There are many varieties and specifications of CNC machine tools, and the classification methods are also different. Generally, according to the function and structure, it can be classified according to the following 4 principles
1. Classification according to the control trajectory of machine tool movement
(1) Point-controlled CNC machine tools
Point control only requires the accurate positioning of the moving parts of the control machine tool to move from one point to another, and the requirements for the movement trajectory between points are not strict, and no processing is carried out in the process of moving, and the movement between the coordinate axes is not related. In order to achieve both fast and accurate positioning, the movement of the displacement between the two points generally moves quickly first, and then approaches the positioning point slowly to ensure the positioning accuracy, as shown in the figure below, which is the movement trajectory of the point control.
Machine tools with point control functions mainly include CNC drilling machines, CNC milling machines, CNC punching machines, etc. With the development of CNC technology and the reduction of the price of CNC system, CNC system used solely for point control is rare.
(2) Linear control CNC machine tools
Linear control CNC machine tool is also known as parallel control CNC machine tool, which is characterized by the fact that in addition to the accurate positioning between the control point and the point, the movement speed and route (trajectory) between the two related points should also be controlled, but its motion route is only parallel to the coordinate axis of the machine tool, that is to say, there is only one coordinate axis controlled at the same time (that is, there is no need to have the interpolation operation function in the numerical control system), and the tool can be cut at the specified feed rate in the process of displacement, and generally can only process rectangular and step-shaped parts.
Its machine tools with linear control function mainly include relatively simple CNC lathes, CNC milling machines, CNC grinding machines, etc. The CNC system of this machine tool is also known as the linear control CNC system. Similarly, CNC machine tools that are used solely for linear control are rare.
(3) Contour control CNC machine tools
Contour control CNC machine tool is also known as continuous control CNC machine tool, and its control feature is that it can control the displacement and speed of two or more motion coordinates at the same time.
In order to meet the requirements of the relative motion trajectory of the tool along the contour of the workpiece and the machining contour of the workpiece, the displacement control and speed control of each coordinate movement must be accurately coordinated according to the specified proportional relationship.
Therefore, in this kind of control mode, the numerical control device is required to have the function of interpolation operation. The so-called interpolation is based on the basic data entered by the program (such as the end coordinates of the straight line, the end coordinates of the arc and the center of the circle or radius), through the mathematical processing of the interpolation combinator in the numerical control system, the shape of the straight line or arc is described, that is, while calculating, the pulse is distributed to the controller of each coordinate axis according to the calculation results, so as to control the linkage displacement of each coordinate axis to conform to the required contour In the process of movement, the tool continuously cuts the surface of the workpiece, and can carry out the processing of various straight lines, arcs and curves. Contour-controlled machining trajectories.
This kind of machine tool mainly includes CNC lathes, CNC milling machines, CNC wire cutting machines, machining centers, etc., and its corresponding CNC devices are called contour control CNC systems, which can be divided into the following forms according to the number of linkage coordinate axes it controls
(1) Two-axis linkage: mainly used for CNC lathe processing of rotating surface or CNC milling machine processing curve cylinder.
(2) Two-axis semi-linkage: mainly used for the control of machine tools with more than three axes, in which two axes can be linked, and the other axis can be used as a periodic feed.
(3) Three-axis linkage: generally divided into two categories, one is the X/Y/Z three linear coordinate axis linkage, which is more used for CNC milling machines, machining centers, etc. The other type is to control the rotation of one of the linear axes around one of the linear axes at the same time, in addition to controlling the two linear coordinates in X/Y/Z at the same time.
For example, in the case of a turning center, in addition to the longitudinal (Z-axis) and transverse (X-axis) linear coordinate axes, it also needs to control the spindle (C-axis) that rotates around the Z-axis at the same time.
(4) Four-axis linkage: control the three linear coordinate axes of X/Y/Z and a certain rotation coordinate axis at the same time.
(5) Five-axis linkage: in addition to controlling the linkage of the three coordinate axes of X/Y/Z at the same time. It also controls the two coordinate axes of A, B and C that rotate around these linear axes at the same time, forming a simultaneous control of five axes linkage, and the tool can be set in any direction of space.
For example, the tool is controlled to swing around the x-axis and y-axis at the same time, so that the tool always maintains the normal direction with the machined contour surface at its cutting point, so as to ensure the smoothness of the machined surface, improve its machining accuracy and processing efficiency, and reduce the roughness of the machined surface.
2. Classification according to the way of servo control
(1) Open-loop control CNC machine tools
The feed servo drive of this kind of machine tool is open-loop, that is, there is no detection and feedback device, generally its driving motor is a stepper motor, the main feature of the stepper motor is that the control circuit changes the command pulse signal every time, the motor rotates a step angle, and the motor itself has the ability to lock itself.
