This paper introduces the mechanism of precision grinding and ultra-precision grinding, explains the research status of precision grinding machines and precision grinding and ultra-precision grinding technologies, and analyzes the development trend of precision grinding and ultra-precision grinding.
[Abstract] This paper introduces the mechanism of precision grinding and ultra-precision grinding, explains the research status of precision grinding machines and precision grinding and ultra-precision grinding technologies, and analyzes the development trend of precision grinding and ultra-precision grinding.
0 Introduction
Grinding is the main precision and ultra-precision processing method, which is generally divided into ordinary grinding, precision grinding, and ultra-precision grinding. The grinding accuracy they can achieve has different accuracy ranges in different periods of production development.
At present, ordinary grinding generally refers to the grinding method with a surface roughness of Ra between 0.16 and 1.25 μm and a processing accuracy of > 1 μm. The current precision that can be achieved by precision grinding is generally a surface roughness of Ra between 0.04 and 1.25 μm and a processing accuracy of 1 to 0.5 μm. Ultra-precision grinding is the contemporary grinding method that can achieve the lowest grinding surface roughness value and the highest processing accuracy. The surface roughness can reach Ra≤0.01 μm, the accuracy ≤0.01 μm, and even enter the nanometer level.
1. Mechanism of precision and ultra-precision grinding
Precision grinding generally uses high-hardness abrasive grinding wheels such as diamond and cubic boron nitride. It mainly relies on fine dressing of the grinding wheel. Diamond dressing tools are used with extremely small and uniform micro-feed (10-15 mm/min) to obtain numerous equal-height micro-edges. The grinding marks on the processed surface are very fine. Finally, spark-free polishing is used. Due to the combined effects of micro-cutting, sliding and friction, low surface roughness values and high precision requirements are achieved. Ultra-precision grinding uses a smaller dressing lead and cutting amount to dress the grinding wheel, and relies on ultra-fine abrasives to grind high micro-edges [1]. The mechanism of precision and ultra-precision grinding is somewhat different from that of ordinary grinding.
1) Ultra-micro removal. Use a smaller dressing lead and dressing depth to finely dress the grinding wheel, so that the abrasive grains are finely broken to produce micro-edges. One abrasive grain becomes multiple abrasive grains, which is equivalent to the grinding wheel becoming finer in grain size. The micro-cutting action of the micro-edge forms low roughness.
2) Contour cutting effect of micro-edges. Micro-edges are formed by fine dressing of the grinding wheel. There are many micro-edges distributed at the same depth on the surface of the grinding wheel, and the contour is good, so the residual height of the machined surface is extremely small [ .
3) Single-grain grinding process. Abrasive grains are elastic bodies with elastic support and large negative rake cutting edges. When a single abrasive grain is in contact with the workpiece during grinding, it starts with an elastic zone, followed by a plastic zone, a cutting zone, a plastic zone, and finally an elastic zone, which is consistent with the shape of chip formation. Ultra-precision grinding has micro-cutting action, plastic flow and elastic destruction, as well as sliding friction. When the blade is sharp and has a certain grinding depth, the micro-cutting action is stronger; if the blade is not sharp enough, or the grinding depth is too shallow, the abrasive cutting edge cannot cut into the workpiece, resulting in plastic flow, elastic destruction and sliding friction.
4) Continuous grinding process. The workpiece rotates continuously, the grinding wheel cuts in continuously, and the entire grinding system begins to deform elastically. The difference between the grinding cut-in (grinding depth) and the actual workpiece size reduction is the elastic cut-out. After that, the grinding cut-in gradually becomes equal to the actual workpiece size reduction, and the grinding system is in a stable state. Finally, the grinding cut-in reaches the given value, but the elastic deformation of the grinding system gradually returns to the non-cutting grinding state.
2. Development of precision and ultra-precision grinders
Precision grinders are the basis of precision grinding. The development direction of today's precision grinding machine technology is high precision, integration, and automation.
