In CNC milling machining, g code for milling is the core bridge between design drawings and machine tool processing, which directly determines machining accuracy, efficiency and workpiece quality. Whether you’re an experienced CNC operator optimizing your machining process or a beginner learning programming, mastering the core logic and practical instructions for milling G-code is an essential skill. This article will start from basic cognition, systematically disassemble the definition, core instructions, application scenarios, and common problems of milling G-code, integrate real machining cases and practical skills, and share optimization solutions based on industry experience to help readers fully grasp the application essence of G-code for milling and solve programming and operation problems in actual machining.
1. Basic cognition of G code milling
1.1 What is G-code in CNC milling?
G-code is a standardized programming language used to control the motion trajectory and machining parameters of machine tools in CNC (computer numerical control) systems, while G code for milling is a collection of instructions customized for milling processing scenarios (such as plane milling, contour milling, hole machining, etc.). It transmits core instructions such as “where to move”, “how to move” and “how fast to move” to the machine tool through a simple code combination, so as to realize the precise relative movement of the tool and the workpiece, and finally complete the preset machining task.
Core facts: According to the ISO 6983 standard, the numbering rules of G-codes are universal, of which the numbering range of 00-99 corresponds to different functional types, and the commonly used G-codes in milling are concentrated in 00-04, 20-21, 40-42, 54-59, 90-91 and other ranges, and the world’s mainstream CNC systems (such as Fanuc, Siemens, Mazak) are compatible with these core instructions, with a compatibility coverage rate of more than 95%.
1.2 Why do milling operations have to rely on G-code?
The core requirement of milling is “precision forming”, and manual operation cannot meet the machining requirements of complex contours and small dimensions, and the core value of G-code is to solve this pain point:
- Controllable accuracy: The G-code can control the machining error at the level of 0.001mm, far exceeding the 0.1mm error of manual operation, and is suitable for precision parts processing (such as mold cavities, aviation parts);
- Efficiency improvement: Through programming to achieve automatic processing, the processing efficiency of a single process is 3-5 times higher than that of manual labor, and 24-hour uninterrupted operation can be realized.
- Strong consistency: the same set of G code can be reused for mass production, and the qualification rate of the workpiece is stable at more than 99%, avoiding individual differences in manual operation;
- Complex machining adaptation: It can realize complex trajectory processing such as straight lines, arcs, and spiral lines, covering various milling scenarios such as planes, curved surfaces, and hole systems.
Experience case: A precision mold factory once tried to manually mill the mold cavity (complex surface + 0.005mm accuracy requirement), the pass rate was only 30%, and after switching to standardized G Code for Milling programming, the pass rate increased to 99.2%, and the processing time of a single set of molds was shortened from 8 hours to 2.5 hours.
1.3 How difficult is it to learn milling G-code?
Many novices will worry about the difficulty of learning G code, but in fact, there is no need to worry too much: there are only more than 20 core instructions for milling G code, and the logic is highly bound to the processing process, and you can quickly get started by mastering the correspondence of “process logic + instruction function”.
Professional analysis: From the perspective of the learning curve, basic instructions (such as G00, G01) can be mastered in 1-2 days of learning; Complex instructions (such as arc interpolation, tool compensation) can be used proficiently in combination with practical practice for 1-2 weeks; Advanced loop instructions (such as hole processing cycles) need to accumulate experience based on specific process scenarios, and the overall learning difficulty is lower than that of programming software (such as Python). According to industry research data, novices can independently complete the programming and processing of simple milled parts in an average of 1 month through systematic learning + practical training.
2. Detailed explanation of milling core G code: functions, usage and practical cases
The following is the most commonly used core G-code in milling processing, combined with function definitions, programming formats, practical precautions and real-world examples, to help readers quickly grasp the key points of application.