The feed command signal output by the numerical control system controls the driving circuit through the pulse divider, which controls the coordinate displacement by the number of change pulses, the displacement speed by the frequency of the change pulse, and the direction of displacement by the distribution sequence of the change pulse.
Therefore, the biggest feature of this control method is that it is convenient to control, simple in structure and cheap in price. The command signal flow issued by the numerical control system is unidirectional, so there is no stability problem of the control system, but because the error of the mechanical transmission is not corrected by feedback, the displacement accuracy is not high.
Early CNC machine tools all use this control mode, but the failure rate is relatively high, at present due to the improvement of the driving circuit, so that it has still been more used. Especially in the mainland, the general economic CNC system and the CNC transformation of old equipment mostly use this control method. In addition, this control method can be configured with a single-chip microcomputer or a single-board microcomputer as a numerical control device, so that the price of the whole system is reduced.
(2) Closed-loop control of the machine tool
The feed servo drive of this kind of CNC machine tool works according to the closed-loop feedback control mode, and its drive motor can adopt DC or AC two servo motors, and needs to be configured with position feedback and speed feedback, the actual displacement of the moving parts is detected at any time in the processing, and timely feedback to the comparator in the numerical control system, it is compared with the instruction signal obtained by the interpolation operation, and its difference is used as the control signal of the servo drive, and then the displacement part is driven to eliminate the displacement error.
According to the different installation parts 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.
(1) Full closed-loop control
As shown in the figure, its position feedback device adopts linear displacement detection element (at present generally adopts grating ruler), is installed in the saddle part of the machine tool, that is, the linear displacement of the machine tool coordinates is directly detected, and the transmission error in the entire mechanical transmission chain from the motor to the machine tool saddle can be eliminated through feedback, so as to obtain a high static positioning accuracy of the machine tool.
However, because the friction characteristics, rigidity and clearance of many mechanical transmission links are nonlinear in the whole control ring, and the dynamic response time of the whole mechanical transmission chain is very large compared with the electrical response time. This brings great difficulty to the stability correction of the whole closed-loop system, and the design and adjustment of the system are also quite complex, therefore, this full closed-loop control mode is mainly used for CNC coordinate building beds and CNC precision grinding machines with high precision requirements.
(2) Semi-closed-loop control
As shown in the figure, the position feedback is directly mounted on the end of the servo motor or lead screw using a corner detection element (currently mainly using an encoder, etc.). Since most of the mechanical transmission links are not included in the closed-loop loop of the system, it is called to obtain more stable control characteristics. The mechanical transmission error such as the lead screw can not be corrected at any time by feedback, but the software value compensation method can be used to improve its accuracy appropriately. At present, most CNC machine tools adopt semi-closed-loop control
(3) Hybrid control CNC machine tools
The characteristics of the above control methods are selectively concentrated, and a hybrid control scheme can be formed. As mentioned earlier, due to the good stability, low cost and poor accuracy of the open-loop control mode, and the poor stability of the full closed-loop, in order to make up for each other and meet the control requirements of some machine tools, it is advisable to adopt the hybrid control mode. There are two types of open-loop compensation and semi-closed-loop compensation
3. Classification according to the functional level of the numerical control system
According to the functional level of the CNC system, the CNC system is usually divided into three categories: low, medium and high. This classification method is more commonly used in the mainland. The boundaries of low, medium and high grades are relative, and the division criteria will be different in different periods. At the current level of development, various types of CNC systems can be divided into three categories: low, medium and high-grade according to some functions and indicators. Among them, medium and high-grade are generally called full-function CNC or standard CNC.
(1) Metal cutting
Refers to CNC machine tools that use various cutting processes such as turning, milling, bumping, hinging, drilling, grinding, and planing. It can be divided into the following two categories.
(1) Ordinary CNC machine tools, such as CNC lathes, CNC milling machines, CNC grinding machines, etc.
(2) Machining center Its main feature is that it has a tool magazine with an automatic tool change mechanism, and the workpiece passes through one time. After clamping, through the automatic replacement of various tools, on the same machine tool to each processing surface of the workpiece to continuously carry out milling (turning) key, reaming, drilling, tapping and other processes of processing, such as (building/milling) machining center, turning center, drilling center, etc.
(2) Metal forming
It refers to CNC machine tools that use extrusion, punching, pressing, drawing and other forming processes, and commonly used CNC presses, CNC bending machines, CNC pipe bending machines, CNC spinning machines, etc.
(3) Special processing
There are mainly CNC EDM wire cutting machines, CNC EDM forming machines, CNC flame cutting machines, CNC laser processing machines, etc.
(4) Surveying and drawing
There are mainly three-coordinate measuring instrument, CNC tool setting instrument, CNC plotter, etc.