There are many types of CNC machine tools and different classification methods. Generally, it can be classified according to the following principles according to function and structure.

1. Classification according to the control trajectory of the machine tool movement.

(1) Point-controlled CNC machine tools

Point position control only needs to accurately move the moving part of the machine tool from one point to another point, and the requirements for the movement path between points are not strict, and no processing is performed during the movement process, and the movement between each coordinate axis is irrelevant. To achieve fast and accurate positioning, generally move quickly first, and then slowly approach the positioning point to ensure positioning accuracy. As shown in the figure below, the point position control method is used for positioning.

The machine tools with point control function mainly include CNC drilling machines, CNC milling machines, CNC punching machines, etc. Due to the development of numerical control technology and the decline of numerical control system prices, it is not common to simply use point control in numerical control systems.

(2) Linear control CNC machine tools

Linear CNC machine tools, also known as parallel control CNC machine tools, are characterized in that in addition to controlling the precise positioning between two points, they can also control the moving speed and path (track) between two points, but the movement path is only parallel to the machine tool coordinate axis. At the same time, there is only one control coordinate axis (that is, no interpolation operation is required) (that is, no interpolation operation is required in the CNC system), and no interpolation operation function is required in the CNC system), and the cutting process is performed at the specified feed rate.

Machine tools with linear control functions include relatively simple CNC lathes, CNC milling machines, and CNC grinding machines. The CNC system of the machine tool is also called linear control CNC system. Similarly, CNC machine tools with simple linear control are also rare.

⑶ contour control CNC machine tools

Contour control CNC machine tools, also known as continuous control CNC machine tools, are characterized by the ability to simultaneously control the speed and displacement of two or more moving coordinates.

Such machine tools mainly include CNC lathes, CNC milling machines, CNC wire cutting machines, machining centers, etc. According to the number of connecting rod coordinate axes controlled by them, they can be divided into the following forms: CNC machine tools.

In order to meet the relative motion trajectory of the tool along the workpiece contour and meet the requirements of the workpiece machining contour, the displacement control and speed control of each coordinate movement must be properly coordinated.

Therefore, in this control mode, the numerical control device needs to insert and supplement the operation function. The so-called insertion and supplementation is to describe the shape of the line or arc through the mathematical processing of inserting and supplementing the operator in the numerical control system according to the basic data entered in the program (such as the coordinates of the end point of the line, the coordinates of the end point of the circular arc, the coordinates of the circle center or the radius). That is, according to the calculation results, pulses are distributed to each coordinate axis controller, so as to control the linkage displacement and required contour of each coordinate axis. During the movement, the tool can continuously cut the workpiece surface, and can process various straight lines, arcs and curves. Contour-controlled machining path.

①Two-axis linkage: It is mainly used to process rotating surfaces on CNC lathes or curved columns on CNC milling machines.

②Two-axis semi-linkage: mainly in the control of machine tools with more than three axes, two of which can be linked, and the other axis can be used as a periodic feed.

③Three-axis linkage: generally divided into two categories, one is the linkage of X/Y/Z three linear coordinate axes, which are mostly used in CNC milling machines, machining centers, etc. The other type is that in addition to controlling the two linear coordinates in X/Y/Z at the same time, it can also control the rotating coordinate axis that rotates around a linear coordinate axis.

For example, a turning machining center needs to control the spindle (Z axis) to rotate around the Z axis in addition to the linkage with the longitudinal axis (Z axis) and the horizontal axis (X axis).

④Four-axis linkage: control the three linear coordinate axes of X/Y/Z at the same time to connect with the same rotating coordinate axis

⑤ Five-axis linkage: In addition to the simultaneous control of Z, the two coordinate axes of the a, b, and c coordinate axes also rotate at the same time. When the five-axis linkage is formed at the same time, the tool can be set in any direction in space.

For example, control the tool to rotate in the two directions of the x-axis and the y-axis at the same time, and the tool always maintains the direction of the normal to the contour surface to be processed at its cutting point, so as to ensure the smoothness of the processed surface and improve its machining accuracy and machining. efficiency, reducing the roughness of the machined surface.

2. Classification according to the way of servo control

(1) Open-loop control of CNC machine tools

The feed servo drive of this kind of machine tool is open-loop and has no detection and feedback device. Generally, its drive motor is a stepper motor. The main feature of the stepper motor is that every time the control circuit converts the command pulse signal, the motor will rotate the step angle. , the motor itself has self-locking ability.

The feed command signal output by the CNC system controls the drive circuit through the pulse distributor, which controls the coordinate displacement by changing the number of pulses, thereby changing the frequency of the pulses to control the displacement speed, and thus changing the distribution order of the pulses to control the direction of displacement .

Therefore, the biggest features of this control method are convenient control, simple structure and low price. The command signal sent by the numerical control system is one-way, so there is no stability problem of the control system, but because there is no feedback correction for the error of the mechanical transmission, so The displacement accuracy is not high.

In the past, CNC machine tools used this control method, and the failure rate was relatively high. At present, due to the improvement of the driving circuit, it is still used to a large extent. Especially in our country, the general economical CNC system and the CNC transformation of old equipment mostly use this control method. In addition, in this way, a single-chip computer or a single-board computer can be configured as a numerical control device, thereby reducing the price of the entire system.

(2) Closed-loop control of machine tools

The feed servo drive of the CNC machine tool works in a closed-loop feedback control mode. The drive motor can be a DC or AC servo motor, and position feedback and speed feedback need to be configured. During the machining process, the actual displacement of the moving parts can be detected at any time and fed back to the comparator in the numerical control system in time. Compared with the command signal generated by the interpolation operation, the difference is used as the control signal of the servo drive to drive the displacement element to eliminate the displacement error.

According to the installation position of the position feedback detection element and the feedback device used, it is divided into two control modes: full-closed loop and semi-closed loop…