2.1 Motion control G-code: positioning and trajectory forming core
| G-code | Functional definition | Programming format | Practical precautions | Practical cases |
|---|---|---|---|---|
| G00 | Fast movement (empty stroke positioning) | G00 X_ Y_ Z_ (X/Y/Z as target coordinates) | 1. There is no feed speed command, and the speed is set by the machine tool parameters; 2. To avoid collision with workpieces and fixtures, a safety distance should be reserved (5-10mm is recommended); 3. Prioritize short path movements | Plane milling of an aluminum plate: quickly move from the origin (X0Y0Z50) of the machine tool to the starting point of workpiece processing (X10Y10Z5), programmed as “G00 X10 Y10 Z5; The movement time is only 0.8 seconds, which is 10 times faster than manual positioning. |
| G01 | Linear interpolation (cutting feed) | G01 X_ Y_ Z_ F_ (F is the feed rate, in mm/min) | 1. It must be cooperated with the F command, and the feed speed needs to be adjusted according to the material and tool (such as aluminum milling F800-1200, steel parts F300-500); 2. The trajectory is a straight line between two points, which is suitable for plane, step, and straight contour machining; 3. The speed can be appropriately reduced at the corner to avoid impact | Step milling of steel parts (45#): from (X10Y10Z5) to (X50Y10Z5) at the speed of F400, programmed as “G01 X50 Y10 Z5 F400; “, the step flatness error after machining is 0.01mm, which meets the design requirements. |
| G02/G03 | Arc interpolation (clockwise/counterclockwise arc cutting) | G02 X_ Y_ I_ J_ F_ (G02 clockwise; I/J is the incremental coordinate of the center of the circle relative to the starting point); G03 X_ Y_ I_ J_ F_ (G03 counterclockwise) | 1. I/J cannot be omitted, if the center of the circle coincides with the starting point, I0J0 should be written; 2. The arc radius should match the tool radius to avoid overcutting; 3. The feed speed should be lower than that of straight cutting (reduced by 20-30%) | Arc contour machining of aluminum parts (radius R10, clockwise): starting point (X50Y10), end point (X30Y30), circle center (X50Y30), programmed as “G02 X30 Y30 I0 J20 F1000; “, after machining, the error of arc contour is 0.008mm, and the surface roughness is Ra1.6. |
| G04 | Pause (time-lapse cutting) | G04 P_ (P is pause time, in ms) or G04 X_ (X is pause time, in s) | 1. It is often used for hole bottom smoothing and corner pressure retention; 2. The pause time is set according to the processing demand (0.1-5s), which will affect the efficiency if it is too long; Pause only at the current position and do not change the trajectory of movement | Deep hole milling (hole depth 20mm): pause for 0.5s after reaching the bottom of the hole to ensure that the bottom of the hole is flat, programmed as “G01 Z-20 F300; G04 P500; After processing, the flatness of the bottom of the hole is increased by 40%. |
2.2 Programming and setting G-code: basic configuration of processing parameters
| G-code | Functional definition | Programming format | Application scenarios | Professional considerations |
|---|---|---|---|---|
| G20/G21 | Unit setting (G20 imperial; G21 Metric) | G20; or G21; | G21 is suitable for most domestic processing scenarios (unit mm); G20 is suitable for export parts processing (unit inch) | 1. It must be set at the beginning of the program and cannot be switched throughout the process; 2. If the unit is set incorrectly, it will lead to the deviation of the processing size (1inch=25.4mm), resulting in scrap products; 3. It is recommended to mark the unit at the beginning of the program for easy verification |
| G90/G91 | Coordinate mode setting (G90 absolute coordinates; G91 incremental coordinates) | G90; or G91; | G90 for most milling operations (coordinates based on workpiece origin); G91 for local fine-tuning, repetitive motion (coordinates based on current position) | 1. You can switch between the same program, but you need to be careful to avoid coordinate confusion. 2. It is recommended for beginners to give priority to G90 to reduce programming errors. 3. When programming incremental coordinates, it is necessary to accurately calculate the relative displacement |
2.3 Tool compensation and coordinate system G-code: the core of accuracy assurance
Tool compensation and coordinate system setting are the key to milling accuracy, and the following core codes directly affect the accuracy of workpiece dimensions and need to be mastered.