Cranfield University Precision Engineering (CUPE) in the UK is one of the earliest companies engaged in the development of ultra-precision grinding machine tools. The OAGM2500 large ultra-precision grinder developed by the company is the largest ultra-precision grinding equipment to date, mainly used for ultra-precision grinding of hard and brittle materials such as optical glass [3, 6]. The Nanocentre (nano-machining center) produced by CUPE is equipped with a grinding head and can perform ultra-precision grinding. The shape accuracy of the workpiece can reach 0.1 μm and the surface roughness Ra < 10 nm [3]. In 2003, Cranfield University in the UK and Cranfield Precision Engineering Co., Ltd. jointly developed a new type of ultra-precision grinder that can complete the ductile domain nano-grinding of silicon wafers in one process with high processing efficiency, and obtain good surface and sub-surface integrity. It is said that the use of this ultra-precision grinder to grind large-diameter silicon wafers can completely replace the grinding and etching processes of traditional processes, and is even expected to replace polishing.
The ultra-precision grinder produced by Moore Nanotechnology System in the United States uses an ultra-precision hydrostatic guideway to maintain a straightness of 0.3 μm, a machining geometric accuracy of 0.1 μm, and a surface roughness of Ra = 5 nm［8］. In order to meet the needs of ultra-precision grinding of larger-caliber optical parts and optical parts made of hard and brittle materials, the LLNL laboratory in the United States developed the next-generation ultra-precision optical processing equipment POGAL (Optical Grinder and Lathe) in 2006. The axial and radial precision technical indicators of its spindle are 50 nm.
Japan started research on ultra-precision machining technology later than the United States. It was developed in response to the needs of civilian industries such as electronics and optics. Based on ultra-precision lathes and combined with ELID mirror grinding technology, it developed ELID precision CNC mirror grinders for machining non-spherical surfaces of rotating bodies. Later, it developed three-coordinate linkage CNC ELID precision mirror grinders, which can achieve mirror machining of precision free-form surfaces. Its ultra-precision grinding equipment mainly includes the ULG-100A (H) ultra-precision non-spherical machining machine tool produced by Toshiba Machinery Company in the 1990s. The machine tool spindle adopts high-rigidity air static pressure bearings, two-axis full closed-loop control, and the axis displacement resolution is 0.01 μm. It can process various optical parts and non-spherical lens injection molding metal (copper, non-electrolytic nickel) molds, and compression molding ceramic (WC) molds. The molding molds are turned and ground using diamond tools or grinding wheels to achieve mirror quality.
The most typical precision grinding equipment currently produced by Schneider in Germany is the SCGA121 ultra-precision grinding center for aspheric surfaces. This machine uses a high-rigidity concrete polymer as the bed and is multi-axis CNC-controlled. It can perform both large-removal ordinary grinding wheel grinding and cup-shaped grinding wheel grinding. At the same time, it is integrated with the SCGA121 aspheric polishing machine and the AU aspheric online detection system to achieve ultra-precision, efficient and flexible automated processing of aspheric optical components [6]. The Multi2Nano fully automatic series nano-grinder developed by the German G&N company adopts the principle of self-rotating grinding and is equipped with two grinding wheel spindles for rough and fine grinding respectively. It has 3 (or 4) operating stations and can automatically complete the rough grinding, fine grinding, cleaning or loading and unloading of silicon wafers. It can be used for ultra-precision grinding of 300 mm silicon wafers to obtain nano-level mirror surfaces, and can be used for back grinding to thin silicon wafers to 100 to 150 μm.
The FSGJ-I developed by the State Key Laboratory of Applied Optics at the Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, integrates milling, edge grinding, precision polishing and testing. The Ministry of Education Key Laboratory of Precision and Special Processing at Dalian University of Technology is developing and researching large-diameter silicon wafer ultra-precision grinding systems and equipment based on the principle of self-rotation grinding, and has achieved initial results.
