Top 20 G-Codes Every CNC Programmer Should Know

Mastering G-code is like learning the grammar of manufacturing. While there are hundreds of codes, a small core group does 90% of the heavy lifting in CNC programming.

Here is a breakdown of the top 20 G-codes every programmer should have memorized, categorized by their function.

1. Motion Codes (The “Big Three”)

These codes tell the machine how to move from Point A to Point B.

• G00: Rapid Positioning – Moves the tool at the maximum speed of the machine. Used for non-cutting moves.
• G01: Linear Interpolation – The most used code. It moves the tool in a straight line at a specific feed rate (F).
• G02: Circular Interpolation (Clockwise) – Moves the tool in a clockwise arc.
• G03: Circular Interpolation (Counter-Clockwise) – Moves the tool in a counter-clockwise arc.

2. Compensation & Offsets

These codes ensure the machine knows exactly where the tool tip is relative to the part.

• G40: Tool Radius Compensation Cancel – Turns off cutter compensation.
• G41: Cutter Compensation Left – Offsets the tool to the left of the programmed path (used for climb milling).
• G42: Cutter Compensation Right – Offsets the tool to the right of the programmed path (used for conventional milling).
• G43: Tool Length Compensation – Tells the machine to account for the specific length of the tool currently in the spindle (usually paired with an H code).
• G54 to G59: Work Coordinate Systems – These define the “Zero” point of your part. G54 is the standard primary offset.

3. Plane & Unit Selection

Before the machine starts moving, it needs to know the “rules” of the environment.

• G17: XY Plane Selection – Sets the workspace to the top view (standard for most milling).
• G18: XZ Plane Selection – Used primarily in turning or specific 3D milling.
• G19: YZ Plane Selection – Used for side-profile machining.
• G20: Inches – Sets units to inches.
• G21: Millimeters – Sets units to metric.
• G90: Absolute Programming – All coordinates are relative to the part zero.
• G91: Incremental Programming – Coordinates are relative to the current position of the tool.

4. Canned Cycles

These codes simplify repetitive tasks like drilling into a single line of code.

• G80: Canned Cycle Cancel – Stops any active drilling or tapping cycle.
• G81: Simple Drilling – A basic plunge-and-retract cycle.
• G83: Peck Drilling – Drills in steps, retracting to clear chips. Vital for deep holes.
• G98: Retract to Initial Plane – Tells the tool to pull back to its starting height after a cycle.
• G99: Retract to R-Plane – Tells the tool to pull back to a “clearance” height (usually closer to the part) to save time.

Summary Table

Category	Codes	Purpose
Motion	G00, G01, G02, G03	Moving the tool
Offsets	G41, G42, G43, G54	Accuracy and positioning
Setup	G17, G20, G21, G90	Defining the environment
Holemaking	G81, G83, G80	Automating drilling

Pro Tip: The “Safety Block”

Most professional programmers start their code with a “Safety Block” that resets the machine to a known state. It often looks like this: G00 G17 G20 G40 G80 G90 This ensures no leftover offsets or cycles from the previous job ruin your new part.