1. Introduction to CNC Milling

CNC Milling, short for Computer Numerical Control Milling, is a core subtractive manufacturing technology widely used in the machining industry. It realizes automated, high-precision processing of workpieces by using computer-programmed instructions to control the movement of milling tools and worktables. Unlike traditional manual milling, CNC milling eliminates the need for manual operation of machine tools, greatly improving processing efficiency, accuracy and consistency, and has become an indispensable key technology in fields such as aerospace, automotive manufacturing, precision instrumentation and mold processing.

The development of CNC milling has gone through several stages, from the early numerical control systems based on hardware logic to the modern computer numerical control systems with powerful software functions. Today, CNC milling machines have achieved integration with technologies such as CAD/CAM (Computer-Aided Design/Computer-Aided Manufacturing), intelligent monitoring and flexible manufacturing, further expanding their application scope and processing capabilities, and promoting the transformation and upgrading of the manufacturing industry towards intelligence and refinement.

2. Basic Principles of CNC Milling

2.1 Core Working Principle

The core of CNC milling lies in the closed-loop control of "programming-instruction-execution-feedback". First, engineers use CAD software to design the 2D or 3D model of the workpiece; then, CAM software converts the model into a numerical control program (usually G-code and M-code) that the CNC system can recognize by setting processing parameters (such as cutting speed, feed rate, depth of cut) and tool path. After the program is input into the CNC system, the system parses the instructions, drives the servo motor to control the precise movement of the tool and the workpiece, and uses the rotating milling tool to remove excess material from the workpiece, finally forming the workpiece that meets the design requirements.

In the processing process, the CNC system will real-time collect the position, speed and other information of the tool and workpiece through sensors, and compare it with the preset program parameters. If there is a deviation, it will automatically adjust the movement parameters to ensure the processing accuracy. This closed-loop control mechanism is the key to CNC milling's high precision.

2.2 Key Motion Modes

CNC milling machines have multiple motion axes, and the common ones are 3-axis (X, Y, Z axes), 4-axis and 5-axis. The motion mode is determined by the number of axes and the coordination between axes:

• 3-axis Milling: The most widely used motion mode, where the X and Y axes control the horizontal movement of the workpiece (or tool), and the Z axis controls the vertical movement. It is suitable for processing simple planar parts, grooves, holes and other structures.

• 4-axis Milling: On the basis of 3-axis, an A-axis (rotating around the X-axis) or B-axis (rotating around the Y-axis) is added, which can process parts with complex curved surfaces and inclined planes, such as impellers and cams, and improve processing efficiency compared with 3-axis multi-angle processing.

• 5-axis Milling: Integrates 3 linear axes (X, Y, Z) and 2 rotating axes (usually A-axis and C-axis, or B-axis and C-axis), which can realize arbitrary angle processing of the tool relative to the workpiece. It is suitable for high-precision, complex-shaped parts such as aerospace engine blades and medical implants, and can complete processing that cannot be achieved by 3-axis or 4-axis milling.

3. Main Components of CNC Milling Machine

A complete CNC milling machine is composed of multiple functional components, which cooperate with each other to complete the processing task. The main components include the following parts:

3.1 CNC System

As the "brain" of the CNC milling machine, the CNC system is responsible for receiving, parsing and executing numerical control programs, and controlling the coordinated movement of each component. The modern CNC system usually includes a host computer (man-machine interaction interface), a processor, a memory, a servo control module and an input/output interface. It has functions such as program editing, parameter setting, tool compensation, fault diagnosis and online monitoring, which can greatly improve the operability and reliability of the machine tool.

3.2 Servo System

The servo system is the executive mechanism of the CNC milling machine, which is composed of a servo motor, a servo driver and a position/speed sensor. It receives the control signal from the CNC system, drives the tool and the workpiece to move according to the preset speed and position, and feeds back the actual motion information to the CNC system to form a closed-loop control. The performance of the servo system directly affects the processing accuracy, speed and stability of the milling machine. High-precision servo systems usually adopt permanent magnet synchronous motors and high-resolution encoders to achieve micron-level position control.

3.3 Mechanical Structure

The mechanical structure is the foundation of the CNC milling machine, including the bed, worktable, spindle, tool holder and transmission mechanism:

• Bed: It bears the weight of the entire machine tool and each component, and requires high rigidity and stability to avoid deformation during processing, which affects the processing accuracy.

• Worktable: Used to place the workpiece, and realize the movement of X and Y axes under the drive of the servo system. Some worktables are equipped with a fixture interface to fix the workpiece firmly.

• Spindle: It is used to install the milling tool and drive the tool to rotate at high speed. The spindle speed and rigidity are important indicators affecting cutting efficiency and surface quality. High-speed CNC milling machines usually adopt electric spindles to achieve higher rotational speeds.

• Tool Holder: Used to store and replace tools automatically. The automatic tool change system (ATC) can quickly switch tools according to the program requirements, reducing the auxiliary processing time and improving production efficiency.

