What Is G-Code?
A Complete Guide
to CNC Programming,
Commands, and Applications

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Introduction

G-code is the fundamental programming language used in CNC machining, including CNC turning and milling processes.

It tells a CNC machine how to move, where to cut, and what operations to perform. Without G-code, automated machining would not be possible.

Understanding G-code is essential for:

• CNC programming
• Machining automation
• Process optimization

It forms the foundation of modern manufacturing systems.

G-code (Geometric Code) is a programming language used to control CNC machines.

It defines machine movements and operations such as:

• Tool positioning
• Cutting paths
• Spindle speed
• Feed rate

Each command in G-code represents a specific action that the machine must perform.

　Related ：NC Turning Machine / Turning

A CNC machine reads G-code line by line and executes commands sequentially.

• Coordinates define tool movement
• Commands control machine functions
• The program guides the machining process

G21       ; Set units to millimeters
G90       ; Absolute positioning
G00 X0 Z0 ; Rapid move to starting position
G01 X50 Z-20 F100 ; Linear cutting move
M03 S1500 ; Spindle on (clockwise)

💡 Each line instructs the machine to perform a specific action.

　Related ：Machining

• G00 → Rapid positioning
• G01 → Linear cutting
• G02 / G03 → Circular interpolation

• G90 → Absolute positioning
• G91 → Incremental positioning

• M03 → Spindle on (clockwise)
• M05 → Spindle stop

• Feed rate (F)
• Spindle speed (S)

💡 These commands control the entire machining process.

　Related ：Cutting Speed / Feed Rate in Turning

In CNC turning, G-code controls:

• Diameter and length machining
• Thread cutting
• Grooving and facing

Because turning involves continuous rotating motion, precise control of feed and spindle movement is essential.

　Related ：Turning Center

G-code directly defines machining conditions such as:

• Set through spindle speed (S value)

• Defined using F codes

• Controlled by coordinate commands

💡 Proper programming ensures stable machining and good surface quality.

　Related ：Coolant in Machining

• Accurate control of tool movement

• Same program produces identical parts

• Enables unmanned machining

　Related ：Bar Feeder / Bar Machine

• Easy modification of programs

G-code is used in:

• CNC turning
• CNC milling
• Drilling and tapping
• Multi-axis machining

It is essential for producing precision components across industries such as automotive, aerospace, and medical manufacturing.

• Incorrect coordinates
• Wrong command usage

💡 Can cause machining defects or crashes

• Poor feed and speed settings
• Inefficient tool paths

💡 Leads to reduced productivity

　Related ：Chatter in Machining / Stiffness

• Different machines may use variations of G-code

• Verify programs before machining
• Use simulation when possible
• Optimize feed rate and cutting speed
• Monitor machining conditions

👉 Proper programming minimizes risk and improves efficiency.

G-code is the foundation of CNC machining, providing precise control over machine operations.

By understanding and optimizing G-code programming, manufacturers can achieve:

• High machining accuracy
• Improved productivity
• Consistent part quality

It is an essential skill for modern CNC turning and manufacturing processes.