The rapid advancement of CNC system technology has provided conditions for the technological progress of CNC machine tools. In order to meet the needs of the market and meet the higher requirements of modern manufacturing technology for CNC technology, the current development of world CNC technology and its equipment is mainly reflected in the following technical characteristics:
1. High speed
The development of CNC machine tools towards high-speed direction can not only significantly improve machining efficiency and reduce machining costs, but also improve the surface machining quality and accuracy of parts. Ultra high speed machining technology has broad applicability for achieving low-cost production in the manufacturing industry.
Since the 1990s, countries in Europe, the United States, and Japan have been competing to develop and apply a new generation of high-speed CNC machine tools, accelerating the pace of high-speed development of machine tools. New breakthroughs have been made in the high-speed spindle unit (electric spindle, speed 15000-100000 r/min), high-speed and high acceleration/deceleration feed motion components (fast moving speed 60-120m/min, cutting feed speed up to 60m/min), high-performance CNC and servo systems, and CNC tool systems, reaching new technological levels. With the resolution of key technologies in a series of technical fields such as ultra high speed cutting mechanism, ultra hard wear-resistant long-life tool materials and abrasive grinding tools, high-power high-speed electric spindle, high acceleration/deceleration linear motor driven feed components, high-performance control systems (including monitoring systems) and protective devices, a technical foundation has been provided for the development and application of the new generation of high-speed CNC machine tools.
At present, in ultra high speed machining, the cutting speed of turning and milling has reached over 5000-8000m/min; The spindle speed is above 30000 rpm (some can reach up to 100000 r/min); The movement speed (feed rate) of the workbench: above 100m/min (some up to 200m/min) at a resolution of 1 micrometer, and above 24m/min at a resolution of 0.1 micrometer; Automatic tool changing speed within 1 second; The feed rate for small line interpolation reaches 12m/min.
2. High precision
The development of CNC machine tools from precision machining to ultra precision machining is a direction that industrial powers around the world are committed to. Its accuracy ranges from micrometer level to submicron level, and even to nanometer level (<10nm), and its application range is becoming increasingly widespread.
Currently, under the requirement of high-precision machining, the machining accuracy of ordinary CNC machine tools has increased from ± 10 μ Increase m to ± 5 μ M; The machining accuracy of precision machining centers ranges from ± 3 to 5 μ m. Increase to ± 1-1.5 μ m. Even higher; The ultra precision machining accuracy has entered the nanometer level (0.001 micrometers), and the spindle rotation accuracy is required to reach 0.01~0.05 micrometers, with a machining roundness of 0.1 micrometers and a machining surface roughness of Ra=0.003 micrometers. These machine tools generally use vector controlled variable frequency drive electric spindles (integrated with the motor and spindle), with radial runout of the spindle less than 2 µ m, axial displacement less than 1 µ m, and shaft unbalance reaching G0.4 level.
The feed drive of high-speed and high-precision machining machine tools mainly includes two types: "rotary servo motor with precision high-speed ball screw" and "linear motor direct drive". In addition, emerging parallel machine tools are also easy to achieve high-speed feed.
Due to its mature technology and wide application, ball screws not only achieve high precision (ISO3408 level 1), but also have relatively low cost of achieving high-speed machining. Therefore, they are still used by many high-speed machining machines to this day. The current high-speed machining machine tool driven by ball screw has a maximum movement speed of 90m/min and an acceleration of 1.5g.
Ball screw belongs to mechanical transmission, which inevitably involves elastic deformation, friction, and reverse clearance during the transmission process, resulting in motion hysteresis and other nonlinear errors. In order to eliminate the impact of these errors on machining accuracy, linear motor direct drive was applied to machine tools in 1993. As it is a "zero transmission" without intermediate links, it not only has small motion inertia, high system stiffness, and fast response, It can achieve high speed and acceleration, and its stroke length is theoretically unrestricted. The positioning accuracy can also reach a high level under the action of high-precision position feedback system, making it an ideal driving method for high-speed and high-precision machining machine tools, especially medium and large machine tools. At present, the maximum fast moving speed of high-speed and high-precision machining machines using linear motors has reached 208 m/min, with an acceleration of 2g, and there is still room for development.
3. High reliability
With the development of networked applications of CNC machine tools, the high reliability of CNC machine tools has become a goal pursued by CNC system manufacturers and CNC machine tool manufacturers. For an unmanned factory that works two shifts a day, if it is required to work continuously and normally within 16 hours with a failure free rate of P (t)=99% or more, the average time between failures (MTBF) of the CNC machine tool must be greater than 3000 hours. For only one CNC machine tool, the failure rate ratio between the host and the CNC system is 10:1 (the reliability of CNC is one order of magnitude higher than that of the host). At this point, the MTBF of the CNC system must be greater than 33333.3 hours, and the MTBF of the CNC device, spindle, and drive must be greater than 100000 hours.
The MTBF value of current foreign CNC devices has reached over 6000 hours, and the driving device has reached over 30000 hours. However, it can be seen that there is still a gap from the ideal target.
4. Compounding
In the process of parts processing, a large amount of useless time is consumed in workpiece handling, loading and unloading, installation and adjustment, tool change, and spindle speed up and down. In order to minimize these useless times as much as possible, people hope to integrate different processing functions on the same machine tool. Therefore, compound function machine tools have become a rapidly developing model in recent years.
The concept of machine tool composite machining in the field of flexible manufacturing refers to the ability of a machine tool to automatically perform multi process machining of the same or different types of process methods according to a CNC machining program after clamping the workpiece in one go, in order to complete various machining processes such as turning, milling, drilling, boring, grinding, tapping, reaming, and expanding a complex shaped part. As for prismatic parts, machining centers are the most typical machine tools that perform multi process composite processing using the same process method. It has been proven that machine tool composite machining can improve machining accuracy and efficiency, save space, and especially shorten the machining cycle of parts.