3.4 Cutting Tools

Cutting tools are the direct components that remove material during CNC milling, and their performance and selection are closely related to processing quality and efficiency. Common CNC milling tools include end mills, face mills, ball end mills, slotting cutters and reamers. The tool material is usually high-speed steel, cemented carbide, cubic boron nitride (CBN) or diamond, which is selected according to the processed material (such as steel, aluminum alloy, copper, composite material) and processing requirements. For example, cemented carbide tools are suitable for high-speed cutting of steel parts, while diamond tools are suitable for processing non-ferrous metals and non-metallic materials.

4. CNC Milling Process and Key Parameters

4.1 Typical Processing Process

The CNC milling process needs to go through a series of steps to ensure the processing quality and efficiency. The typical process is as follows:

1. Workpiece Design and Modeling: Use CAD software to design the 3D model of the workpiece, and determine the processing surface, size and tolerance requirements.

2. Process Planning: Formulate processing plans, including determining the processing sequence (rough machining → semi-finish machining → finish machining), selecting tools and fixtures, and setting processing parameters.

3. Programming and Simulation: Use CAM software to generate numerical control programs according to the process plan, and simulate the tool path to check for collisions between the tool and the workpiece, tool holder or machine tool, and correct the program in time.

4. Machine Tool Setup: Install the fixture and workpiece on the worktable, calibrate the workpiece coordinate system (use tools such as edge finders or probes to determine the origin of the workpiece), and install the corresponding tools on the spindle.

5. Test Processing: Conduct a small-batch test processing, measure the size and surface quality of the workpiece, adjust the processing parameters if there is a deviation, and confirm that the program and setup are correct.

6. Formal Processing: Start the formal batch processing, and the CNC system automatically executes the program to complete the processing of the workpiece. During the processing, pay attention to monitoring the operation status of the machine tool and the processing quality of the workpiece.

7. Post-processing: After the processing is completed, take out the workpiece, remove burrs, clean the surface, and conduct quality inspection to ensure that the workpiece meets the design requirements.

4.2 Key Processing Parameters

The selection of CNC milling processing parameters directly affects the processing efficiency, surface quality, tool life and processing accuracy. The key parameters include:

• Cutting Speed: The linear speed of the tool cutting edge relative to the workpiece surface, which is determined by the tool material, processed material and tool diameter. Too high cutting speed will accelerate tool wear and even damage the tool; too low will reduce processing efficiency.

• Feed Rate: The speed at which the tool moves relative to the workpiece along the feed direction, which affects the surface roughness of the workpiece and the cutting force. High feed rate can improve efficiency, but it may increase surface roughness; low feed rate can improve surface quality, but it will reduce efficiency.

• Depth of Cut: The thickness of the material removed by the tool in one feed, which is divided into axial depth of cut (along the Z-axis direction) and radial depth of cut (along the X/Y-axis direction). The depth of cut is related to the rigidity of the machine tool, tool and workpiece. Too large depth of cut will cause excessive cutting force, leading to deformation of the workpiece or damage to the tool; too small will increase the number of feeds and reduce efficiency.

• Tool Compensation: Including tool length compensation and tool radius compensation. Tool length compensation is used to correct the deviation caused by the difference in tool length; tool radius compensation is used to adjust the tool path according to the tool radius, ensuring that the processed size meets the requirements.

5. Application Fields of CNC Milling

Due to its advantages of high precision, high efficiency and strong flexibility, CNC milling is widely used in various fields of manufacturing, and has become an important support for the development of high-end manufacturing industry.

5.1 Aerospace Industry

The aerospace industry has extremely high requirements for the precision and reliability of parts. CNC milling, especially 5-axis CNC milling, is widely used in the processing of key parts such as engine blades, impellers, casings, and aircraft structural parts. For example, the processing of aerospace engine blades requires high-precision curved surface processing, which can only be achieved by 5-axis CNC milling, ensuring the aerodynamic performance and structural strength of the blades.

5.2 Automotive Manufacturing

In the automotive manufacturing industry, CNC milling is mainly used in the processing of molds (such as stamping molds, injection molds) and key automotive parts (such as engine cylinder blocks, cylinder heads, gearboxes, and chassis parts). With the development of new energy vehicles, CNC milling is also used in the processing of battery casings, motor parts and other components, which improves the production efficiency and quality stability of automotive parts.

5.3 Precision Instrumentation Industry

Precision instruments (such as meters, sensors, optical instruments) require parts to have micron-level or even sub-micron-level precision. CNC milling can achieve high-precision processing of small and complex parts, ensuring the accuracy and stability of the instruments. For example, the processing of sensor cores and optical instrument lenses requires high-precision plane and curved surface processing, which relies on CNC milling technology.

5.4 Medical Device Industry

With the development of medical technology, the demand for personalized and high-precision medical devices is increasing. CNC milling is used in the processing of medical implants (such as artificial joints, dental implants, bone plates) and medical instruments (such as surgical instruments, diagnostic equipment parts). For example, artificial joints need to be processed according to the patient's bone structure, and 5-axis CNC milling can achieve personalized processing, improving the compatibility and comfort of implants.

5.5 Other Fields

In addition to the above fields, CNC milling is also widely used in the processing of mold manufacturing, electronic product parts (such as mobile phone casings, circuit board brackets), and military industry parts. It has become an indispensable processing technology in modern manufacturing.