There is also a class of machining centers that have been developed on the basis of lathes, with axis-type parts as the main processing object. In addition to turning and boring, it can also perform drilling, milling and tapping on any part of the end face and peripheral surface. This kind of machining center also has a magazine, which can install 4 to 12 tools. It is customary to call this kind of machine a turning center (TC: Turning Center).
Figure 1 CNC machining with five-axis linkage
(3) Multi-coordinate CNC machine tools. Some complex-shaped parts can't be machined with three-axis CNC machine tools, such as propellers and aircraft curved parts. It takes more than three coordinate movements to produce the desired shape. So there has been a multi-coordinate CNC machine tool, which is characterized by the number of axes controlled by the numerical control device is more, the machine tool structure is also more complex, and the number of coordinate axes usually depends on the processing requirements of the processing parts. Now commonly used are CNC machines with 4, 5, 6 coordinates. Figure 1 shows the five-axis CNC machining schematic. At this time, the three coordinates of x, y, and z and the rotation of the turntable and the swing of the tool can be simultaneously linked to process the wing and other parts.
2, according to the classification of the trajectory of CNC machine tools
According to the trajectory of the relative motion between the tool and the workpiece that can be controlled, the CNC machine tool can be divided into point-controlled CNC machine tools, point-position linear control CNC machine tools, and contour control CNC machine tools. The breakdown is as follows:
(1) Point control of CNC machine tools. The numerical control device of this kind of machine tool can only control the movement of the moving part of the machine tool from one position (point) to another position (point) precisely, that is, only the coordinate value of the end point of the stroke is controlled, and no cutting processing is performed during the movement. The speed and route of travel between related points depends on productivity. In order to have as high productivity as possible on the basis of precise positioning, the movement between the two correlation points is first to move quickly to a new position, and then to slow down 1-3 steps to bring the slow approach to the positioning point. Ensure its positioning accuracy.
This type of machine tools are mainly CNC coordinate boring machines, CNC drilling machines, CNC punch presses, and numerical control measuring machines. The corresponding numerical control devices are called point position control devices.
(2) Point-position linear control of CNC machine tools. When such machines work, they not only control the position (ie, distance) between two related points, but also control the speed and the route (ie, trajectory) between two related points. The route generally consists of straight sections that are parallel to the axes. The difference between it and the point-controlled CNC machine tool is that when the moving parts of the machine tool move, they can be cut in the direction of one coordinate axis (generally, they can also be cut along a 45-degree diagonal line, but they cannot be cut along any straight-line slope). Processing, and its auxiliary functions more than point control of CNC machine tools, for example, to increase the spindle speed control, circulation feed processing, tool selection and other functions.
This type of machine tools mainly includes simple CNC lathes, CNC boring and milling machines, and CNC machining centers. The corresponding numerical control device is called a point position linear control device.
(3) Contour control of CNC machine tools. The control device of this type of machine can continuously control two or more coordinate axes at the same time. During processing, not only the starting point and the end point must be controlled, but also the speed and position of each point in the entire machining process should be controlled so that the machine tool can produce complex parts that meet the requirements of the drawings. Its auxiliary functions are also relatively complete.
Such machine tools mainly include CNC lathes, CNC milling machines, CNC grinders and electric machining machines. The corresponding numerical control device is called a contour control device (or continuous control device).
3, according to the servo control system classification
CNC machine tools can be divided into two types, open-loop and closed-loop, according to whether or not the controlled device has a feedback device. In the closed-loop system, according to the location of the measuring device, it can be divided into two types: full-closed and semi-closed. Based on the open-loop system, an open-loop compensation type CNC system has also been developed.
(1) Open-loop control of CNC machine tools. In open-loop control, the machine does not detect the feedback device (see Figure 2).
Figure 2 Open-loop control system block diagram
The signal sent by the numerical control device is one-way, so there is no system stability problem. It is also because of the one-way flow of the signal that it does not test the actual position of the moving parts of the machine tool, so the machining accuracy of the machine tool is not high, and its accuracy mainly depends on the performance of the servo system. The working process is: The input data is distributed by the numerical control device to calculate the command pulse, and the controlled worktable is moved by the servo mechanism (the servo element is often a stepping motor).
