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On the development of CNC machine tools

With the development of science and technology and the advancement of manufacturing technology, the requirements for product quality and variety are increasing, and the proportion of small and medium batch production has increased significantly. Direction of development. In addition, CNC machine tools, as the basic equipment of flexible manufacturing units, flexible manufacturing systems and computer integrated manufacturing systems, have put forward higher requirements on the components such as numerical control devices, servo drive systems, programming, testing and monitoring, and machine tools.
1. Development of CNC system. The development of CNC system is the key to the development of CNC technology and CNC machine tools. The development of electronic components and computer technology has promoted the development of numerical control systems. The first numerical control system used electronic tube devices, and later used transistors and printed circuit boards. In the late 1960s, small-scale integrated circuit devices were used. These are so-called hard-wired numerical control systems. Since the 1970s, with the development of computer technology, a computer numerical control system (CNC) with a small computer and a microprocessor as its core has appeared. Now it has been widely adopted and has the absolute advantage

(1) Central processing unit of numerical control system. The central processing unit (CPU) of the numerical control system has been increased from 8-bit word length to 16-bit or 32-bit, and the clock frequency has been increased from 2MHz to 16MHz, 20MHz, or 32MHz. Recently, 64-bit CPUs have also appeared and began to use reduced instruction set operations The chip RSC acts as a CPU, which further improves the operation speed. In addition, the application of large-scale and ultra-large-scale integrated circuits and multiple microprocessors makes the hardware structure of the numerical control system standardized, modular, and generalized, so that the numerical control functions can be combined and expanded as required.
(2) The numerical control system is equipped with a variety of remote control and intelligent interfaces. Interfaces such as RS-232C serial interface, Rs-422 high-speed long-distance serial interface and DNC interface. The CNC system equipped with DNC interface can realize data communication between several CNC machine tools, and can also directly control several CNC machine tools. In addition, the use of advanced industrial control networks such as MAP or Ethernet in the numerical control system has created conditions for solving the networking of numerical control machine tools produced by different types and different manufacturers and the access of numerical control machine tools to manufacturing systems such as FMS and CMS.

(3) The numerical control system has good operation performance. A good man-machine interface is set on the numerical control system, and membrane buttons are generally used to reduce the number of indicators and keys; a large number of menu selection operations are used; a color CRT display screen can display not only characters, flat graphics, but also 3D dynamics Three-dimensional graphics make operation more and more simple.
(4) The reliability of the numerical control system is greatly improved. The numerical control system uses a large number of highly integrated chips, dedicated chips and synthetic integrated circuits, reducing the number of components. Electronic components use surface-mount technology (SMT), and three-dimensional high-density mounting has emerged. The components are strictly selected, which improves the quality of the hardware, reduces the power consumption, greatly improves the reliability of the system, and makes the mean time between failures (MTBF) of the CNC system reach 10,000 ~ 36000h.
(5) Development-type system. The structural hardware, software, and bus specifications of the CNC system that appeared in the late 1980s and early 1990s were all open to the outside world, providing powerful support for CNC equipment manufacturers and users to develop systems with their own technical characteristics.

2. Development of feed servo system. Feed servo system is an important part of CNC machine tools, and its circuit, motor and detection device technology have been greatly improved.
(1) The permanent magnet synchronous AC servo motor gradually replaces the DC servo motor, which improves the reliability of the motor, reduces the manufacturing cost, and basically requires no maintenance.
(2) The position, speed and current control part of the servo drive circuit has been digitized, and even the digital control is based on a single-chip microcomputer or high-speed digital signal processor. It has two-way communication with the computer of the CNC system. This avoids zero drift and improves the accuracy and stability of position and speed control. Because software control is used, the system can reference multiple control strategies and easily change the structure and parameters of the system to meet the requirements of different mechanical loads. Some It can even identify the load inertia automatically, and automatically adjust and optimize the system parameters to obtain the best static and dynamic control performance and effects.

