G-code is the language of CNC machines. Key commands and practices include:

• Basic movement codes:

  • G00: Rapid move (non-cutting move to a position at maximum speed). Use for quick positioning above the part.

  • G01: Linear interpolation (straight-line cutting move) at a specified feed rate. This is the workhorse for controlled cuts.

  • G02/G03: Circular interpolation (arc moves) – G02 for clockwise arcs, G03 for counterclockwise. Specify center or radius to cut circles or curves.

• Units and distance mode:

  • G20/G21: Sets units (G20 = inches, G21 = millimeters). Always include one of these at the start to avoid unit confusion.

  • G90/G91: Absolute vs. incremental positioning. G90 means coordinates are absolute from the program’s origin; G91 means relative (each move adds to the previous position). Know which mode you’re in when coding arcs or repetitive moves.

• Miscellaneous commands (M-codes):

  • M03/M04/M05: Control the spindle (M03 = spindle on clockwise, M04 = counterclockwise, M05 = stop). Pair them with S commands (S1000, for example) to set spindle speed.

  • M06: Tool change. Use this to call a specific tool (e.g. T1) and then manually or automatically change to that cutter.

  • M08/M09: Coolant control (M08 on, M09 off) if your machine uses flood or mist coolant. M00/M01 for program pause or optional pause. M30 ends the program and rewinds.

• Best practices:

  • Comment your code: Use parentheses (or semicolons, depending on the controller) to annotate sections of G-code. For example, (Starting pocket cut) or (Material is aluminum). Comments don’t affect machining but help you or others understand the program.

  • Line numbers: Though not always required, adding N numbers (N10, N20, etc.) at the start of lines can make it easier to restart or edit.

  • Check sequence: Always begin with a setup block (setting units, offsets, and initial spindle speed). For example: G21 G17 G90 G40 (mm, XY plane, absolute, cancel cutter comp). End with a safe retraction and M30.

  • Simulate or dry-run: Before cutting, run the program in a simulator or with the spindle off to catch any major errors. Verify tool changes and ensure the right tool is loaded.

  • Use incremental moves for loops: If machining repetitive patterns or cycles, G91 (incremental mode) can simplify code. For instance, a drilling cycle can repeat G81 commands with incremental Y-moves.

  • Safety moves: Raise the Z-axis high (clearance plane) before rapid XY moves. For example, before G00 to a new cut location, lift up to a safe height. Many programs insert a G00 Z10.0 (go to 10 mm above part) before XY moves.

  • Consistent formatting: Keep one command per line (e.g. “G01 X10 Y10 F150”). This clarity prevents mistakes. Lowercase or uppercase (G00 vs g00) is tolerated by most controllers, but be consistent.

By mastering these commands and following good programming habits, you ensure reliable CNC operation. Well-written G-code is self-explanatory, reduces downtime, and makes debugging much easier. Whether hand-coding small jobs or reviewing CAM outputs, these practices lead to smoother runs and better finished parts.