The common ones are classified by the following four methods.
- Classification by process use
(1) General CNC machine tools. This kind of machine tools is the same as the traditional general machine tools. There are CNC turning, milling, boring, drilling, grinding machines, etc., and there are many kinds of each kind. For example, CNC milling machines include vertical milling, horizontal milling, tool milling, gantry milling, etc. The technological possibility of this kind of machine tool is similar to that of general machine tool, but the difference is that it can process complex shape parts.
(2) CNC machine tool center machine tool. This kind of machine tool is developed on the basis of general CNC machine tool. It is a kind of NC machine tool with automatic tool changing device (also called multi process NC machine tool or boring milling machining center, conventionally referred to as machining center) which is composed of a tool magazine (which can hold more than 10-100 tools) and automatic tool changing device on the general NC machine tool, which makes the NC machine tool further develop towards automation and high efficiency.
The difference between NC machining center machine tool and general NC machine tool is: after the workpiece is clamped once, the NC device can control the machine tool to automatically change the cutting tool, and continuously complete the milling (turning), boring, drilling, reaming, tapping and other processes on each processing surface of the workpiece. This kind of machine tools are mostly boring and milling, mainly used to process box parts. Compared with general NC machine tools, it has the following advantages
① Reduce the number of machine tools, easy to manage, for multi process parts as long as a machine can complete all processing, and can reduce the inventory of semi-finished products;
② Since the workpiece is clamped only once, the positioning error caused by multiple installation is reduced, and the machining quality can be guaranteed by the accuracy of the machine tool;
③ Process concentration reduces the auxiliary time and improves the productivity;
④ As the parts can be processed in multiple processes in one clamping on one machine tool, the number of special fixtures is greatly reduced, and the production preparation time is further shortened.
Due to the advantages of CNC machine tools, it is very popular with users, so it plays an important role in the production of CNC machine tools.
In addition, there is a kind of machining center, which is developed on the basis of lathe, with shaft parts as the main processing object. In addition to turning and boring, it can also drill, mill and tap any part of the end face and peripheral surface. This kind of machining center also has a tool magazine, which can install 4-12 tools. It is used to call this kind of machine tool turning center (TC).
(3) Multi coordinate CNC machine tool. Some complex shape parts can’t be processed with three-dimensional CNC machine tools, such as propeller, aircraft surface parts, etc. it needs more than three coordinate synthetic motion to process the required shape. Therefore, the emergence of multi coordinate CNC machine tools, which is characterized by the number of axes controlled by the CNC device is more, the structure of the machine tool is more complex, the number of coordinate axes usually depends on the processing requirements of parts. Now commonly used is 4, 5, 6 coordinate CNC machine tools. Figure 1 is the schematic diagram of five axis NC machining. At this time, the X, y, Z coordinates can be linked with the turntable rotation and tool swing at the same time to process wing and other parts.
- According to the movement track classification of CNC machine tools
According to the relative motion path between the tool and the workpiece, the CNC machine tools can be divided into point control CNC machine tools, point line control CNC machine tools, contour control CNC machine tools, etc. It is described as follows:
(1) Point position control CNC machine tool. The numerical control device of this kind of machine tool can only control the moving parts of the machine tool to move accurately from one position (point) to another position (point), that is, it only controls the coordinate value of the end of the stroke, and does not carry out any cutting during the moving process. As for the moving speed and route between the two related points, it depends on the productivity. In order to have as high productivity as possible on the basis of accurate positioning, the movement between the two related points first moves to the new position quickly, and then decelerates by 1-3 levels to make it approach the positioning point slowly, so as to ensure its positioning accuracy. This kind of machine tool mainly includes CNC coordinate boring machine, CNC drilling machine, CNC punch machine and CNC measuring machine, and its corresponding CNC device is called point position control device.
(2) CNC machine tool controlled by point and line. When this kind of machine tool works, it is necessary to control not only the position (i.e. distance) between two related points, but also the moving speed and route (i.e. trajectory) between two related points. Its route is generally composed of straight lines parallel to each axis. It is different from the point control CNC machine tool in that when the moving parts of the machine tool move, it can cut along the direction of a coordinate axis (generally, it can also cut along a 45 ° oblique line, but it can not cut along a straight line with any slope), and its auxiliary functions are more than those of the point control CNC machine tool, for example, to increase the spindle speed control, cycle feed processing, cutting tool Selection and other functions. This kind of machine tool mainly has the simple numerical control lathe, the numerical control boring milling machine and the numerical control processing center and so on. The corresponding numerical control device is called point position linear control device.
(3) Contour control CNC machine tool. The control device of this kind of machine tool can continuously control two or more coordinate axes at the same time. When machining, it is necessary to control not only the starting point and the end point, but also the speed and position of each point in the whole machining process, so that the machine tool can process complex shape parts that meet the requirements of the drawing. Its auxiliary functions are also quite complete.
This kind of machine tools mainly include CNC lathes, CNC milling machines, CNC grinding machines and electrical machining machines. The corresponding numerical control device is called contour control device (or continuous control device).
- According to the control mode of servo system
CNC machine tools can be divided into open-loop and closed-loop according to whether there is a detection feedback device for the controlled quantity. In the closed-loop system, according to the position of the measuring device, it can be divided into full closed-loop and half closed-loop. Based on the open-loop system, an open-loop compensation CNC system is developed.
(1) Open loop control of CNC machine tools. In the open loop control, there is no feedback device
The signal process of numerical control device is unidirectional, so there is no system stability problem. Because of the unidirectional flow of the signal, it does not check 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 calculated by the numerical control device, the command pulse is distributed, and the controlled worktable is moved by the servo mechanism (the servo element is usually a stepping motor).
This kind of machine tool works stably, reacts quickly, debugs conveniently and maintains simply, but its control precision is limited. It is suitable for medium and small CNC machine tools with general requirements.
(2) Closed loop control of CNC machine tools. Because the accuracy of open-loop control can not meet the requirements of precision machine tools and large-scale machine tools, it is necessary to detect its actual working position. Therefore, the detection feedback device is added to the open-loop control CNC machine tools to detect the position of the moving parts of the machine tools at any time during processing, so as to make it consistent with the position required by the CNC equipment, so as to achieve high processing accuracy.
- Classified by numerical control device
CNC machine tools can be divided into hard wire (part) CNC and soft wire (part) CNC according to the CNC devices that realize the control of CNC logic function.
(1) Hard wire numerical control (called ordinary numerical control, namely NC). The input, interpolation and control functions of this kind of CNC system are realized by integrated circuits or discrete components. Generally speaking, different CNC machine tools have different control circuits, so the universality of the system is poor. Because it is all composed of hardware, the function and flexibility are also poor. This kind of system was widely used before 1970s.
(2) Flexible wire numerical control (also known as computer numerical control or microcomputer numerical control, namely CNC or MNC). This kind of system uses medium, large scale and super large scale integrated circuits to form CNC device, or microcomputer and special integrated chip. Its main NC functions are almost completely realized by software. For different NC machine tools, only different software can be programmed, while hardware can be almost universal. Therefore, it has strong flexibility and adaptability, and is also convenient for mass production. Modular software and hardware improve the quality and reliability of the system. Therefore, modern CNC machine tools all use CNC device.
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