CNC machine tools were first born in the United States. In 1948, Parsons of the United States put forward the idea of a CNC machine tool when developing a prototype machine tool for processing helicopter blade contour inspection. Later, it was commissioned by the US Air Force to cooperate with the Massachusetts Institute of Technology, and in 1952 trial-produced the world's first machine. Three-coordinate CNC vertical milling machine, whose CNC system uses electronic tubes. Starting in 1960, Germany, Japan, China, etc. have successively developed, produced and used CNC machine tools. In 1968, Beijing No. 1 Machine Tool Factory developed the first CNC machine tool in China. In 1974, microprocessors were directly used in CNC machine tools, which further promoted the popularization and rapid development of CNC machine tools.
Due to the continuous development of microelectronics and computer technology, the numerical control system of numerical control machine tools has been constantly updated. So far, it has experienced the following generations of changes:
The first generation of numerical control (1952~1959): a hardware numerical control system composed of electronic tubes;
The second generation of numerical control (1959~1965): a hardware numerical control system based on transistor circuits;
The third generation of CNC (started in 1965): hardware CNC system using small and medium-scale integrated circuits;
The fourth generation of numerical control (since 1970): a small general-purpose electronic computer numerical control system using large-scale integrated circuits;
The fifth generation of numerical control (started in 1974): a numerical control system controlled by a microcomputer;
The sixth generation of CNC (started in 1990): General CNC system using industrial PCs.
The first three generations are the first stage. The numerical control system is mainly composed of hardware connected structure, called hardware numerical control; the latter three generations are called computer numerical control, and its functions are mainly completed by software.
In the past 20 years, with the development of science and technology, the rise and maturity of advanced manufacturing technology have put forward higher requirements for CNC technology. At present, CNC technology is mainly developing in the following directions:
1) Development towards high speed and high precision
Speed and accuracy are two important indicators of CNC machine tools, which are directly related to the quality and grade of the product, the production cycle of the product, and the competitiveness in the market.
In terms of machining accuracy, in the past 10 years, the machining accuracy of ordinary CNC machine tools has increased from 10μm to 5μm, and the precision machining center has increased from 3~5μm to 1~1.5μm, and ultra-precision machining accuracy has begun to enter the nanometer level ( 0.001μm). The improvement of machining accuracy is not only the use of ball screw pairs, hydrostatic guides, linear rolling guides, magnetic floating guides and other components, which improve the control accuracy of the CNC system, and the application of high-resolution position detection devices, but also the use of various errors Compensation technology, such as screw pitch error compensation, tool error compensation, thermal deformation error compensation, spatial error comprehensive compensation, etc.
In terms of processing speed, high-speed processing originated in the early 1990s. It is characterized by the application of electric spindle and linear motor, which greatly increases the spindle speed. The feed speed is above 60m/min, and the feed acceleration and deceleration reach 1~ Above 2g, the spindle speed can reach above 100000r/min. High-speed feed requires the numerical control system to have fast calculation speed and short sampling period. It also requires the numerical control system to have sufficient advance path acceleration (deceleration) speed optimization preprocessing capabilities (forward processing), and some systems can process 5000 program segments in advance. In order to ensure the processing speed, the high-end CNC system can change the feed speed 2000 to 10000 times per second.
2) Development towards flexibility and functional integration
While increasing the flexibility of a single machine, CNC machine tools are developing towards unit flexibility and systemization. For example, CNC multi-axis machining centers, tool-changing box-changing machining centers and other flexible and efficient processing equipment have appeared; The machine tool constitutes the flexible manufacturing cell (Flexible Manufacturing Cell, FMC), the flexible manufacturing system (FMS), and the flexible manufacturing line (FML) of the underlying processing equipment.
On modern CNC machine tools, automatic tool changers and automatic worktable exchange devices have become basic devices. With the development of CNC machine tools in the direction of flexibility, the integration of functions is more reflected in: automatic loading and unloading of workpieces, automatic positioning of workpieces, automatic tool setting, automatic measurement and compensation of workpieces, integration of drilling, turning, boring, milling, and grinding. Integrated "universal processing" and "complete processing" integrating loading and unloading, processing and measurement.
3) Development towards intelligence
With the continuous penetration and development of artificial intelligence in the computer field, numerical control systems are developing in the direction of intelligence. In the new generation of CNC systems, due to the adoption of control mechanisms such as "Evolutionary Computation" (Evolutionary Computation), "Fuzzy System" and "Neural Network", the performance is greatly improved, and the machining process is self-contained. Adaptive control, automatic load identification, self-generation of process parameters, dynamic compensation of motion parameters, intelligent diagnosis, intelligent monitoring and other functions.