3. Development of precision and ultra-precision grinding technology
In recent years, foreign countries have made many achievements in the development and research of precision and ultra-precision grinding technology, mainly in the research of new ELID (Electrolytic In process Dressing) mirror grinding technology and the application of processing silicon wafers and non-spherical parts. The average particle size of diamond abrasive grains of resin bonded grinding wheels used for ultra-precision mirror grinding can be as small as 4 μm. The integrated manufacturing system of 300 mm diameter silicon wafers uses single crystal diamond grinding wheels to enable ductile grinding and finishing processing to be performed on the same device, so that the surface roughness of silicon wafers reaches Ra <1 nm (Ry <5 ~ 6 nm) and the flatness reaches <0.2 μm/300 mm.
In 1987, Professor Masaru Omori of the RIKEN Institute of Physical and Chemical Research in Japan successfully developed a new ELID mirror grinding process for online dressing of grinding wheels. ELID mirror grinding technology uses online electrolytic dressing to continuously dress the grinding wheel to obtain a constant cutting edge height and good chip space. At the same time, a passivation film is gradually formed on the surface of the grinding wheel. When the abrasive particles on the grinding wheel surface are worn, the passivation film is scraped off by the chips on the workpiece surface. The electrolytic process continues to dress the grinding wheel surface. The machined surface roughness Ra reaches 0.02 to 0.01 μm, and the surface gloss is like a mirror [2]. Professor Masaru Omori applied ELID technology to the self-rotating grinding process of silicon wafers and achieved ductile domain grinding of silicon wafers. The depth of the subsurface damage layer is < 0.14 μm, which is only 1/3 to 1/10 of the damage layer depth of traditional grinding silicon wafers [10-11]. H. Eda of Ibaraki University et al. studied an integrated grinding system based on the principle of self-rotation grinding. The system uses a giant magnetostrictive micro-drive device to adjust the angle between the grinding wheel spindle and the workpiece axis to control the surface accuracy of the silicon wafer. The system uses a precision cylinder and a grinding force detection system to control pressure grinding. The ductile domain grinding of silicon wafers and the polishing-like grinding process to reduce the damage layer can be completed in one process. The surface roughness Ra < 1 nm and the flatness < 0.2 μm of 300 mm silicon wafers are processed. The surface damage layer is reduced to 0.1 ~ 0.12 μm, and the energy consumption is reduced by 70% compared with the traditional process.
The United States has made breakthrough progress in the application of ELID grinding technology to process electronic computer semiconductor microprocessors, and application research in the fields of national defense, aerospace and nuclear industry is also underway. Pei ZJ et al. conducted a systematic experimental study on the processing process of precision grinding silicon wafers by self-rotation grinding and the influence of processing parameters, grinding wheel particle size, coolant supply and other processing conditions on grinding force, silicon wafer surface accuracy, surface grinding texture and surface roughness.
Germany is one of the first countries to study ELID grinding technology. In 1991, a German machine tool manufacturer designed a series of ELID special machine tools. In addition, Britain, France and other countries have also conducted in-depth research on ELID grinding technology.
Ultra-precision composite processing has developed rapidly, such as fluid polishing, ultrasonic vibration grinding, electrochemical polishing, ultrasonic electrochemical polishing, discharge grinding, electrochemical discharge dressing grinding, dynamic suspension grinding, magnetic fluid grinding, magnetic abrasive polishing, dynamic magnetic abrasive polishing, soft particle grinding, mechanical chemical polishing, swing abrasive flow polishing and electrophoresis grinding technology. The grinding diameter of micro-carbide tools processed by ultrasonic vibration grinding can be reduced by 10% to 20% compared with those without ultrasonic vibration, and the aspect ratio can be increased by 50%, and cylindrical tools with a diameter of 11 to 23 μm and a length of 50 to 320 μm can be obtained. Electrochemical polishing can obtain a surface roughness of 50 nm, free abrasive polishing can reach 8 nm, and the combination of the two can reach 6 nm. If the abrasive particle size is increased from 21 μm is replaced by 0.51 μm, and the surface roughness can reach 2 nm. Vibration magnetic abrasive polishing with a grain size of 90 μm can achieve a surface roughness Ra of 8 nm. The surface roughness Ra of the zero thermal expansion glass ceramic specimen processed by fluid polishing is less than 0.1 nm, and the fracture strength is 546 MPa. The surface roughness of the inner wall of the stainless steel capillary processed by fluid polishing is better than Ra = 0.5 μm.
