What Is G-Code? Introduction To CNC Programming

G-Code is an integral part of computer numerical control (CNC) machining. This programming language is used to direct the movements and actions of CNC machines, including mills, lathes, and routers. Essentially, G-Code comprises a sequence of commands that instruct the machine on how to move, where to cut, and at what speed. Mastering G-Code is crucial for everyone involved in CNC machining, from operators to programmers.

What is CNC Machining?

Before exploring G-code, we need to understand the core concept of CNC machining itself. CNC (Computer Numerical Control) machining is the process of using computer-controlled machines to shape, cut, and create precision parts out of metal, plastic, wood, or other materials. It's the backbone of industries like aerospace, automotive, medical devices, and advanced manufacturing, where accuracy and efficiency are critical.

The CNC machining process begins with creating a 2D vector or 3D solid part computer-aided design (CAD) either in-house or by a CAD/CAM design service company. CAD software allows designers and manufacturers to produce a model or rendering of their parts and products and the necessary technical specifications, such as dimensions and geometries, for producing the part or product¹.

The Language of CNC Machines

CNC machines operate using two main types of code: G-code and M-code. Once more, before moving on to G-code, we need to examine its companion, M-code.

Understanding M-Code

M-code or M code, short for 'Miscellaneous code', is often referred to as machine code. This code is used to operate the machine's auxiliary or non-motion functions, such as spindle control, coolant, and tool changes.

Here are some common examples of M-codes that you'll see often (bearing in mind variation across machines):

M-Code / Function

• M00 - Program stop
• M01 - Optional program stop
• M02 - End of program
• M03 - Spindle on, clockwise rotation
• M04 - Spindle on, counterclockwise rotation
• M05 - Spindle off / stop
• M06 - Tool change
• M08 - Coolant on
• M09 - Coolant off
• M30 - End of program + reset / rewind
• M98/M99 - Call subprogram / return from subprogram

Understanding the Basics of G-Code

G-Code or G code, also known as geometric code, is a programming language employed to control machine tools, frequently used in computer-aided manufacturing (CAM). It consists of commands that guide the machine tool in performing specific tasks.

At its core, G-Code is a set of instructions that dictate a machine tool's movements, speed, and actions. These instructions, usually written in a text format, are interpreted by the machine's controller.

The significance of G-Code lies in its capability to convert design files into tangible objects. Through G-Code, operators can automate the manufacturing process, ensuring precise and accurate outcomes.

G-Code finds applications across various industries, including automotive, aerospace, and robotics. It is used to control machine tools such as lathes, mills, routers, and 3D printers. Whether the task involves cutting, drilling, milling, or engraving, G-Code provides the necessary instructions to perform these operations with precision.

Machine tools are controlled by G-Code through numerical values and commands. These values specify the coordinates, speeds, and tool movements needed to accomplish a task. The machine's controller interprets these instructions and converts them into mechanical actions.

Overall, G-Code plays a pivotal role in computer-controlled manufacturing, enabling operators to program complex tasks, automate processes, and achieve consistent production quality.

Different Types of G-Code

G-Code is a programming language used in CNC machining to control the machine's movement and operation. Various types of G-Code serve different purposes in CNC machining:

Overview of various G-Code types

• G0: Rapid positioning command to move the machine quickly to a specific location.
• G1: Linear interpolation command to move the machine along a straight line at a specified feed rate.
• G2 and G3: Circular interpolation commands to move the machine along an arc or circle, creating a curve at a selected degree.
• G4: Dwell command to pause the machine for a specified time.
• G17, G18, and G19: Plane selection commands to specify the operating plane (XY, XZ, or YZ).

Examples of G-Code usage in CNC machining

• G1 X10 Y20 Z30: Moves the machine to coordinates X=10, Y=20, and Z=30.
• G2 X50 Y30 I20 J10: Moves the machine along an arc from the current position to X=50, Y=30, with a center at X=20, Y=10.
• G4 P500: Pauses the machine for 500 milliseconds.

By understanding these different types of G-Code and their applications, CNC operators can effectively program and control machines to perform precise machining operations.

Reading and Interpreting G-Code Commands

G-Code commands are essential for programming CNC machines. To read G-Code commands effectively, it is crucial to understand the syntax and structure of the code and interpret it for machine operations.

Understanding G-Code syntax and structure is vital for accurately deciphering the commands. G-Codes typically consist of letters and numbers representing specific actions or functions. Each command comprises a letter followed by a number, specifying the desired action or movement. For instance, the G01 command indicates linear interpolation or a straight-line move².

