Classification of CNC Machine Tools
CNC machines, or Computer Numerical Control machines, represent the pinnacle of modern manufacturing technology. The classification of cnc machines is essential for understanding their capabilities and applications across various industries. This comprehensive guide explores the different categorization methods for cnc machines, providing detailed insights into their functionalities and characteristics.
Advanced CNC machines in a modern manufacturing facility
Introduction to CNC Machine Classification
CNC machines come in many varieties, and different classification methods can be applied when examining them from various perspectives. These classification systems help manufacturers, engineers, and operators select the right cnc machines for specific applications. The classification of cnc machines typically follows several approaches as shown in Table 1-1.
Table 1-1: Classification Methods of CNC Machines
| Classification Method | Types of CNC Machines |
|---|---|
| By Motion Control Method | Point-to-point control CNC machines, Straight-line control CNC machines, Contour control CNC machines |
| By Servo System | Open-loop CNC machines, Semi-closed loop CNC machines, Fully closed loop CNC machines |
| By Functional Level | Economic CNC machines, Popular CNC machines, High-end CNC machines |
| By Process Method | Metal cutting CNC machines, Metal forming CNC machines, Special processing CNC machines |
Classification by Control Function
1) Point-to-Point Control CNC Machines
These CNC machines can only control two coordinate axes to drive the tool relative to the workpiece, moving quickly from one coordinate position to the next, then controlling the third axis for drilling, boring and other cutting processes with high positioning accuracy.
2) Straight-line Control CNC Machines
These CNC machines can control the tool or worktable to move and cut along directions parallel to the coordinate axes at an appropriate feed rate, which can be adjusted within a certain range according to cutting conditions.
3) Contour Control CNC Machines
These CNC machines have the ability to control several coordinate axes for simultaneous coordinated movement, enabling multi-axis linkage, allowing the tool to move relative to the workpiece according to the programmed trajectory and speed.
Detailed Explanations
1) Point-to-Point Control CNC Machines
Point-to-point control CNC machines are designed for precise positioning rather than controlling the path between points. These cnc machines are optimized for operations where the tool moves from one position to another without cutting during transit. To improve productivity, positioning movements use the maximum feed rate set by the CNC system, with no cutting performed during movement, so there are no requirements for the movement trajectory.
Point-to-point control CNC machines are used for machining hole systems in planes. These machines mainly include CNC drilling machines, printed circuit board drilling machines, CNC boring machines, CNC punching machines, and coordinate measuring machines. The precision of these cnc machines is particularly important for applications like circuit board manufacturing, where thousands of holes must be drilled with exact positioning.
2) Straight-line Control CNC Machines
Straight-line control CNC machines represent an advancement over point-to-point systems, as they can maintain control during movement along straight paths. These cnc machines allow for cutting operations while moving along axes, making them suitable for creating flat surfaces and simple shapes. Early simple two-axis CNC lathes can be used for machining stepped shafts. Simple three-axis CNC milling machines can be used for plane milling operations.
Modern transfer machines use CNC feed servo drive systems, which can drive the power head with a multi-axis box for axial feed for drilling and boring operations. These can also be considered a type of straight-line control CNC machine. The versatility of these cnc machines makes them popular in mass production environments where consistent, linear cuts are required.
3) Contour Control CNC Machines
Contour control CNC machines represent the most advanced category, capable of simultaneous multi-axis movement to create complex shapes. The essential feature of these cnc machines is their ability to achieve联动加工 (linkage processing). These CNC machines include CNC lathes, CNC milling machines, machining centers and other CNC machines used for processing parts with curved and surface shapes.
Most modern CNC machines are of this type. They can be further divided into 2-axis, 3-axis, 4-axis, and 5-axis linkage CNC machines based on the number of linkage axes. For example, 4-axis 3-linkage means that only any three axes can be controlled for linkage at any time, and the more linkage axes there are, the more difficult it is to program the machining program. These sophisticated cnc machines are essential for aerospace, automotive, and mold-making industries where complex geometries are common.