Research on precision grinding in my country is still in its infancy, mainly concentrated in colleges and universities. The ELID research group headed by Professor Yuan Zhejun of Harbin Institute of Technology has successfully developed pulse power supplies, grinding fluids and grinding wheels for ELID grinding, and developed plane, outer and inner ELID grinding devices on domestic machine tools, achieving precision mirror grinding of a variety of difficult-to-process materials. Currently, this technology is being actively promoted and popularized to achieve its commercialization [8, 15]. Researchers from the School of Mechanical Engineering of Donghua University used the finishing process of low-frequency vibration (frequency f is 0.5 to 20 Hz, amplitude is 0.5 to 3 mm) of fixed abrasive particles and pressure feeding to study the appropriate economic processing conditions and related parameters, and verified that the surface roughness of ceramic workpieces after grinding can be further reduced by 2 to 4 levels after superfinishing [16-17]. Tsinghua University has conducted in-depth research in integrated circuit ultra-precision processing equipment, disk processing, ultra-precision belt grinding and polishing, and ultra-precision grinding of diamond micro-powder grinding wheels, and has launched corresponding products.
4. Issues that should be paid attention to in future research
Precision and ultra-precision grinding technology has achieved rapid development in all aspects and has become one of the key technologies of advanced manufacturing technology. In today's research, we should focus on the following issues: 1) Basic theory and process research of ultra-precision grinding, focusing on the research of multi-particle grinding mechanism, grinding surface generation and influencing factors; 2) Development of high-precision, high-performance, and highly automated processing machinery and mobile guide mechanisms and bearings of testing devices; 3) The current problem of ELID mirror grinding technology is that it is very difficult to supply power to the high-speed rotating grinding wheel. Contact brush power supply equipment is usually used. This equipment is complex and expensive, which affects the promotion and application of ELID mirror grinding technology. Therefore, solving the problem of power supply to the high-speed rotating grinding wheel in ELID mirror grinding is also a problem that should be paid attention to; 4) Development of new materials suitable for ultra-precision processing and can obtain ultra-high precision and ultra-high surface quality, such as ultra-fine powder sintered metal, ultra-fine powder ceramics, non-crystalline semiconductor ceramics, new polymer materials, etc.
The mechanism of precision grinding and ultra-precision grinding is introduced, the research status of precision grinding machines and precision grinding and ultra-precision grinding technologies is explained, and the development trend of precision grinding and ultra-precision grinding is analyzed.
The surface roughness is a grinding method with a precision Ra of 0.16 to 1.25 μm and a processing accuracy of > 1 μm. The current precision that can be achieved by precision grinding is generally a surface roughness Ra of 0.04 to 1.25 μm and a processing accuracy of 1 to 0.5 μm. Ultra-precision grinding is a contemporary grinding method that can achieve the lowest grinding surface roughness value and the highest processing accuracy. The surface roughness can reach Ra≤0.01 μm, the accuracy ≤0.01 μm, and even enter the nanometer level.
1. Mechanism of precision and ultra-precision grinding
Precision grinding generally uses high-hardness abrasive grinding wheels such as diamond and cubic boron nitride. It mainly relies on fine dressing of the grinding wheel. Diamond dressing tools are used with extremely small and uniform micro-feed (10-15 mm/min) to obtain numerous equal-height micro-edges. The grinding marks on the processed surface are very fine. Finally, spark-free polishing is used. Due to the combined effects of micro-cutting, sliding and friction, low surface roughness values and high precision requirements are achieved. Ultra-precision grinding uses a smaller dressing lead and cutting amount to dress the grinding wheel, and relies on ultra-fine abrasives to grind high micro-edges [1]. The mechanism of precision and ultra-precision grinding is somewhat different from that of ordinary grinding.