Interpreting G-Code for machine operations requires knowledge of the specific CNC machine in use. Different machines may interpret certain commands differently or support additional functionalities. Therefore, it is essential to consult the machine's documentation or the manufacturer for guidance on interpreting G-Code commands specific to your machine.

Background of G-Code

The first numerical control (NC) machines were introduced in the late 1940s, by inventor John T. Parsons. The origins of G-Code trace back to the late 1950s. These early machines used punched tape to provide machining instructions. G-Code emerged as a standardized language to communicate these instructions, enabling greater automation and precision.

Over the years, G-Code has evolved significantly. A key milestone in G-Code history was the introduction of the APT (Automatically Programmed Tools) language in the late 1950s. APT was a high-level language that could generate G-Code instructions automatically, simplifying the programming process.

Another important advancement came with the creation of the RS-274 standard (opens a PDF document) in the 1980s by The EIA (Electronic Industries Alliance). This standard defined G-Code's syntax and structure, making it more consistent and compatible across different CNC machines.

Advantages and Limitations of G-Code

Advantages of G-Code

• Standardization Through RS-274
• Explicitly stated code - a list of commands in their logical order makes it easy to follow what the machine's tool path and actions.
• Efficiency - Simple Parts with basic geometries can be created through manual coding and lead to a faster and more efficient process than proceeding through a full CAD/CAM workflow.
• Longevity - G-code has been in use since the late 1950s and is still the backbone of modern CNC industry, which means plenty of training resources and documentation is available.

Disadvantages of G-Code

• Complexity - Advanced Parts become a limitation for manual coding, especially for 3-D surfaces and complex geometry, so reliance on CAD/CAM may be required.
• Manufacturer-Specific Syntax - While RS-274 defines a standard, different controllers (FANUC, Haas, Siemens) have vendor-specific codes and syntax differences.
• Manual coding can be error-prone.
• Steep Learning Curve - Beginners must memorize a large set of G and M codes, understand coordinate systems, how feeds and speed control works.
• Limited Abstraction - G-code is procedural and low-level: it tells the machine how to move, not what the part looks like. High-level programming languages (like APT or modern CAM) are better for defining complex shapes and letting software handle the path planning.

G-Code is a powerful programming language for CNC systems, offering universality and flexibility. It is essential to weigh its advantages and limitations when deciding whether to use G-Code or explore alternative programming languages. Overcoming challenges with G-Code implementation can be achieved through training, practice, and utilizing helpful software tools.

G-Code & M-Code in Lincoln Tech Training Programs

Lincoln Tech has integrated G-Code programming into our CNC Machining and Manufacturing program curriculum, so students can prepare for employment in the manufacturing industry.

Our programs provide hands-on training in G-Code programming, enabling students to develop the skills necessary to operate and control CNC machines. G-Code, the programming language of CNC machines, controls these machines' movement and functions. By learning G-Code, students gain a solid foundation in CNC machining, equipping them with the knowledge to bring designs to life.

Through our comprehensive training, students become proficient in G-Code programming and gain valuable experience in operating CNC machines. They learn to read and interpret G-Code instructions, create and modify programs, and troubleshoot any issues that may arise. This hands-on approach allows students to develop practical skills necessary for success in the manufacturing industry.

Upon completing our programs, graduates are well-prepared for a wide range of career opportunities. With proficiency in G-Code programming, they can pursue roles as CNC operators, CNC programmers, manufacturing technicians, or even start their own businesses. The demand for professionals with G-Code skills is high, as CNC machining is crucial in various industries, including aerospace, automotive, and medical device manufacturing.

At Lincoln Tech, we are committed to providing our students with the knowledge and skills needed for success in their chosen careers. By integrating G-Code into our curriculum, we ensure that our graduates are prepared to meet the demands of the modern manufacturing industry.

¹ Edwards, Ed. "What Is CNC Machining? | Definition, Processes, Components & More." Thomasnet, Thomas Publishing Company, 2 Dec. 2023, https://www.thomasnet.com/articles/custom-manufacturing-fabricating/understanding-cnc-machining/. Accessed 7 Oct. 2025.

² "G01 Linear Interpolation Motion (Group 01)." Haas Automation, Haas Automation, Inc., https://www.haascnc.com/service/codes-settings.type=gcode.machine=mill.value=G01.html. Accessed 7 Oct. 2025.