There are many varieties and specifications of CNC machine tools, and the classification methods are also different. Generally, they can be classified according to the following 4 principles based on functions and structures.

I. Classification by Control Trajectory of Machine Tool Motion

(1) CNC Machine Tools with Point-to-Point Control

Point-to-point control only requires accurate positioning of the moving parts of the machine tool from one point to another, and there is no strict requirement for the motion trajectory between points. No machining is performed during the movement, and the movements between the coordinate axes are independent. To achieve fast and accurate positioning, the displacement between two points generally moves quickly first, then approaches the positioning point at a slow speed to ensure positioning accuracy. As shown in the figure below is the motion trajectory of point-to-point control.

Machine tools with point-to-point control function mainly include CNC drilling machines, CNC milling machines, CNC punching machines, etc. With the development of CNC technology and the reduction in the price of CNC systems, pure point-to-point control CNC systems are rarely seen now.

(2) CNC Machine Tools with Linear Control

CNC machine tools with linear control are also known as parallel control CNC machine tools. Their characteristic is that in addition to controlling the accurate positioning between points, they also control the moving speed and path (trajectory) between two related points. However, their movement path is only parallel to the machine tool coordinate axes, that is, only one coordinate axis is controlled at the same time (i.e., the CNC system does not need to have interpolation calculation function). During the displacement process, the tool can perform cutting at the specified feed rate, and generally only rectangular and step-shaped parts can be processed. Machine tools with linear control function mainly include relatively simple CNC lathes, CNC milling machines, CNC grinding machines, etc. The CNC system of such machine tools is also called linear control CNC system. Similarly, pure linear control CNC machine tools are also rare.

(3) CNC Machine Tools with Contouring Control

Processing Schematic of CNC Machine Tools with Contouring Control

CNC machine tools with contouring control, also known as continuous control CNC machine tools, are characterized by being able to control the displacement and speed of two or more motion coordinates simultaneously.

To meet the requirement that the relative motion trajectory of the tool along the workpiece contour conforms to the workpiece processing contour, the displacement control and speed control of each coordinate motion must be accurately coordinated according to the specified proportional relationship.

Therefore, in this type of control mode, the CNC device is required to have interpolation calculation function. The so-called interpolation is to describe the shape of a straight line or arc through the mathematical processing of the interpolation calculator in the CNC system based on the basic data input by the program (such as the end coordinates of a straight line, the end coordinates and center coordinates or radius of an arc). That is, while calculating, pulses are distributed to each coordinate axis controller according to the calculation results, thereby controlling the linkage displacement of each coordinate axis to conform to the required contour. During the movement, the tool continuously cuts the workpiece surface, and various straight lines, arcs, and curves can be processed, which is the processing trajectory of contouring control.

Such machine tools mainly include CNC lathes, CNC milling machines, CNC wire cutters, machining centers, etc. The corresponding CNC devices are called contouring control CNC systems. According to the number of linked coordinate axes it controls, it can be divided into the following forms:

① Two-axis linkage: Mainly used for processing rotating surfaces on CNC lathes or curved cylindrical surfaces on CNC milling machines.

② 2.5-axis linkage: Mainly used for the control of machine tools with more than three axes, where two axes can be linked, and another axis can perform periodic feeding.

③ Three-axis linkage: Generally divided into two categories. One category is the linkage of three linear coordinate axes X/Y/Z, which is mostly used for CNC milling machines, machining centers, etc. The other category is to control two linear coordinates among X/Y/Z at the same time, as well as a rotary coordinate axis rotating around one of the linear coordinate axes.

For example, a turning machining center requires the linkage of two linear coordinate axes (longitudinal Z-axis and transverse X-axis) as well as the spindle (C-axis) rotating around the Z-axis at the same time.

④ Four-axis linkage: Simultaneously controlling the three linear coordinate axes X/Y/Z and one rotary coordinate axis.

⑤ Five-axis linkage: In addition to simultaneously controlling the linkage of three linear coordinate axes X/Y/Z, it also controls two of the A, B, and C coordinate axes rotating around these linear coordinate axes, forming simultaneous control of five-axis linkage. At this time, the tool can be positioned in any direction in space.For example, controlling the tool to swing around both the X-axis and Y-axis at the same time, so that the tool always maintains a normal direction to the machined contour surface at its cutting point, to ensure the smoothness of the machined surface, improve its machining accuracy and efficiency, and reduce the roughness of the machined surface.

II. Classification by Servo Control Mode

(1) Open-Loop Control CNC Machine Tools

The feed servo drive of such machine tools is open-loop, that is, there is no detection and feedback device. Generally, its drive motor is a stepping motor. The main characteristic of a stepping motor is that the motor rotates by one step angle each time the control circuit changes the command pulse signal, and the motor itself has self-locking ability.