1) Ultra-micro removal. Use a smaller dressing lead and dressing depth to finely dress the grinding wheel, so that the abrasive grains are finely broken to produce micro-edges. One abrasive grain becomes multiple abrasive grains, which is equivalent to the grinding wheel becoming finer in grain size. The micro-cutting action of the micro-edge forms low roughness.
2) Contour cutting effect of micro-edges. Micro-edges are formed by fine dressing of the grinding wheel. There are many micro-edges distributed at the same depth on the surface of the grinding wheel, and the contour is good, so the residual height of the machined surface is extremely small [ .
3) Single-grain grinding process. Abrasive grains are elastic bodies with elastic support and large negative rake cutting edges. When a single abrasive grain is in contact with the workpiece during grinding, it starts with an elastic zone, followed by a plastic zone, a cutting zone, a plastic zone, and finally an elastic zone, which is consistent with the shape of chip formation. Ultra-precision grinding has micro-cutting action, plastic flow and elastic destruction, as well as sliding friction. When the blade is sharp and has a certain grinding depth, the micro-cutting action is stronger; if the blade is not sharp enough, or the grinding depth is too shallow, the abrasive cutting edge cannot cut into the workpiece, resulting in plastic flow, elastic destruction and sliding friction.
4) Continuous grinding process. The workpiece rotates continuously, the grinding wheel cuts in continuously, and the entire grinding system begins to deform elastically. The difference between the grinding cut-in (grinding depth) and the actual workpiece size reduction is the elastic cut-out. After that, the grinding cut-in gradually becomes equal to the actual workpiece size reduction, and the grinding system is in a stable state. Finally, the grinding cut-in reaches the given value, but the elastic deformation of the grinding system gradually returns to the non-cutting grinding state.
2. Development of precision and ultra-precision grinders
Precision grinders are the basis of precision grinding. The development direction of today's precision grinding machine technology is high precision, integration, and automation.
Cranfield University Precision Engineering (CUPE) in the UK is one of the earliest companies engaged in the development of ultra-precision grinding machine tools. The OAGM2500 large ultra-precision grinder developed by the company is the largest ultra-precision grinding equipment to date, mainly used for ultra-precision grinding of hard and brittle materials such as optical glass [3, 6]. The Nanocentre (nano-machining center) produced by CUPE is equipped with a grinding head and can perform ultra-precision grinding. The shape accuracy of the workpiece can reach 0.1 μm and the surface roughness Ra < 10 nm [3]. In 2003, Cranfield University in the UK and Cranfield Precision Engineering Co., Ltd. jointly developed a new type of ultra-precision grinder that can complete the ductile domain nano-grinding of silicon wafers in one process with high processing efficiency, and obtain good surface and sub-surface integrity. It is said that the use of this ultra-precision grinder to grind large-diameter silicon wafers can completely replace the grinding and etching processes of traditional processes, and is even expected to replace polishing.
The ultra-precision grinder produced by Moore Nanotechnology System in the United States uses an ultra-precision hydrostatic guideway to maintain a straightness of 0.3 μm, a machining geometric accuracy of 0.1 μm, and a surface roughness of Ra = 5 nm［8］. In order to meet the needs of ultra-precision grinding of larger-caliber optical parts and optical parts made of hard and brittle materials, the LLNL laboratory in the United States developed the next-generation ultra-precision optical processing equipment POGAL (Optical Grinder and Lathe) in 2006. The axial and radial precision technical indicators of its spindle are 50 nm.
Japan started research on ultra-precision machining technology later than the United States. It was developed in response to the needs of civilian industries such as electronics and optics. Based on ultra-precision lathes and combined with ELID mirror grinding technology, it developed ELID precision CNC mirror grinders for machining non-spherical surfaces of rotating bodies. Later, it developed three-coordinate linkage CNC ELID precision mirror grinders, which can achieve mirror machining of precision free-form surfaces. Its ultra-precision grinding equipment mainly includes the ULG-100A (H) ultra-precision non-spherical machining machine tool produced by Toshiba Machinery Company in the 1990s. The machine tool spindle adopts high-rigidity air static pressure bearings, two-axis full closed-loop control, and the axis displacement resolution is 0.01 μm. It can process various optical parts and non-spherical lens injection molding metal (copper, non-electrolytic nickel) molds, and compression molding ceramic (WC) molds. The molding molds are turned and ground using diamond tools or grinding wheels to achieve mirror quality.