(1) The introduction of adaptive control technology. In the actual machining process, there are many factors that affect the machining accuracy, such as uneven workpiece allowance, uneven material hardness, tool wear, workpiece deformation, and thermal deformation of machine tools. These factors are difficult to predict in advance, so that in actual machining, it is difficult to use the best parameters for cutting. The purpose of introducing adaptive control technology is to enable the processing system to automatically adjust the cutting parameters and other parameters according to the changes in cutting conditions, so that the processing process can maintain the best working state, so as to obtain higher processing accuracy and smaller surface roughness. It can improve the service life of the tool and the production efficiency of the equipment.
(2) Fault self-diagnosis and self-repair function In the entire working state of the system, the CNC system's built-in program is used to perform self-diagnosis and self-inspection on the CNC system itself and various equipment connected to it at any time. Once a failure occurs, immediately take measures such as shutdown, and carry out a failure alarm, prompting the location and cause of the failure, etc., and use "redundancy" technology to automatically take the failed module offline and connect the standby module.
(3) Automatic tool life detection and automatic tool change function Use infrared, acoustic emission, laser and other detection methods to detect tools and workpieces. If the workpiece is out of tolerance, tool wear and damage, etc., it will alarm, automatically compensate or replace the tool in time to ensure product quality.
(4) Pattern recognition technology Application of image recognition and voice control technology enables the machine tool to recognize the pattern itself and process it according to natural language commands.
(5) Intelligent AC servo drive technology At present, intelligent servo systems that can automatically identify loads and automatically adjust parameters have been studied, including intelligent spindle AC drive devices and feed servo drive devices, so that the drive system can get the best operation.
4) Development towards high reliability
The reliability of CNC machine tools has always been the main indicator that users are most concerned about, and it mainly depends on the reliability of each servo drive unit of the CNC system. In order to improve reliability, the following measures are currently taken:
(1) The use of higher integrated circuit chips, the use of large-scale or ultra-large-scale dedicated and hybrid integrated circuits to reduce the number of components and improve reliability.
(2) Through the softwareization of hardware functions to meet the requirements of various control functions, at the same time, through the modularization, standardization, generalization and serialization of the hardware structure, the production batch and quality of the hardware are improved.
(3) Enhance fault self-diagnosis, self-recovery and protection functions, and perform fault diagnosis and alarm on the hardware, software and various external devices in the system. When processing overtravel, tool damage, interference, power failure and other accidents occur, corresponding protections are automatically performed.
5) Development towards network
The networking of CNC machine tools will greatly meet the needs of flexible production lines, flexible manufacturing systems, and manufacturing companies for information integration. It will also realize new manufacturing modes, such as Agile Manufacturing (AM) and Virtual Enterprise (VE). , The basic unit of Global Manufacturing (GM). The current advanced CNC system provides users with powerful networking capabilities. In addition to the RS232C interface, it also has a DNC interface with remote buffering function, which can realize data communication between multiple CNC machine tools and directly control multiple CNC machine tools. Some have been equipped with the function of communicating with the industrial local area network and the network interface, which promotes system integration and information integration, making remote online programming, remote simulation, remote operation, remote monitoring and remote fault diagnosis possible.
6) Development towards standardization
Numerical control standards are a trend in the development of manufacturing informatization. More than 50 years after the birth of CNC technology, the information exchange is based on the ISO6983 standard, that is, the G and M codes are used to describe the machining process. Obviously, this process-oriented description method has become increasingly unable to meet the rapid development of modern CNC technology. need. For this reason, a new CNC system standard ISO14649 (STEP-NC) is being studied and formulated internationally. Its purpose is to provide a neutral mechanism that does not depend on a specific system and can describe a unified data model throughout the product life cycle. Realize the standardization of the entire manufacturing process and even product information in various industrial fields.
7) Development towards driving parallelization
Parallel machine tools (also known as virtual axis machine tools) are the most revolutionary breakthroughs in the motion structure of machine tools in the 20th century and have attracted widespread attention. The parallel machine tool (see Figure 1-7) consists of a base, a platform, and multiple telescopic rods. The two ends of each rod are respectively connected to the base by a spherical support, and are composed of a servo motor and a ball screw. The telescopic motion is realized according to the numerical control instructions, so that the motion platform drives the spindle component or the worktable component to move at any track. The structure of the parallel machine tool is simple but the mathematics is complicated. The movement of the entire platform involves considerable mathematical operations. Therefore, the parallel machine tool is a knowledge-intensive mechanism. Compared with traditional tandem machine tools, parallel machine tools have the advantages of high rigidity, high load-bearing capacity, high speed, high precision, light weight, simple mechanical structure, low manufacturing cost, and high degree of standardization. They have been successfully applied in many fields.
The hybrid CNC machine tool composed of parallel and series at the same time not only has the advantages of parallel machine tools, but also has more practical value in use. It is a kind of promising CNC machine tools.