| G-code | Functional definition | Programming format | Practical points | Fault resolution cases |
|---|---|---|---|---|
| G40/G41/G42 | Tool radius compensation (G40 cancelled; G41 left compensation; G42 Right Compensation) | G41 G01 X_ Y_ D_ (D is the tool radius compensation number); G42 G01 X_ Y_ D_ ； G40 G01 X_ Y_ ; | 1. The compensation should be turned on before the cutting movement and closed after the cutting is completed; 2. The D size should be consistent with the tool radius set in the machine tool; 3. The initial path of opening compensation should be greater than the tool radius to avoid overcutting; 4. G41 is preferred for inner contour machining, and G42 is preferred for outer contouring | When milling the outer contour of a part, the size is 0.8mm smaller (tool radius 4mm), and the tool compensation is not turned on, and “G42 G01 X10 Y10 D1 F800; “(D1 corresponds to the tool radius of 4mm), and the dimensional error is controlled within 0.01mm. |
| G54-G59 | Workpiece coordinate system setting (6 basic coordinate systems can be set) | G54; (Activate the G54 coordinate system, and subsequent coordinates are based on this) | 1. The origin of the coordinate system needs to be set by tool setting (manual tool setting or automatic tool setter); 2. When processing multiple workpieces, the origin of different workpieces can be set to G54-G59, which is convenient to switch; 3. It is recommended to activate the coordinate system at the beginning of the program to avoid default coordinate system errors | When batch processing 3 different parts, set the origin of the 3 parts at G54, G55, and G56 respectively, and switch “G54; ”“G55;” “G56;” It can quickly switch between machining objects without reprogramming, and the changeover time can be reduced by 60%. |
3. The core difference between G code and M code: the division of labor between processing and auxiliary functions
Many CNC operators confuse G-code with M-code, which are functionally completely different but work together to support the entire machining process. In short, the G code is responsible for “machining movement” (such as tool movement and trajectory forming), and the M code is responsible for “auxiliary functions” (such as machine tool switches and coolant control), and the two work together to achieve automated machining.
| Contrast dimensions | G-code (prepare function code) | M Code (Accessibility Code) |
|---|---|---|
| Core features: | Control tool trajectory, machining parameters (e.g. positioning, cutting, coordinate setting) | Control of machine tool auxiliary equipment (e.g. spindle, coolant, fixture) |
| Application scenarios | Core stages of the machining process (cutting, positioning, precision control) | Preparation before processing, assistance during processing, and finishing after processing |
| Examples of common instructions | G00, G01, G02, G21, G41, G54, etc | M03 (spindle forward), M08 (coolant on), M30 (end of program), etc |
| Execution logic | Continuous execution, linked with feed speed and motion trajectory | Single execution, feedback signal after completion, and then subsequent instructions are executed |
| Associations with G Code for Milling | It directly constitutes the core programming content of milling processing | Guarantee the execution of milling G-code (e.g. spindle rotation before cutting) |
Case study: In the complete program fragment of milling a part, the synergistic logic of G code and M code: “G21 G90 G54; (Meric, absolute coordinates, G54 coordinate system setting) M03 S2000; (Spindle forward rotation, speed 2000r/min) M08; (Coolant on) G00 X10 Y10 Z5; (Quick move to starting point) G41 G01 X10 Y10 Z-5 D1 F800; (Left compensation on, cutting under the tool)…… G00 Z50; (Tool Retraction) M05; (Spindle Stop) M09; (Coolant Off) M30; (End of procedure)”, which shows that the division of labor between the two is clear and closely coordinated.
4. The versatility and particularity of G code in CNC milling machine tools
4.1 Versatility: Cross-system compatibility of core G-code
According to the ISO 6983 international standard, the core G-codes in milling machining (such as G00, G01, G02, G21, G40-G42, G54-G59, etc.) are highly versatile in mainstream CNC systems around the world, which means that a set of milling programs written based on the core G-code can be run directly on mainstream machines such as Fanuc, Siemens, Mazak, Haas, etc., without major modifications.
Core facts: Industry data shows that the cross-system compatibility rate of core G-code exceeds 95%, which provides convenience for CNC operators to operate across equipment and reuse programs, and also reduces industry learning costs. For example, an operator wrote an aluminum plane milling program (based on the Fanuc system) that can be processed normally by adjusting the parameters of spindle speed and feed speed to adapt to the Siemens 828D system machine tool, and the program modification amount is less than 5%.
4.2 Specificity: The difference between system-specific G-code
Although the core G-code is common, there are still some exclusive G-codes (non-standard codes) in different CNC systems, mainly focusing on advanced functions (such as macro programs, special loops), which is the focus that novices need to pay attention to.
| CNC system | Exclusive G-code examples | Function | Notes: |
|---|---|---|---|
| Fanuc | G73 | High-speed deep hole pecking cycle | Only Fanuc 0i, 31i and other systems are supported, and Siemens systems need to be replaced with CYCLE83 |
| Siemens | G71 | Cylindrical rough turning cycle (can be used for contour roughing in milling) | Unlike the G71 function of the Fanuc system, the programming format is very different and cannot be directly reused |
| Haas | G87 | Back boring cycle | Exclusive loop instructions, other systems need to achieve the same function through the basic G-code combination |
Professional advice: When writing milling programs, if they need to be used across systems, ISO standard core G code should be used first; If advanced functions must be used, the manual for the target machine system should be consulted and the programming format should be adjusted to avoid machining failures due to system differences.
