Ultra-precision machining technology is an important way to improve the performance, quality, service life and reliability of electromechanical products, as well as saving materials and energy. For example: improving the machining accuracy of cylinders and pistons can improve the efficiency and horsepower of automobile engines and reduce fuel consumption; improving the machining accuracy of rolling elements and raceways of rolling bearings can increase the speed of bearings, reduce vibration and noise; improve flatness of processing of magnetic disks, thereby reducing the gap between it and the magnetic head, can greatly increase the storage capacity of the magnetic disk; improving the marking accuracy of the semiconductor device (reducing the line width, increasing the density) can improve the integration of the microelectronic chip, etc.
Introduction to Ultra-Precision Machining
Generally, according to the machining accuracy, machining can be divided into three stages: general machining, precision machining, and ultra-precision machining. Ultra-precision machining refers to sub-micron (dimensional error is 0.3 ~ 0.03µm, surface roughness is Ra0.03 ~ 0.005µm) and nanoscale (accuracy error is 0.03µm, surface roughness is less than Ra0.005µm) precision machining. The process methods and technical measures taken to realize these processing are called ultra-finishing technology. Coupled with measurement technology, environmental protection and materials and other issues, this technology is generally referred to as ultra-precision engineering. Ultra-precision machining mainly includes three fields: ultra-precision machining of diamond tools, which can process various mirror surfaces. It has successfully solved the processing of large parabolic mirrors for laser fusion systems and astronomical telescopes; ultra-precision grinding processing such as coating surface processing of high-density hard disks and processing of large-scale integrated circuit substrates; ultra-precision special processing such as large-scale integrated circuit wafers are processed by electron beam and ion beam etching methods, and the line width can reach 0.1µm. If processed by scanning tunneling electron microscope (STM), the line width can reach 2-5nm.
a. Ultra-precision cutting
Ultra-precision cutting starts with SPDT technology, which is supported by air bearing spindles, pneumatic slides, high rigidity, high precision tools, feedback control and ambient temperature control to achieve nanoscale surface roughness. Diamond cutters are mostly used for milling, which is widely used in the processing of copper plane and aspherical optical components, plexiglass, plastic products (such as plastic lenses for cameras, contact lens lenses, etc.), ceramics and composite materials. The future development trend is to use coating technology to improve the wear of diamond tools when working hardened steel. In addition, the processing of tiny parts such as MEMS components requires tiny tools. At present, the size of tiny tools can reach about 50-100 μm. However, if the machining geometric features are at the sub-micron or even nano-scale, the tool diameter must be reduced. The development trend is to use nanomaterials. Such as carbon nanotubes to make ultra-small diameter turning tools or milling cutters.
b. Ultra-precision abrasive machining
Ultra-precision abrasive machining is a mirror grinding method developed on the basis of general precision grinding. The processing objects of ultra-precision grinding are mainly brittle and hard metal materials, semiconductor materials, ceramics, glass, etc. After grinding, a large number of extremely fine grinding marks are left on the machined surface, and the residual height is extremely small. In addition to the sliding, friction and polishing effects of the micro-blade, a machined surface with high precision and low surface roughness can be obtained. Grinding can process cylindrical parts with a roundness of 0.01μm, a dimensional accuracy of 0.1μm and a surface roughness of Ra0.005μm.
c. Ultra-precision grinding
Ultra-precision grinding includes processing methods such as mechanical grinding, chemical mechanical grinding, floating grinding, elastic emission processing, and magnetic grinding. The key conditions for ultra-precision grinding are a virtually vibration-free grinding motion, precise temperature control, a clean environment, and a fine and uniform abrasive. The sphericity of ultra-precision grinding reaches 0.025μm, and the surface roughness Ra reaches 0.003μm.
d. Ultra-precision special machining
Ultra-precision special machining mainly includes laser beam machining, electron beam machining, ion beam machining, micro EDM, fine electrolytic machining and electrolytic grinding, ultrasonic electrolytic machining, ultrasonic electrolytic grinding, ultrasonic EDM and other composite machining. Laser and electron beam processing can achieve drilling, precision cutting, forming cutting, etching, lithography exposure, and processing laser anti-counterfeiting signs; ion beam processing can achieve atomic and molecular level cutting; the removal of fine metal materials can process fine shafts, holes, narrow planes and curved surfaces; fine electrolytic machining can achieve nano-level precision, and the surface will not generate machining stress, and is often used for mirror polishing, mirror thinning, and some applications that require no stress processing.
