There are many varieties and specifications of CNC machine tools, and the classification methods are also different. Generally, it can be classified according to the following four principles based on function and structure
Point-to-point control require precise positioning of the moving parts from one point to another without strict requirements on the trajectory between points. No machining occurs during the movement, and the motions of each axis are independent. To achieve both speed and accuracy in positioning, the movement between two points typically involves rapid initial motion followed by slow approach to the target point to ensure precise positioning, as shown in the diagram below. This represents the motion trajectory of point-to-point control.
CNC machines with point-to-point control functionality mainly include CNC drilling machines, CNC milling machines, CNC punching machines, etc. With the development of CNC technology and the decrease in prices of CNC systems, standalone CNC systems solely for point-to-point control are becoming less common.
Linear control CNC machine tools, also known as parallel control CNC machine tools, are characterized by controlling not only the precise positioning between points but also the movement speed and route (trajectory) between two related points. However, the motion trajectory is limited to parallel movement along the machine coordinate axes. In other words, only one coordinate axis is controlled simultaneously (meaning no interpolation operation is required within the CNC system). During displacement, the tool can cut at a specified feed rate. Generally, these machines can only process rectangular or stepped parts.
Common examples of machine tools with linear control functionality include relatively simple CNC lathes, CNC milling machines, CNC grinders, etc. The CNC systems used in these machines are also referred to as linear control CNC systems. Similarly, standalone CNC machines solely for linear control purposes are becoming less common.”
Contour control CNC machine tools, also known as continuous control CNC machine tools, are characterized by the ability to simultaneously control the displacement and speed of two or more motion coordinates.
To ensure that the relative motion trajectory of the tool along the workpiece contour meets the requirements of the workpiece processing contour, the displacement and speed control of each coordinate motion must be precisely coordinated according to a specified proportional relationship.
Therefore, in this type of control method, it is necessary for the CNC device to have interpolation operation capability. Interpolation involves mathematically processing the basic data input by the program (such as endpoint coordinates of lines, endpoint coordinates and center coordinates or radius of arcs). It describes the shape of the lines or arcs, calculating and allocating pulses to the respective axis controllers based on the calculated results, thus controlling the coordinated displacement of each axis to match the required contour during motion. The tool continuously cuts the workpiece surface during movement, enabling processing of various straight lines, arcs, and curves. This constitutes the machining trajectory of contour control.
These types of machines mainly include CNC lathes, CNC milling machines, CNC wire cutting machines, and machining centers. The corresponding CNC devices are referred to as contour control CNC systems. Depending on the number of coordinated axes they control, they can be classified into several forms:
1. Two-axis linkage: Mainly used for machining rotating surfaces on CNC lathes or machining curved cylindrical surfaces on CNC milling machines.
2. Two-and-a-half-axis linkage: Primarily used for controlling machine tools with three or more axes. Two axes can be linked while the other axis can perform periodic or continuous feed motions.
3. Three-axis linkage: Generally divided into two types. One type involves simultaneous control of the X/Y/Z three linear coordinate axes, commonly used in CNC milling machines, machining centers, etc. The other type involves simultaneous control of two linear coordinates (X/Y/Z) and a rotary coordinate axis revolving around one of the linear coordinate axes.
For example, in turning machining centers, besides the longitudinal (Z-axis) and transverse (X-axis) linear coordinate axes linkage, simultaneous control of the spindle revolving around the Z-axis (C-axis) is also required.
4. Four-axis linkage: Simultaneous control of the X/Y/Z three linear coordinate axes along with a specific rotary coordinate axis.
5. Five-axis linkage: In addition to controlling the X/Y/Z three linear coordinate axes simultaneously, it also controls two of the A/B/C coordinate axes revolving around these linear coordinate axes. This enables simultaneous control of five axes. With this setup, the tool can be oriented in any direction in space. For instance, controlling the tool to swing simultaneously around the X and Y axes ensures that it maintains a normal direction to the machined contour surface at its cutting point. This ensures the smoothness of the machined surface, enhances machining precision and efficiency, and reduces surface roughness.
