How to Write Code for CNC Machine

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  Key takeaways
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  What You Actually Write: G-Code and M-Code
• 3
  The CAM Workflow Behind Most Production Code
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  Feeds, Speeds and the Numbers That Break Tools
• 5
  When Hand-Written Code Beats CAM Output
• 6
  7 Steps to Write and Prove a CNC Program
• 7
  Hand-Written Code vs CAM Output

A CNC program is a list of blocks. Each block is one line, and each line holds a letter address and a number: G for preparatory commands, M for miscellaneous machine commands, X Y Z for coordinates, F for feed, S for spindle speed, T for tool number. The control reads the list top to bottom and executes it. Nothing is implied; if you leave out a feed rate, the machine uses whatever modal value was active from the previous block.

G-code covers motion. G00 is rapid positioning at machine maximum, G01 is straight feed at the programmed F value, G02 and G03 are clockwise and counterclockwise arcs, G81 to G89 are canned drilling and tapping cycles. G17, G18 and G19 select the working plane for arcs and cutter compensation. G20 and G21 set inch or metric mode, and that single line changes how every coordinate after it is read.

M-code covers everything that is not a path. M03 starts the spindle clockwise, M05 stops it, M08 and M09 control coolant, M06 calls a tool change, M30 ends the program and rewinds. On a mill, M03 S8000 means 8,000 rpm clockwise. On a lathe with live tooling, the same M03 may need a station address first. The meaning is machine-specific, so always check the control manual.

One more distinction matters for how you write code for CNC machine work. G90 and G91 choose absolute or incremental positioning. In absolute mode every move goes to a fixed point from the part zero. In incremental mode each move is measured from the current position. Mixing the two by accident is one of the most common crash causes on a proven program.

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  Block structureN-number is optional on modern controls; most CAM posts skip it.
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  Modal stateG94 feed per minute, G95 feed per revolution. Pick one and stay in it.
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  Cutter compensationG41 left, G42 right, G40 cancel. Apply on a straight lead-in move only.

Hand-written code still has a place, but it is a small place. Simple 2.5D plates, a drilling pattern, a repair on an existing program. Everything with curved surfaces, multiple setups or more than about twenty tools goes through CAM. The CAM system holds the geometry model, the stock model, the tool library and the post-processor, and it writes the coordinate lines you would otherwise type.

The chain starts with CAD. The model must be the finished part at nominal size. Tolerance and finish callouts live in the drawing or the 3D annotation, not in the model, so the programmer has to read them and pick cutting strategies that can hold ±0.005 mm where the drawing demands it. A 0.5 mm finishing pass cannot correct for a roughing strategy that left 0.05 mm of uneven stock.

Then the programmer sets the setup. That means choosing the workholding, defining the WCS origin, and deciding how many operations are needed. For a part that fits a 5-axis machine in one setup, the same geometry may need three setups on a 3-axis mill. Each extra setup adds an origin, an operator step and a chance for position error.

Finally the post-processor converts the internal toolpath into control-specific code. A Fanuc post uses G43 H for tool length compensation and M08 for flood coolant. A Heidenhain control accepts conversational cycles instead of G81 blocks. Same part, same tool, different text. The post is where a good CAM setup becomes a program the operator can actually run.

Before release, the programmer runs a stock simulation and a machine simulation. The first catches gouges and leftover material. The second catches holder collisions, table clearance and rotary axis limits. Both are faster than a crash.

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  CAD modelNominal geometry, correct units, no scaling. Check the model against the drawing revision.
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  Tool libraryReal holder and stick-out lengths. Gauge length errors cause holder crashes.
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  Post-processorTied to the control family, not the machine brand. Verify with a known-good test part.
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  SimulationStock removal plus machine kinematics. Run both before the program reaches the floor.

Most beginner programs fail on cutting data, not on coordinates. The two variables that drive everything are surface speed and chip load. Surface speed is the speed at which the cutting edge travels through the material, expressed in m/min or surface feet per minute. Chip load is the thickness of material each tooth removes per revolution, in mm per tooth. Spindle speed and feed rate are derived from those two.

For aluminum, a coated carbide end mill runs around 300 to 500 m/min surface speed. For 304 stainless, drop to 120 to 180 m/min. Titanium Ti-6Al-4V sits near 40 to 60 m/min. Inconel is lower still. These are starting points, not limits; tool coating, coolant delivery and rigidity move them. The formula for spindle speed is rpm = (surface speed × 1000) ÷ (π × tool diameter).

Feed rate follows from chip load: feed = rpm × number of teeth × chip load. A 10 mm three-flute carbide end mill in 6061 aluminium at 0.05 mm per tooth and 8,000 rpm gives 1,200 mm/min. The same tool in 304 stainless at 0.03 mm per tooth and 3,000 rpm gives 270 mm/min. Cut the feed in half and the tool rubs instead of cutting, which dulls it faster than a heavy chip load.

Radial and axial depth of cut set how much of the tool is engaged. Full-width slotting in aluminium is fine at 0.5 × diameter axial depth with good coolant. In stainless, keep radial engagement below 30 percent of the tool diameter and use trochoidal paths. Adaptive clearing strategies trade a wider path for a lighter load per pass, which keeps tool temperature down.

Plunge moves deserve their own feed. A tool entering vertically at cutting feed in steel will chatter or snap. Use a ramp or helix entry at 2 to 5 degrees, and set a separate plunge feed at roughly one third of the cutting feed when a vertical entry is unavoidable.

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  Chip thinningWhen radial engagement drops below 50 percent, raise the feed per tooth to keep chip thickness.
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  CoolantThrough-spindle coolant helps past 5 × diameter depth. Air blast suits aluminium and plastics.
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  Tool runout0.01 mm of runout cuts effective tool life roughly in half. Check with a dial indicator.

Hand-written code wins on small, repeatable jobs. A drilling pattern on a flat plate, a facing pass, a chamfer run around a rectangular pocket. The program is 30 lines, the geometry is trivial, and setting up a CAM file takes longer than typing the code. Programmers keep a library of these snippets and edit the coordinates.

It also wins on the floor. When a tool breaks mid-run and the replacement is 0.2 mm shorter, the operator edits the tool length offset, not the program. When a finishing pass leaves 0.03 mm of stock, the programmer adds a spring pass in G01 at the same coordinates with a smaller radial step. That is a two-line edit, and it is faster than re-posting the whole job.

The trade-off is risk. Hand-typed coordinates have no simulation behind them, so a single wrong digit sends the tool into the vise. Keep manual edits inside a clearly marked section, back up the original program, and dry-run any edit on a part that matters. For anything with 3D surfaces, undercuts or more than two setups, CAM output is the safer path.

There is a middle route that many shops use: write the code in a text editor with a parametric macro. Fanuc custom macros with variables and IF statements let one program handle a family of parts by changing a few values at the top. A bolt-circle drilling program with a radius variable and a count variable covers every flange size without a new CAM file.

For prototypes at GreatLight, programmers often combine both. CAM handles the contoured surfaces; a short hand-written section handles the fixture bolt pattern and the part marking. The two are merged before the first run.

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  Good candidatesDrilling grids, facing, chamfers, repair passes, fixture plates.
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  Bad candidatesSculpted surfaces, 5-axis swarf cuts, anything with a collision risk near the holder.