The United States, the United Kingdom and Japan are internationally leading countries in ultra-precision machining technology. The ultra-precision machining technology in these countries not only has a high overall level of complete sets, but also has a very high degree of commercialization.
In the 1950s, the United States developed the ultra-precision cutting technology of diamond tools, called "SPDT technology" (SinglePoint Diamond Turning) or "micro-inch technology" (1 micro-inch = 0.025μm), and developed the corresponding ultra-precision air bearing spindle. Precision machine tool, used to process large spherical and aspherical parts for laser fusion mirrors, tactical missiles and manned spacecraft.
The Cranfield Institute of Precision Engineering (CUPE), which belongs to the Cranfield Institute of Technology in the United Kingdom, is a unique representative of the British ultra-precision machining technology level. For example, the Nanocentre (Nano Machining Center) produced by CUPE can be used for ultra-precision turning, with grinding head, and ultra-precision grinding.
Compared with the United States and the United Kingdom, Japan's research on ultra-precision machining technology started relatively late, but it is the fastest-growing country in the world in ultra-precision machining technology.
Development Trend of Ultra-Precision Machining
It is developing towards the direction of high precision, high efficiency, large-scale, miniaturization, intelligence, process integration, online processing and testing integration, and greening.
a. High precision and high efficiency
With the continuous progress of science and technology, the requirements for precision, efficiency and quality are getting higher and higher, and high precision and high efficiency have become the eternal theme of ultra-precision machining. Ultra-precision cutting and grinding technology can effectively improve processing efficiency, CMP and EEM technology can ensure processing accuracy, and semi-fixed abrasive processing methods, electrolytic magnetic grinding, magnetorheological abrasive flow processing and other composite processing methods can take into account the efficiency and will become the trend of ultra-precision machining.
b. Large-scale and miniaturized
Due to the development of aerospace and other technologies, large-scale optoelectronic devices require large-scale ultra-precision machining equipment, such as the ultra-precision machining tool machine for large-scale optical devices with a processing diameter of 2.4-4m developed in the United States. At the same time, with the development of micro-mechanical electronics, optoelectronic information and other fields, ultra-precision processing technology is developing towards miniaturization, such as micro-sensors, micro-drive components and power units, micro-aviation and space flight devices, etc. require micro ultra-precision processing equipment.
Using intelligent equipment to reduce the dependence of processing results on manual experience has always been the goal pursued in the manufacturing field. The intelligent degree of processing equipment is directly related to the stability and efficiency of processing, which is more obvious in ultra-precision processing.
d. Process integration
Today's competition among enterprises tends to be fierce, and high production efficiency has increasingly become a condition for enterprises to survive. On the other hand, there is an obvious growing trend towards using one machine for multiple operations (eg. turning, drilling, milling, grinding, finishing).
e. Integration of online processing and testing
Due to the high precision of ultra-precision machining, it is necessary to develop the integrated technology of online processing and testing to ensure product quality and improve productivity. At the same time, because the accuracy of the processing equipment itself is sometimes difficult to meet the requirements, the use of online detection, working condition monitoring and error compensation methods can improve the accuracy and ensure the processing quality requirements.
f. Green technology
Abrasive machining is the main method of ultra-precision machining. The manufacture of abrasives, the consumption of abrasives in machining, the consumption of energy and materials during machining, and the large amount of machining fluids used in machining have caused great burdens on the environment. To this end, countries are actively investing in green ultra-precision machining technology to reduce the environmental burden.