5. Polyaxialization
With the popularization of 5-axis linkage CNC systems and programming software, 5-axis linkage controlled machining centers and CNC milling machines (vertical machining centers) have become a current development hotspot. Due to the simplicity of 5-axis linkage control in CNC programming for ball end milling cutters when machining free surfaces, and the ability to maintain a reasonable cutting speed for ball end milling cutters during the milling process of 3D surfaces, As a result, the roughness of the machining surface is significantly improved and the machining efficiency is greatly improved. However, in 3-axis linkage controlled machine tools, it is impossible to avoid the end of the ball end milling cutter with a cutting speed close to zero from participating in cutting. Therefore, 5-axis linkage machine tools have become the focus of active development and competition among major machine tool manufacturers due to their irreplaceable performance advantages.
Recently, foreign countries are still researching 6-axis linkage control using non rotating cutting tools in machining centers. Although their machining shape is not restricted and the cutting depth can be very thin, the machining efficiency is too low and it is difficult to be practical.
6. Intelligence
Intelligence is a major direction for the development of manufacturing technology in the 21st century. Intelligent machining is a type of machining based on neural network control, fuzzy control, digital network technology, and theory. It aims to simulate the intelligent activities of human experts during the machining process, in order to solve many uncertain problems that require manual intervention. The content of intelligence includes various aspects in CNC systems:
To pursue intelligent processing efficiency and quality, such as adaptive control and automatic generation of process parameters;
To improve driving performance and facilitate intelligent connection, such as feedforward control, adaptive calculation of motor parameters, automatic identification of loads, automatic selection of models, self tuning, etc;
Simplified programming and intelligent operation, such as intelligent automatic programming, intelligent human-machine interface, etc;
Intelligent diagnosis and monitoring facilitate system diagnosis and maintenance.
There are many intelligent cutting and machining systems under research in the world, among which the Japan Intelligent CNC Device Research Association's intelligent machining solutions for drilling are representative.
7. Networking
The networked control of machine tools mainly refers to the network connection and network control between the machine tool and other external control systems or upper computers through the equipped CNC system. CNC machine tools generally first face the production site and internal LAN of the enterprise, and then connect to the outside of the enterprise through the Internet, which is called Internet/Intranet technology.
With the maturity and development of network technology, the industry has recently proposed the concept of digital manufacturing. Digital manufacturing, also known as "e-manufacturing", is one of the symbols of modernization in mechanical manufacturing enterprises and the standard supply method for international advanced machine tool manufacturers today. With the widespread adoption of information technology, more and more domestic users require remote communication services and other functions when importing CNC machine tools. On the basis of the widespread adoption of CAD/CAM, mechanical manufacturing enterprises are increasingly using CNC machining equipment. CNC application software is becoming increasingly rich and user-friendly. Virtual design, virtual manufacturing and other technologies are increasingly being pursued by engineering and technical personnel. Replacing complex hardware with software intelligence is becoming an important trend in the development of contemporary machine tools. Under the goal of digital manufacturing, a number of advanced enterprise management software such as ERP have emerged through process reengineering and information technology transformation, creating higher economic benefits for enterprises.
8. Flexibility
The trend of CNC machine tools towards flexible automation systems is to develop from point (CNC single machine, machining center, and CNC composite machining machine), line (FMC, FMS, FTL, FML) to surface (independent manufacturing island, FA), and body (CIMS, distributed network integrated manufacturing system), and on the other hand, to focus on application and economy. Flexible automation technology is the main means for the manufacturing industry to adapt to dynamic market demands and rapidly update products. It is the mainstream trend of manufacturing development in various countries and the fundamental technology in the advanced manufacturing field. Its focus is on improving the reliability and practicality of the system, with the goal of easy networking and integration; Emphasize the development and improvement of unit technology; CNC single machine is developing towards high precision, high speed, and high flexibility; CNC machine tools and their flexible manufacturing systems can be easily connected with CAD, CAM, CAPP, MTS, and develop towards information integration; The development of network systems towards openness, integration, and intelligence.
9. Greenization
The metal cutting machine tools of the 21st century must prioritize environmental protection and energy conservation, that is, to achieve the greening of cutting processes. At present, this green processing technology mainly focuses on not using cutting fluid, mainly because cutting fluid not only pollutes the environment and endangers worker health, but also increases resource and energy consumption. Dry cutting is generally carried out in an atmospheric atmosphere, but it also includes cutting in special gas atmospheres (nitrogen, cold air, or using dry electrostatic cooling technology) without the use of cutting fluid. However, for certain machining methods and workpiece combinations, dry cutting without the use of cutting fluid is currently difficult to apply in practice, so quasi dry cutting with minimal lubrication (MQL) has emerged. Currently, 10-15% of large-scale mechanical processing in Europe uses dry and quasi dry cutting. For machine tools such as machining centers that are designed for multiple machining methods/workpiece combinations, quasi dry cutting is mainly used, usually by spraying a mixture of extremely small amounts of cutting oil and compressed air into the cutting area through the hollow channel inside the machine spindle and tool. Among various types of metal cutting machines, the gear hobbing machine is the most commonly used for dry cutting.
In short, the progress and development of CNC machine tool technology have provided favorable conditions for the development of modern manufacturing industry, promoting the development of manufacturing towards a more humanized direction. It can be foreseen that with the development of CNC machine tool technology and the widespread application of CNC machine tools, the manufacturing industry will usher in a profound revolution that can shake the traditional manufacturing model.