Comparison of Control Functions in CNC Machines
Classification by Feed Servo Drive System
Based on whether the CNC system's feed servo drive system has a position measuring device (no position measuring device is an open-loop system, and with a position measuring device is a closed-loop system), CNC machines can be divided into open-loop CNC machines and closed-loop CNC machines. Closed-loop CNC machines can be further divided into fully closed-loop CNC machines and semi-closed-loop CNC machines according to the installation position of the position measuring device.
1) Open-loop CNC Machines
Open-loop CNC machines use an open-loop feed servo drive system. This system has no position measuring device, and the signal flow is one-way (CNC device → feed system), so the system stability is good.
Without position feedback, their accuracy is relatively lower than closed-loop systems. These cnc machines typically use power stepping motors as servo drive components, offering advantages of simple structure, stable operation, convenient debugging, easy maintenance, and low cost.
2) Semi-closed Loop CNC Machines
The feed servo drive system of semi-closed loop CNC machines uses position detection points derived from the drive motor (usually AC or DC servo motors) or the end of the lead screw.
These cnc machines indirectly detect the displacement of the worktable by detecting the rotation angle of the motor and lead screw, rather than directly detecting the actual position of the worktable. This design offers a balance between performance and cost, making these cnc machines popular in many manufacturing applications.
3) Fully Closed Loop CNC Machines
Fully closed-loop feed servo drive systems directly detect the actual position of the worktable. In theory, fully closed-loop feed servo drive systems can eliminate errors, gaps, and lost motion in the entire drive and transmission chain, providing high position control accuracy.
These high-precision cnc machines are more complex and expensive due to their sophisticated feedback systems. The design, installation, and debugging of fully closed-loop systems are quite challenging, making these cnc machines suitable for applications requiring exceptional precision.
Performance Characteristics
Open-loop CNC Machines
The accuracy of open-loop CNC machines mainly depends on the performance and accuracy of the servo drive system and mechanical transmission mechanism. These cnc machines find widespread application in situations where precision and speed requirements are not high and driving torque is not large, generally used for economic CNC machines and the CNC transformation of old machine tools. Their simplicity makes open-loop cnc machines reliable and easy to maintain, though their precision limitations restrict their use in high-tolerance applications.
Semi-closed Loop CNC Machines
Since semi-closed loop systems do not include or only include a few mechanical transmission links in the loop, semi-closed loop feed servo drive systems can achieve relatively stable control performance. Their stability is not as good as open-loop feed servo drive systems but better than fully closed-loop feed servo drive systems.
Additionally, errors in various components within the position loop can be compensated to some extent, but errors in components outside the position loop such as lead screw pitch errors and motion errors caused by gear backlash cannot be eliminated. These errors can be compensated through software to improve motion accuracy of these cnc machines. Overall, semi-closed loop feed servo drive systems offer better accuracy than open-loop systems but less than fully closed-loop systems, while also featuring simple structure and convenient debugging, making them widely used in CNC machines.
Fully Closed Loop CNC Machines
Due to the fact that many mechanical transmission links in the position loop have non-linear characteristics such as friction, rigidity, and backlash, it is easy to cause system instability. Therefore, the design, installation, and debugging of fully closed-loop feed servo drive systems are quite challenging.
Fully closed-loop feed servo drive systems have high requirements for the accuracy, rigidity, and dynamic characteristics of their components, so they are expensive. These high-performance cnc machines are mainly used in boring and milling machines, ultra-precision lathes, ultra-precision grinders, and larger CNC machines that require very high precision. The exceptional accuracy of these cnc machines justifies their higher cost in applications where precision is critical.
Servo System Comparison in CNC Machines
Classification by Functional Level
CNC machines can be classified into three categories by their functional level: high-end, popular, and economic models. This classification does not have a clear definition or exact boundaries. CNC machine grades are usually evaluated by the main CPU (central processing unit) grade, resolution and feed rate, number of linkage axes, servo level, communication functions, and human-machine interface. These distinctions help manufacturers choose the right cnc machines based on their specific needs and budget constraints.
High-end CNC Machines
- 32-bit or 64-bit high-performance CPU
- 5 or more linkage axes
- Resolution up to 0.1μm
- Feed rate exceeding 15m/min
- Advanced AC servo system with full digital control
- High-speed communication interfaces (Ethernet, etc.)