(3) A high-speed and high-resolution position detection device is used to form a semi-closed-loop and closed-loop position control system. Incremental position detection encoder can reach 10,000 pulses / r, absolute encoder can reach 100,000 pulses / r and 0.01m / pulse resolution. When the resolution is 0.1μm / pulse, the displacement speed can reach 240m / min, which greatly improves the accuracy of position control, that is, the positioning accuracy of the machine tool.
(4) The feed servo system can not only realize the compensation of lead screw pitch error, but also make significant achievements in thermal deformation error compensation and inter-error compensation. The application of comprehensive error compensation technology can reduce machining errors by about 60%.
3. Development of CNC machine tool programming technology

(1) In addition to the language programming system for the automatic programming system of CNC machine tools, graphics programming has also made considerable progress, and automatic programming methods have been added. Physical programming and language programming have also evolved.
(2) From offline programming to online programming. Offline programming refers to the completion of programming by a manual or programming computer system, and then input into the numerical control system through an input device. Modern CNC systems have strong computing power, high computing speed, and large storage capacity. Many functions of automatic programming can be implanted into the CNC system, so that the machining programs of parts can be on the operation panel of the CNC system. Online programming, such as SymbolicFAP of FANUC Company, adopts such a programming method, which can also be called graphical human-computer dialogue programming. Some CNC systems also have the function of spatial surface interpolation. The interpolation software can process the contour of the surface according to the mathematical model of the spatial surface stored in the CNC system, which greatly simplifies programming and program input and improves processing reliability. .

(3) In the process of online programming, the CNC system can not only process geometric information, but also process information. The CNC system is equipped with a small process database or expert system related to the machining process of the machine tool. The system can automatically select the best process parameters .
4. Working condition detection, monitoring and fault diagnosis of CNC machine tools. Modern CNC machine tools are equipped with a workpiece size detection device, which periodically detects the workpiece processing size, and sends out an alarm or compensation signal in time if it finds out of tolerance. Infrared, ultrasonic emission and other monitoring devices can monitor the working conditions of the tool. When the tool wear exceeds the standard or the tool is broken, the system can give a timely alarm to change the tool to ensure the quality of the processed product.
At present, the CNC system has adopted fault self-diagnosis technologies such as startup diagnosis, running diagnosis, communication diagnosis, and expert diagnosis system to automatically find, classify, display and alarm the faults, in order to find and eliminate system faults in time.

5, using a very powerful programmable controller. For the control of auxiliary functions of CNC machine tools, relay logic hardware circuits were used before, and they were designed and manufactured by users. Modern CNC machine tools widely use built-in or independent programmable controllers (Programmable Controllers). It has a dedicated 32-bit microprocessor, the basic instruction execution time is 0.2 us / step, and there is a ladder language program above 16000tf. C can be used. Language or Pas language to compile the PC program, the program capacity is 68-256KB, there is a high-speed window between the PC and CNC. When programming in C, you can work in the development environment of a personal computer. Utilizing the high-speed processing function of the PC, the CNC and PC are organically combined, and the monitoring function of the ladder diagram (Ladder) can be used to make the fault diagnosis and maintenance of the machine tool more convenient.
6. The host of the machine tool. There are also many new developments in the host of CNC machine tools. The performance is as follows:

(1) The main moving parts continue to achieve high-speed electrification. In order to increase the speed and speed of the main movement, and reduce the mechanical transmission chain, in addition to using DC speed motors and AC variable frequency speed motors to drive the spindle components, in recent years, more machine tools using built-in spindle motors have appeared. On the motor rotor, the spindle speed is greatly increased. The maximum spindle speed can reach 10000-1000ymin, and it can be raised from zero to the maximum speed in only 1.8s.
(2) Increase processing functions. Centralizing processes can increase productivity and form accuracy of the workpiece. For example, the use of automatic tool changers, automatic workpiece change mechanisms, CNC fixtures, etc. to develop milling and boring machining centers, turning machining centers and other machine tools; the use of indexing spindle headstock, forming five-sided machining capabilities

(3) Adopt mechatronics and fully enclosed structure. The CNC machine tool integrates the NC device, high-power control device and hydraulic transmission oil tank, which were separated from the host machine in the past, with the host machine, making the structure compact, reducing the pipeline, and reducing the footprint; the part processing area is completely closed to the peepable Inside the casing, an automatic chip removal device is used to improve the processing environment and conditions; pneumatic and hydraulic mechanisms are used to control various auxiliary motion mechanisms, and centralized compressed air power is used to eliminate the energy consumption, heat and noise of the hydraulic pump, etc. defect.
(4) The large parts of the main machine adopt a welded structure and a reasonable structural form, which can obtain extremely high structural rigidity and seismic resistance while reducing the weight of the machine tool. The stepless speed regulation motor is adopted to shorten the length of the mechanical transmission chain, reduce the noise and improve the mechanical efficiency. Transmission and guiding elements such as ball screw nut pairs, static pressure screw nut pairs, rolling guides, hydrostatic guides, and plastic-guided guides with low frictional resistance greatly improve transmission rigidity, reduce frictional resistance, and thereby improve Dynamic response performance of feed motion and smooth performance of low speed motion.

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