These types of CNC machine tools operate with closed-loop feedback control for feed servo drive. The drive motor can use either DC or AC servo motors and requires position and velocity feedback. During machining, the actual displacement of the moving components is continuously monitored, and this data is promptly fed back to the comparator within the CNC system. It compares this feedback with the instruction signals obtained from interpolation operations. The difference is then used as the control signal for the servo drive to eliminate displacement errors. Depending on the installation location of the position feedback detection components and the type of feedback device used, it is further classified into two control modes: full closed-loop and semi-closed-loop.
1. Full Closed-Loop Control: As shown in the diagram, linear displacement detection components (typically using gratings) are installed at the saddle of the machine tool, directly detecting the linear displacement of the machine tool coordinates. Through feedback, it can eliminate transmission errors throughout the entire mechanical transmission chain from the motor to the machine tool saddle, thereby achieving high static positioning accuracy of the machine tool. However, due to the nonlinear friction characteristics, rigidity, and clearance in many mechanical transmission links within the entire control loop, and the significantly larger dynamic response time of the entire mechanical transmission chain compared to the electrical response time, it poses considerable challenges to the stability calibration of the entire closed-loop system. Consequently, this full closed-loop control mode is mainly used for CNC coordinate boring machines and CNC precision grinders with high precision requirements.
2. Semi-Closed-Loop Control: As illustrated in the diagram, angular detection components (mainly encoders) are used for position feedback, directly installed at the servo motor or screw end. Since most of the mechanical transmission links are not included in the system’s closed-loop circuit, it achieves relatively stable control characteristics. The transmission errors, such as those in the screw, cannot be corrected in real-time through feedback. However, software compensation methods can be employed to enhance its accuracy appropriately. Currently, most CNC machine tools adopt the semi-closed-loop control mode.
3. Hybrid Control: Hybrid control schemes selectively integrate the characteristics of the above control modes. As mentioned earlier, due to the stable characteristics and low cost of open-loop control and the poor stability of full closed-loop control, a hybrid control mode may be adopted to complement each other, meeting the control requirements of certain machine tools. Commonly used hybrid control modes include open-loop compensation type and semi-closed-loop compensation type.
In terms of functional levels, CNC systems are typically categorized into three classes: low, medium, and high. This classification method is widely used in China. The boundaries between low, medium, and high classes are relative and may vary depending on different periods. Currently, various types of CNC systems can be classified into low, medium, and high classes based on certain functions and indicators. Among them, medium and high classes are generally referred to as full-function CNC or standard CNC.
1. Metal Cutting Category:
This category includes CNC machine tools that employ various cutting processes such as turning, milling, drilling, tapping, grinding, and planing. It can be further divided into two types:
– General CNC Machine Tools: Such as CNC lathes, CNC milling machines, CNC grinders, etc.
– Machining Centers: These have the main feature of a tool magazine with an automatic tool changer. After the workpiece is clamped once, various tools are automatically changed to perform multiple machining operations such as milling (turning), keyway milling, tapping, drilling, and thread tapping on different faces of the workpiece continuously on the same machine tool. Examples include (column/milling type) machining centers, turning centers, drilling centers, etc.
2. Metal Forming Category:
This category comprises CNC machine tools that use forming processes such as extrusion, punching, pressing, and pulling. Common examples include CNC presses, CNC bending machines, CNC pipe bending machines, CNC spinning machines, etc.
3. Special Processing Category:
This category mainly includes CNC machines for special processing tasks such as CNC wire cut EDM machines, CNC EDM forming machines, CNC flame cutting machines, CNC laser processing machines, etc.
4. Measurement and Drawing Category:
This category primarily consists of CNC equipment for measurement and drawing tasks, including coordinate measuring machines, CNC tool setters, CNC drawing machines, etc.