- Advanced graphics interface with 3D simulation
Popular CNC Machines
- 16-bit or higher performance CPU
- Up to 5 linkage axes
- Resolution of 1μm
- Feed rate up to 24 m/min
- AC/DC servo motor drive
- RS232 or DNC communication interface
- CRT display with character and graphics capabilities
Economic CNC Machines
- 8-bit CPU or single-chip microcomputer
- Up to 3 linkage axes
- Resolution of 0.01mm
- Feed rate of 6-8 m/min
- Stepper motor or AC servo drive
- Simple RS232 communication function
- LED digital display or simple CRT character display
Application Scenarios for Different Levels of CNC Machines
The classification by functional level helps match cnc machines to appropriate applications. High-end CNC machines are typically used in industries requiring complex parts with tight tolerances, such as aerospace and precision mold making. These cnc machines offer the greatest flexibility and accuracy but come with a higher price tag.
Popular CNC machines strike a balance between performance and cost, making them suitable for general manufacturing applications across various industries. These versatile cnc machines can handle most common machining tasks with good accuracy and reliability.
Economic CNC machines are ideal for simple machining operations and small-scale production. These affordable cnc machines provide an entry point into CNC technology for small workshops and educational institutions, offering basic CNC capabilities at a lower cost. They are also commonly used for retrofitting older manual machines to extend their useful life with CNC capabilities.
Classification by Process Purpose
CNC machines can be divided into the following four categories according to their process purposes. This classification is particularly useful for manufacturers selecting cnc machines based on the specific operations they need to perform. Each category of cnc machines is optimized for particular manufacturing processes, ensuring efficient and accurate production.
Cutting Processing CNC Machines
Cutting processing CNC machines are those with cutting capabilities. These versatile cnc machines remove material through various cutting operations to achieve the desired part geometry. Examples include CNC milling machines, CNC lathes, CNC grinding machines, machining centers, and CNC gear processing machines.
These cnc machines use different cutting tools and techniques to shape metal and other materials with high precision. Machining centers, a type of advanced cutting CNC machine, offer multiple axes of movement and tool changers, allowing complex parts to be produced in a single setup.
Forming Processing CNC Machines
Forming processing CNC machines are those capable of changing the shape of workpieces through physical methods without removing material. These cnc machines reshape materials using force, pressure, or heat to achieve the desired form.
Examples include CNC bending machines, CNC pipe bending machines, CNC presses, and CNC stamping machines. These cnc machines are essential in the sheet metal industry and for producing parts with complex curves and shapes that would be difficult or impossible to achieve with cutting processes alone.
Special Processing CNC Machines
Special processing CNC machines are those with specialized processing capabilities that don't rely on traditional cutting or forming methods. These unique cnc machines use unconventional techniques to shape materials, often for specialized applications.
Examples include CNC wire cutting machines, CNC electrical discharge machining (EDM) machines, laser processing machines, ultrasonic machining equipment, and plasma cutting machines. These cnc machines are particularly valuable for working with extremely hard materials or creating intricate shapes that conventional machining can't achieve.
Other Types of CNC Machines
This category includes various CNC equipment in a broader sense that don't fit neatly into the previous classifications. These specialized cnc machines support manufacturing processes beyond direct material shaping.
Examples include CNC assembly machines, CNC measuring machines, CNC robots, CNC material handling equipment, and CNC quality inspection systems. These cnc machines play crucial roles in automated manufacturing systems, ensuring precision throughout the production process from material handling to final inspection.
Distribution of CNC Machines by Process Type
Conclusion
The classification of CNC machines provides a framework for understanding the capabilities and applications of these versatile manufacturing tools. From simple economic models to advanced high-end systems, cnc machines continue to revolutionize manufacturing across industries. By understanding the different classification methods, manufacturers can select the most appropriate cnc machines for their specific needs, balancing performance requirements with budget considerations.
As technology advances, the capabilities of cnc machines continue to expand, with improvements in precision, speed, and automation. Whether categorized by control function, servo system, functional level, or process purpose, each type of CNC machine offers unique advantages for specific manufacturing challenges.