First, the concept of digital control machine tools:
Digital control machine: It is a machine tool that uses digital code (program command) to control the tool to automatically process according to the given working program, movement speed and trajectory, referred to as CNC machine.
Second, the application of CNC machine tools:
CNC machine tools have wide adaptability. When the machining object changes, only the input program instructions need to be changed. The machining performance is higher than that of the general automatic machine tool, and the complex profile can be accurately processed. Therefore, it is suitable for small and medium-sized batches, frequent modification, and high precision requirements. More complex shapes and good economic results. With the development of numerical control technology, the number of machine tools using CNC systems is increasing, including lathes, milling machines, boring machines, drilling machines, grinding machines, gear processing machines and EDM machines. In addition, there are machining centers and turning centers that can automatically change tools and perform multiple steps in one-time loading. In 1948, Parsons of the United States was commissioned by the US Air Force to develop processing equipment for aircraft propeller blade profile. Due to the complex and diverse shape of the sample, the high precision requirements, and the general processing equipment are difficult to adapt, so the idea of ​​computer-controlled machine tools is proposed. In 1949, the company began research on CNC machine tools with the assistance of the Massachusetts Institute of Technology's servo research laboratory. In 1952, it successfully produced the first three-axis CNC milling machine modified from a large vertical copy milling machine. That is, official production begins. At that time, the numerical control device used the electronic tube components, which was bulky and expensive. It was only used in a few special departments such as the aviation industry to process complex profile parts. In 1959, transistor components and printed circuit boards were fabricated to make the numerical control device enter. The second generation, the volume is reduced, the cost has decreased; after 1960, the relatively simple and economical point-controlled CNC drilling machine, and the linear control CNC milling machine have developed rapidly, making CNC machine tools gradually promoted in various sectors of the machinery manufacturing industry. . In 1965, the third-generation integrated circuit numerical control device appeared, which not only had small volume, low power consumption, but also improved reliability and further reduced price, which promoted the development of CNC machine tools and output. In the late 1960s, a direct numerical control system (referred to as DNC), which is also called a group control system, was directly controlled by one computer. The computer numerical control system (CNC) controlled by a small computer made the numerical control device enter. Small computerized as the fourth generation. In 1974, a microcomputer numerical control device (MNC) using a microprocessor and a semiconductor memory was successfully developed, which is a fifth-generation numerical control system. Compared with the third generation, the function of the numerical control device has doubled, while the volume has been reduced to 1/20 of the original, the price has been reduced by 3/4, and the reliability has been greatly improved. In the early 1980s, with the development of computer software and hardware technology, there appeared a numerical control device capable of automatic programming of human-machine dialogue. The numerical control device became smaller and smaller, and could be directly installed on the machine tool; the automation degree of the numerical control machine tool was further improved. It has the functions of automatically monitoring tool breakage and automatically detecting workpieces.
Third, the structural design of CNC machine tools:
CNC machine tools are mainly composed of numerical control devices, servo mechanisms and machine tool bodies. The program instructions of the input numerical control device are recorded on the information carrier, received by the program reading device, or manually input directly by the keyboard of the numerical control device. The numerical control device includes a program reading device and an input portion, an arithmetic portion, a control portion, an output portion, and the like, which are composed of electronic circuits. The numerical control device is divided into three types according to the control functions that can be realized: point control, linear control and continuous trajectory control. Point control is to control only the exact positioning of the tool or table from one point to another, and then perform fixed-point machining, and the path between points does not need to be controlled. These types of control are CNC drilling machines, CNC boring machines and CNC coordinate boring machines. The linear control is to control the exact positioning of the starting point and the end point of the linear path, and also to control the straight line cutting between the two points at the specified feed speed. CNC milling machines for face milling with this type of control, as well as CNC lathes and CNC grinding machines for stepped shaft turning and grinding. Continuous trajectory control (or contour control) continuously controls joint motion in two or more coordinate directions. In order to make the tool contour of the workpiece according to the specified trajectory, the numerical control device has the function of interpolation operation, so that the motion trajectory of the tool approaches the specified contour curve with a minimum error, and coordinates the movement speed in each coordinate direction, so as to be in the cutting process. The specified feed rate is always maintained. CNC milling machines, CNC lathes, CNC grinding machines and machining centers for machining surfaces can be used with this type of control. The servo mechanism is divided into three types: open loop, semi-closed loop and closed loop. The open loop servo is composed of a stepper motor drive circuit and a stepper motor. Each pulse signal causes the stepping motor to rotate at a certain angle, and the table is moved by the ball screw to move a certain distance. This servo mechanism is relatively simple, stable in operation, easy to grasp, but the accuracy and speed are limited. The semi-closed loop servo consists of a comparison line, a servo amplifier line, a servo motor, a speed detector and a position detector. The position detector is mounted on the end of the screw or servo motor, and the position of the table is indirectly measured by the angle of rotation of the screw. Commonly used servo motors are wide speed DC motors, wide speed AC motors and electro-hydraulic servo motors. The position detector has a resolver, a photoelectric pulse generator, and a circular grating. The accuracy, speed and dynamics of this servo mechanism are superior to the open-loop servo mechanism and are used by most small and medium-sized CNC machine tools. The working principle and composition of the closed-loop servo mechanism are the same as those of the semi-closed-loop servo mechanism. Only the position detector is installed on the workbench, and the actual position of the workbench can be directly measured. Therefore, the feedback accuracy is higher than the semi-closed loop control, but the difficulty of mastering the debugging is better. Large, often used for high precision and large CNC machine tools. The servo motor used in the closed-loop servo is the same as the semi-closed loop, and the position detector uses a long grating, a long inductive synchronizer or a long magnetic grid.
In order to ensure that the machine tool has great process adaptability and continuous and stable working ability, the numerical control machine tool structure design is characterized by sufficient rigidity, precision, vibration resistance, thermal stability and precision retention. The mechanical drive chain of the feed system uses ball screws, static pressure screws and gapless gear pairs to minimize backlash. The machine adopts plastic anti-friction guide rails, rolling guides or hydrostatic guide rails to improve the smoothness of the movement and prevent creeping at low speeds. Thanks to the wide-speed feed servo motor and the wide-speed spindle motor, gearing and gear shifting can be eliminated or used, which simplifies the machine's transmission. The machine layout is easy to remove chips and workpieces. Some CNC machines have automatic chip conveyors and automatic workpiece exchange devices. Most CNC machine tools use a programmable controller with a microprocessor to replace a large number of relays in the power cabinet, which improves the reliability and flexibility of the machine's high-power control.
Fourth, the development trend of CNC machine tool control system:
With the rapid development of microelectronics technology, computer technology and software technology, the control system of CNC machine tools is increasingly miniaturized and multi-functional, with perfect self-diagnosis function; reliability is also greatly improved; CNC system itself will be universally realized automatically program. In the future, the types of CNC machine tools will be more diversified, and there will be more and more types of CNC machine tools that are processed in multiple processes. Laser processing and other technologies will be applied to cutting machine tools, thereby expanding the process range of multiple processes. The automation of CNC machine tools is even more Improved and with multiple monitoring functions to form a flexible manufacturing unit that is easier to incorporate into highly automated flexible manufacturing systems.
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