G-code for CNC Machines: Guide to Commands and Uses

Whatever CNC machine you are using, they need G-code. Stereotypically, many people think that G-code for CNC machine is complicated. Learn the G-code basics, how it works, and how to make a G-code for your next project is very simple. Read on to learn all there is to know about G-code in CNC machining!

G-code means Geometric Code, that is the standard programming language for all CNC (Computer Numerical Control) machines. As a bridge between design and manufacturing, it converts CAD (Computer-Aided Design) models into machine-understandable instructions that facilitate precise cutting, drilling, milling, and shaping of materials. G-code has become foundational to modern manufacturing driving automation, consistency, and efficiency. G-code consists of individual lines, called blocks, for each of which the machine’s microcontroller interprets the instructions and sends commands to control elements within the machine. This is an easy-to-use temperature-independent language that requires no prior training in advanced mathematics or complex logic, while still supporting deeply sophisticated calculations.

G-code was initially developed to carry out basic machine tasks with manually written commands, making it suitable for simpler operations. As time went on, improvements in both functionality and sophistication of CAM (Computer-Aided Manufacturing) software changed the nature of programming, automatically producing more than just G-code from CAD models alone. This breakthrough facilitated the transfer of digital designs to CNC machines, enabling rapid and accurate production. In most modern systems, G-code is actually used in conjunction with another type of code – M-code (Machine Code) which relates to non-movement functions such as loading the program, changing tools, pausing and controlling coolant. G-code and M-code work in tandem with one another to simplify complex manufacturing processes allowing for broad capability and efficiency within sectors from aerospace to consumer electronics.

One of the keys to CNC programming that can help you optimize machine movement is properly using modal and non-modal G-codes. Since a modal G-code is active and thus executed for the respective blocks until being canceled by some other G-code, which automatically takes precedence over the prior one as soon as it appears. For instance, a modal command such as G01 (linear interpolation) will remain in effect until the next motion command — for example, G00. Conversely, non-modal G-codes only take effect in the block where they are specified, allowing for clearer and individual control without changing the following commands. Knowing where to deploy and employ modal versus non-modal G-codes can really improve machining accuracy and efficiency—particularly with complex operations like drilling or milling.

• Modal G-codesstay active until canceled or replaced by another command (e.g., G01 for linear motion).
• Non-modal G-codesexecute their function once without altering subsequent operations (e.g., G04 for dwell time).

Understanding this distinction ensures efficient programming and prevents errors.

• Motion codes(G00, G01, etc.) dictate how the machine moves.
• Plane selection codes(G17, G18, G19) specify the working plane.
• Canned cycles(G81, G83) simplify repetitive tasks like drilling.

What is G-code: Yes, G-code is a universal programming language for CNC but powers every CNC machining operation. Microcontrollers of each CNC machine are shipped with the firmware that takes care of interpreting G-code commands between software and hardware. G-code formatting varies between manufacturers, though. However, some only employ single zero (G0) for fast placement while others need double zero (G00).

These differences are just implementations of the same principles behind G-code. Its ability to be used for a large variety of CNC systems, from simple 3-axis mills up through complex 5-axis machining centers, makes it a very flexible language. Manufacturers may add proprietary G-codes/extensions for some custom features in machines. These commands are normally listed in a user manual that assists an operator can design their programs to certain machine capabilities.

G-code commands are the foundation of CNC programming, allowing for automation and precision machining. These standardized commands are used by CNC machines to cut, drill, mill, and turn. In contrast to the languages used in a conversation, which is not restricted by any settled principles, there are just specific syntax controlled by G-code that CNC systems can comprehend and because of this reason G-code transforms into a basic instrument for correspondence between programmer and machine.

M-codes and G-code complement each other to provide a complete package for machining. G-codes commands the movement of tools (i.e., linear motion, Circular Path) whereas M Codes drive machine functions such as tool changes, coolant on or off and Spindle controls etc. Combined, they make up the entire CNC programming file that instructs the machine on how to run during production.