5. Yigu Technology’s view: G-code optimization is the key to reducing costs and increasing efficiency in milling processing
As an enterprise focusing on CNC machining technology research and development and solution optimization, Yigu Technology believes that g code for milling is not only a “processing language”, but also the core starting point for milling to reduce costs and increase efficiency, and enhance competitiveness. In the context of the current transformation of the manufacturing industry to intelligence and high precision, the mastery of G code should not stay at the level of “knowing how to use”, but should pursue “optimization” and “accurate matching”.
From the perspective of industry practice, high-quality G-code programming can achieve three major values: first, the accuracy is improved, through reasonable tool compensation, coordinate system setting and trajectory optimization, the defect rate of workpieces can be reduced by more than 30%; The second is to improve efficiency, by optimizing the empty stroke path (such as G00 path streamlined) and reasonably setting the feed speed, the processing time of a single workpiece can be shortened by 15-25%; The third is the extension of tool life, which can reduce tool loss by more than 20% through smooth trajectory transitions (such as G02/G03 arc corners instead of right corners) and uniform cutting load distribution.
In the future, with the integration of AI technology and CNC machining, G-code programming will gradually be upgraded to “intelligent generation and optimization”, but core logic and basic applications are still essential skills for operators. Yigu Technology suggests that industry practitioners should continue to deepen the foundation of G-code, accumulate optimization experience in combination with practical operations, and pay attention to the application of intelligent programming tools to achieve the collaborative improvement of “basic capabilities + intelligent tools” and better adapt to the transformation needs of the manufacturing industry.
6. G Code for Milling Frequently Asked Questions (FAQ)
Q1: What is the core difference between G00 and G01 in milling operations? How to choose and use it in actual processing?
A1: The core difference is function and speed: G00 is fast movement (empty stroke), no feed speed, speed is set by machine tool parameters, used for positioning in non-cutting state; G01 is a linear interpolation (cutting feed), which needs to be set in accordance with the F command to set the speed for cutting processing. Selection principle: use G00 for non-cutting stages (such as from the origin to the starting point of machining, and after the end of processing) to quickly position to improve efficiency; G01 is used for the cutting stage (such as plane milling, contour forming) to ensure cutting accuracy and surface quality. Note: When the G00 moves, a safe distance must be reserved to avoid collision.
Q2: What instructions should beginners start with when learning g code for milling? What are the effective ways to learn?
A2: Novices should follow the order of “from basic to complex” and give priority to learning the core instructions: the first step is to learn the basic setting instructions such as G21 (metric), G90 (absolute coordinates), and G54 (coordinate system); the second step is to learn motion control commands such as G00 (fast movement) and G01 (linear interpolation); The third step is to learn G40-G42 (tool compensation); Step 4: Learn G02/G03 (arc interpolation); Finally, learn the loop instructions. Efficient learning method: “theory + practice” combined, first understand the command function and format through the manual, and then carry out simple part programming exercises (such as plane milling, simple contour milling) on the machine tool, and at the same time accumulate typical cases and summarize programming misunderstandings.
Q3: What are the common faults when using G41/G42 tool radius compensation? How to solve it?
A3: Common faults and solutions: (1) Over-cutting: The reason may be the wrong setting of the tool radius compensation value, the initial path of opening the compensation is too short, and the compensation radius is larger than the contour radius during inner contour machining. Solution: check the tool radius compensation value, extend the initial path of the compensation opening (greater than the tool radius), select a tool smaller than the profile radius during internal contour machining and accurately set the compensation value; (2) Compensation cannot be turned on: The reason may be that the compensation value is not set in the D number, and the instruction format is wrong (such as not cooperating with G01). Solution: Check whether the compensation value corresponding to No. D is correct, and ensure that the compensation instruction is linked with G01; (3) Machining dimension deviation: The cause may be the wrong compensation direction (G41/G42 confusion). Solution: Confirm the compensation direction according to the profile type (inside/outside), with the outer profile giving priority to G42 and the inner profile giving priority to G41.
