CAM post has that offset built in.

Subprograms and loops are worth knowing about even if you never write them. M98 calls a subprogram and M99 returns from it. For a part with 40 identical holes, a loop is shorter and easier to verify than 40 repeated blocks. If a CAM post is generating very long output, ask whether a subprogram would be cleaner.

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  M03 / M05Spindle on clockwise and spindle stop
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  M06Tool change
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  M08 / M09Coolant on and off
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  M30Program end and rewind

Nobody hand-writes a five-axis program for a complex part. The CAM system takes the solid model, applies toolpath strategies, and posts the result as G-code for a specific control. The post processor is the piece that decides how the code looks: which G-codes are modal, how arcs are output, whether the program uses subprograms, and how work offsets and tool numbers are numbered.

Two shops can cut the same part with the same model and produce completely different-looking programs. That is normal. What matters is that the posted code matches the machine, the tooling, and the setup the operator will actually use. A program posted for a three-axis mill will not run correctly on a five-axis machine without the right post.

Simulation catches most geometry and collision problems before anything is cut. We verify toolpath and check for gouges, rapid collisions, and holder clearance in software, then make a first-article part and inspect it. For tight work we hold ±0.005 mm and a finish of Ra 0.8–1.6 μm, and the program has to be stable enough to repeat that across a run.

If you receive a program to review, start with three things: the units line, the work offset, and the tool list. Those three account for a large share of setup errors. After that, check the first rapid move and the retract height. A tool that rapids into a fixture at full speed does not give anyone a second chance.

Editing posted code by hand is sometimes necessary and always risky. Change a feed rate or a wear offset value at the control and you can undo it. Change a coordinate in the file and you may not remember why six months later. Keep a record of any manual edit.

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  Post processorTurns CAM toolpath into control-specific G-code
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  SimulationChecks gouges, collisions, and holder clearance
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  First articleVerifies the program before a full run