More than 100 standardized G-code commands exist, dedicated to a wide variety of machining tasks. There are generic commands like G00 for rapid movement and G01 for linear interpolation, as well as application codes that control threading or canned drilling cycles.

Keep in mind, though, that availability of G-codes can depend on the machine and its manufacturer. Note: Some high-end CNC machines, particularly the multi-axis variety, include special or proprietary G-codes specific to their functions. To see if a specific machine is G-code compatible, operators should refer to the reference manual for a complete list of supported G-codes. With a firm command over these commands, programmers will be able to unlock the full potential of their CNC systems, resulting in enhanced precision and efficiency.

G00: Rapid Positioning for Fast Travel

This command moves the machine tool to a specified position at maximum speed. It’s ideal for non-cutting moves.

G01: Precise Linear Interpolation for Smooth Cuts

G01 provides controlled, straight-line movements, ensuring high-precision cuts during machining.

G02: Clockwise Circular Interpolation for Arcs

Use G02 for clockwise circular movements, crucial for creating rounded shapes or holes.

G03: Counterclockwise Circular Interpolation for Complex Geometry

Similar to G02 but for counterclockwise motion, enabling intricate curves and details.

G17, G18, G19: XY, ZX, YZ Plane Selection

These codes specify the primary working plane, ensuring accurate toolpath programming.

G20: Set Dimensions to Inches

This command changes the measurement units to inches for projects following imperial standards.

G21: Set Dimensions to Millimeters

Conversely, G21 sets the units to millimeters, adhering to metric standards.

G40, G41, G42: Tool Radius Compensation

These codes adjust for the cutting tool’s radius, ensuring accurate machining dimensions.

G54–G59: Work Offset Commands

Work offsets allow multiple setups on a single machine, increasing flexibility.

G81: Drilling Cycle

This code automates simple drilling operations, saving time on repetitive tasks.

G83: Peck Drilling Cycle

G83 is used for deep hole drilling, minimizing tool breakage by retracting after each peck.

G50: Turn Off Scaling or Set Speed Limits

This command resets scaling or limits spindle speeds, improving safety.

G80: Cancel Canned Cycles

G80 halts active canned cycles, preparing the machine for the next operation.

G90 positions based on absolute coordinates, while G91 uses relative positions from the current point.

G-codes dynamically adjust feed rates (F) and spindle speeds (S), balancing precision and productivity.

These codes dictate whether the tool returns to the starting plane or retracts to a clearance height after completing a cycle.

Explore codes like G93 for inverse time feed rates or G92 for setting program origins, unlocking advanced capabilities.

Some CNC machines use proprietary languages, offering specialized features but reduced compatibility.

While most CNC machines use G-code, differences in dialects (e.g., FANUC, Haas) require tailored programming.

Commands like G28 ensure the tool retracts to a safe height, reducing the risk of collisions.

Always simulate G-code before execution to verify toolpaths and prevent costly errors.

M-codes control auxiliary functions like coolant, spindle direction, and tool changes.

T-codes specify the active tool, ensuring compatibility with the program.

These codes set the tool’s feed rate, critical for material-specific machining.

G-code is the backbone of CNC machining, ensuring precision, efficiency, and consistency in manufacturing. Want to master CNC programming? Dive deeper into advanced G-code techniques to elevate your machining skills.

Start with motion codes (G00, G01), plane selection (G17), and unit settings (G20/G21).

Use simulation software to identify and correct issues before running the program.

M-codes (Machine Functions – e.g. M03 = Spindle On)

Secondary functions such as coolant, spindle direction and tool changes are controlled using M-codes.

T-codes for Tool Selection

T-codes to indicate the tool in use and program compatibility

F-codes for Feed Rates

These codes establish the feed rate of the tool, which is a very important parameter depending on the material to be cut.