Q4: What are the core differences in G-code programming when milling different materials (aluminum, steel, copper)?
A4: The core difference is focused on the parameter settings of “feed speed (F)” and “spindle speed (S)”, and there is no difference in the G code instruction itself. Specific adaptation: (1) Aluminum parts (easy to cut): feed speed F800-1200mm/min, spindle speed S1500-3000r/min, higher speed can be used to improve efficiency; (2) Steel parts (45#, alloy steel, difficult to cut): feed speed F300-500mm/min, spindle speed S800-1500r/min, need to reduce the speed to ensure stable cutting; (3) Copper parts (good plasticity): feed speed F600-900mm/min, spindle speed S1200-2500r/min, need to control the speed to avoid surface burrs. When programming, the F and S parameters need to be adjusted according to the material, and the instruction format is consistent.
Q5: What is the setting process for the G54-G59 workpiece coordinate system? How can I ensure accurate settings?
A5: Setting process: (1) Machine tool return to zero (ensure that the machine tool is at the mechanical origin); (2) Installation of workpieces and tools; (3) Manually move the tool to the origin of the workpiece (e.g., X-direction origin: the tool touches the X-direction end face of the workpiece, records the coordinate value of the machine tool, and enters the G54 X coordinate; The same goes for Y; Z direction: the tool touches the upper surface of the workpiece, records the coordinate value, and enters the G54 Z coordinate); (4) Confirm the settings: write a simple program (such as G00 X0 Y0 Z5) and check whether the tool is aligned with the workpiece origin after running. Key to ensuring accuracy: (1) Use high-precision tools (e.g., dial indicators, automatic tool setters) when setting tools; (2) Verify the test cut after the knife, measure the size and adjust it; (3) Before batch processing, check the coordinate system parameters again.
Q6: Do all CNC milling machines support the same G-code? What should I pay attention to when using programs across machines?
A6: Not all G-codes are universal. The core difference is that the core G-codes of ISO standards (such as G00, G01, G21, etc.) have a high compatibility rate (>95%) across machine tools, but the G-codes specific to each system (such as Fanuc G73, Siemens G71) are not common. Precautions for cross-machine tool use procedures: (1) Priority is given to the use of ISO standard core G code; (2) Check whether the program contains exclusive G code, if there is a need to adjust it for the target machine; (3) Check the machine tool parameters (such as spindle speed range, feed speed range) and adjust the corresponding S and F commands; (4) Verify the empty operation before the first use, and confirm that the trajectory is free of collision before trial cutting.
Q7: What is the main purpose of the G04 Suspension Instruction in milling operations? How to set the pause time reasonably?
A7: The main uses of G04: (1) Hole bottom smoothing: pause after deep hole processing to allow chips to fully discharge and ensure that the hole bottom is flat; (2) Corner pressure holding: pause when cutting at right angles to avoid dimensional deviation caused by inertia; (3) Tool adaptation: The tool is initially paused when cutting into the workpiece to ensure stable cutting loads. Pause time setting principle: According to the adjustment of the processing scene, the bottom of the hole is generally set to 0.1-0.5s (shallow hole) and 0.5-2s (deep hole); corner holding pressure setting 0.1-0.3s; tool adaptation setting 0.2-0.5s. Too long a time will reduce efficiency, and too short a time will not achieve the desired effect.
Q8: How can I improve the efficiency of milling by optimizing G-code?
A8: Core optimization direction: (1) Streamline empty stroke: optimize the G00 movement path, choose the shortest path, and avoid unnecessary round-trip movement; (2) Reasonable setting of feed speed: adjust the F-value according to the material, tool, and processing stage (such as high-speed feed for roughing and low speed for finishing to ensure accuracy); (3) Use cyclic instructions: use cyclic instructions for complex machining (such as hole machining) to replace the basic G-code combination with cyclic instructions to reduce the number of program lines and improve machining coherence; (4) Smooth trajectory transition: replace the right angle with G02/G03 arc corner to avoid sudden speed changes and improve cutting efficiency; (5) Batch machining optimization: G54-G59 multi-coordinate system setting is used to reduce programming and tool setting time during changeover. Through the above optimizations, the processing efficiency can generally be increased by 15-25%.