The feed command signal output by the CNC system controls the drive circuit through a pulse distributor. It controls the coordinate displacement by changing the number of pulses, controls the displacement speed by changing the pulse frequency, and controls the displacement direction by changing the pulse distribution sequence.

Therefore, the biggest advantage of this control mode is convenient control, simple structure, and low price. The command signal flow sent by the CNC system is unidirectional, so there is no problem of control system stability. However, since the mechanical transmission error is not corrected by feedback, the displacement accuracy is not high.

Early CNC machine tools all adopted this control mode, but the failure rate was relatively high. At present, due to the improvement of drive circuits, it is still widely used. Especially in China, this control mode is mostly used for general economical CNC systems and CNC transformation of old equipment. In addition, this control mode can be equipped with a single-chip microcomputer or single-board computer as the CNC device, which reduces the price of the entire system.

(2) Closed-Loop Control CNC Machine Tools

The feed servo drive of such CNC machine tools works in a closed-loop feedback control mode. Its drive motor can adopt DC or AC servo motors, and it needs to be equipped with position feedback and speed feedback. During processing, the actual displacement of the moving parts is detected at any time and fed back to the comparator in the CNC system in a timely manner. It is compared with the command signal obtained by interpolation calculation, and the difference is used as the control signal of the servo drive, which then drives the displacement parts to eliminate the displacement error.

According to the installation position of the position feedback detection element and the different feedback devices used, it can be divided into two control modes: full closed-loop and semi-closed-loop.

① Full Closed-Loop Control

As shown in the figure, its position feedback device adopts linear displacement detection elements (currently generally grating scales), installed on the saddle part of the machine tool, that is, directly detecting the linear displacement of the machine tool coordinates. Through feedback, the transmission errors in the entire mechanical transmission chain from the motor to the machine tool saddle can be eliminated, thereby achieving high static positioning accuracy of the machine tool. However, in the entire control loop, the friction characteristics, rigidity and clearance of many mechanical transmission links are nonlinear, and the dynamic response time of the entire mechanical transmission chain is much longer than the electrical response time. This brings great difficulties to the stability correction of the entire closed-loop system, and the design and adjustment of the system are quite complex. Therefore, this full closed-loop control mode is mainly used for CNC coordinate boring machines, CNC precision grinding machines, etc. with high precision requirements.

② Semi-Closed-Loop Control

As shown in the figure, its position feedback adopts angle detection elements (currently mainly encoders, etc.), directly installed at the end of the servo motor or lead screw. Since most mechanical transmission links are not included in the system closed loop, more stable control characteristics can be obtained. The mechanical transmission errors of the lead screw and other parts cannot be corrected at any time through feedback, but the accuracy can be appropriately improved by the software fixed-value compensation method. At present, most CNC machine tools adopt the semi-closed-loop control mode.

(3) Hybrid Control CNC Machine Tools

By selectively integrating the characteristics of the above control modes, a hybrid control scheme can be formed. As mentioned earlier, since the open-loop control mode has good stability, low cost and poor accuracy, while the full closed-loop has poor stability, in order to make up for each other to meet the control requirements of some machine tools, it is advisable to adopt the hybrid control mode. The two commonly used modes are open-loop compensation type and semi-closed-loop compensation type.

III. Classification by Functional Level of CNC System

According to the functional level of the CNC system, CNC systems are usually divided into three categories: low, medium and high. This classification method is widely used in China. The boundaries between low, medium and high grades are relative, and the division standards will be different in different periods. Based on the current development level, various types of CNC systems can be divided into low, medium and high grades according to some functions and indicators. Among them, medium and high grades are generally called full-function CNC or standard CNC.

(1) Metal Cutting Type

Refers to CNC machine tools adopting various cutting processes such as turning, milling, boring, reaming, drilling, grinding, planning, etc. It can be further divided into the following two categories.

① Ordinary CNC machine tools such as CNC lathes, CNC milling machines, CNC grinding machines, etc.

② Machining centers Its main feature is a tool magazine with an automatic tool change mechanism. After the workpiece is clamped once, various tools are automatically replaced to continuously perform multiple processes such as milling (turning), boring, reaming, drilling, tapping on each processing surface of the workpiece on the same machine tool, such as (boring/milling) machining centers, turning centers, drilling centers, etc.

(2) Metal Forming Type

Refers to CNC machine tools adopting forming processes such as extrusion, punching, pressing, drawing, etc. Commonly used ones include CNC presses, CNC bending machines, CNC pipe bending machines, CNC spinning machines, etc.

(3) Special Processing Type

Mainly include CNC EDM wire cutters, CNC EDM forming machines, CNC flame cutting machines, CNC laser processing machines, etc.

(4) Measuring and Drawing Type

Mainly include three-coordinate measuring machines, CNC tool setters, CNC plotters, etc.