This kind of machine tool work is relatively stable, the response is quick, debugging is convenient, maintenance is simple, but its control accuracy is limited. It is suitable for medium and small-sized CNC machine tools that are generally required.
(2) Closed-loop control of CNC machine tools. Since open-loop control precision cannot meet the requirements of precision machine tools and large-scale machine tools, its actual working position must be detected. For this purpose, a detection feedback device is added to the open-loop control numerical control machine tool to detect the position of the moving parts of the machine tool during processing. , To make it consistent with the position required by the numerical control device, in order to achieve high processing accuracy.
Closed-loop control system block diagram shown in Figure 3. In the figure, A is the velocity measuring element and C is the position measuring element. When the command value is sent to the position comparison circuit, if the work table does not move at this time, there is no feedback amount, the command value makes the servo motor rotate, the speed feedback signal is sent to the speed control circuit through A, and the actual displacement of the work table is passed through C. The feedback is returned and compared with the command value in the position comparison circuit, and the difference is used to control until the difference is eliminated, so that the precise positioning of the table is finally achieved. The advantages of this type of machine are high accuracy and speed, but the commissioning and maintenance are more complicated. The key is the stability of the system, so must pay enough attention to the stability in the design.
Fig. 3 Closed-loop control system block diagram
(3) Can be used universally. Therefore, it has strong flexibility and adaptability, and it is also convenient for mass production. The modular software and hardware improve the quality and reliability of the system. Therefore, modern CNC machine tools use CNC devices. Semi-closed-loop control of CNC machine tools. The composition of the semi-closed loop control system is shown in Figure 4.
Fig. 4 Half-closed loop control system block diagram
This kind of control method does not check and measure the actual position of the table, but indirectly detects the rotation angle of the servo motor through measuring elements associated with the servo motor, such as a speed measuring generator A and a photoelectric encoder disk B (or rotary transformer). Calculate the actual displacement of the workbench. The block diagram of the semi-closed-loop control system in Figure 4 is compared with the command value and the difference is used to achieve the control. As can be seen from FIG. 5, since the table is not completely included in the control loop, it is called semi-closed-loop control. This control method is between open-loop and closed-loop, the accuracy is not closed-loop high, debugging is more convenient than closed-loop.
(4) Open-loop compensation CNC machine tools. The characteristics of the above three control modes are selectively gathered to form a hybrid control scheme. This is a topic that has been studied by people for many years in large CNC machine tools and has now become a reality. Because large CNC machine tools require much higher feed rates and return speeds, they also require considerable precision. If only full-closed loop control is used, the machine tool drive chain and the table are all placed in the control link, and the factors are very complicated. Despite numerous installation and commissioning difficulties, it is still difficult. In order to avoid these conflicts, a hybrid control approach can be used. In specific programs, it can be divided into two forms: one is open-loop compensation; the other is semi-closed-loop compensation. Here only the open-loop compensation type control CNC machine tool is introduced.
Figure 5 shows the block diagram of the open-loop compensation control method. Its characteristic is: The basic control chooses the stepping motor's open-loop control servo mechanism and attaches a correction servo circuit. The error of the mechanical system is corrected by the feedback signal of the linear displacement measuring element mounted on the table.
Figure 5 Open-loop compensation control block diagram
4, according to the numerical control device classification
If the numerical control machine tool is divided according to the numerical control device which realizes the numerical control logic function control, there are two kinds of hard line (piece) numerical control and cord (piece) numerical control.
(1) Hard-wired CNC (known as NC, NC). The functions of input, interpolation, and control of this type of CNC system are realized by integrated circuits or discrete components. In general, CNC machine tools have different control circuits. Therefore, the system's versatility is poor. Because it consists entirely of hardware, its functionality and flexibility are also poor. This type of system was widely used before the 1970s.
(2) Cord NC (also known as computer numerical control or computer numerical control, ie CNC or MNC). Such systems use medium-, large-scale, and ultra-large-scale integrated circuits to form CNC devices, or use microcomputers and dedicated integrated chips. Their major numerical control functions are almost entirely implemented by software. Different CNC machines require different software. It can be achieved, and the hardware is almost universal. Therefore, it has strong flexibility and adaptability, and it is also convenient for mass production. The modular software and hardware improve the quality and reliability of the system. Therefore, modern CNC machine tools use CNC devices.
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