The most typical precision grinding equipment currently produced by Schneider in Germany is the SCGA121 ultra-precision grinding center for aspheric surfaces. This machine uses a high-rigidity concrete polymer as the bed and is multi-axis CNC-controlled. It can perform both large-removal ordinary grinding wheel grinding and cup-shaped grinding wheel grinding. At the same time, it is integrated with the SCGA121 aspheric polishing machine and the AU aspheric online detection system to achieve ultra-precision, efficient and flexible automated processing of aspheric optical components [6]. The Multi2Nano fully automatic series nano-grinder developed by the German G&N company adopts the principle of self-rotating grinding and is equipped with two grinding wheel spindles for rough and fine grinding respectively. It has 3 (or 4) operating stations and can automatically complete the rough grinding, fine grinding, cleaning or loading and unloading of silicon wafers. It can be used for ultra-precision grinding of 300 mm silicon wafers to obtain nano-level mirror surfaces, and can be used for back grinding to thin silicon wafers to 100 to 150 μm.
The FSGJ-I developed by the State Key Laboratory of Applied Optics at the Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, integrates milling, edge grinding, precision polishing and testing. The Ministry of Education Key Laboratory of Precision and Special Processing at Dalian University of Technology is developing and researching large-diameter silicon wafer ultra-precision grinding systems and equipment based on the principle of self-rotation grinding, and has achieved initial results.
3. Development of precision and ultra-precision grinding technology
In recent years, foreign countries have made many achievements in the development and research of precision and ultra-precision grinding technology, mainly in the research of new ELID (Electrolytic In process Dressing) mirror grinding technology and the application of processing silicon wafers and non-spherical parts. The average particle size of diamond abrasive grains of resin bonded grinding wheels used for ultra-precision mirror grinding can be as small as 4 μm. The integrated manufacturing system of 300 mm diameter silicon wafers uses single crystal diamond grinding wheels to enable ductile grinding and finishing processing to be performed on the same device, so that the surface roughness of silicon wafers reaches Ra <1 nm (Ry <5 ~ 6 nm) and the flatness reaches <0.2 μm/300 mm.
In 1987, Professor Masaru Omori of the RIKEN Institute of Physical and Chemical Research in Japan successfully developed a new ELID mirror grinding process for online dressing of grinding wheels. ELID mirror grinding technology uses online electrolytic dressing to continuously dress the grinding wheel to obtain a constant cutting edge height and good chip space. At the same time, a passivation film is gradually formed on the surface of the grinding wheel. When the abrasive particles on the grinding wheel surface are worn, the passivation film is scraped off by the chips on the workpiece surface. The electrolytic process continues to dress the grinding wheel surface. The machined surface roughness Ra reaches 0.02 to 0.01 μm, and the surface gloss is like a mirror [2]. Professor Masaru Omori applied ELID technology to the self-rotating grinding process of silicon wafers and achieved ductile domain grinding of silicon wafers. The depth of the subsurface damage layer is < 0.14 μm, which is only 1/3 to 1/10 of the damage layer depth of traditional grinding silicon wafers [10-11]. H. Eda of Ibaraki University et al. studied an integrated grinding system based on the principle of self-rotation grinding. The system uses a giant magnetostrictive micro-drive device to adjust the angle between the grinding wheel spindle and the workpiece axis to control the surface accuracy of the silicon wafer. The system uses a precision cylinder and a grinding force detection system to control pressure grinding. The ductile domain grinding of silicon wafers and the polishing-like grinding process to reduce the damage layer can be completed in one process. The surface roughness Ra < 1 nm and the flatness < 0.2 μm of 300 mm silicon wafers are processed. The surface damage layer is reduced to 0.1 ~ 0.12 μm, and the energy consumption is reduced by 70% compared with the traditional process.
The United States has made breakthrough progress in the application of ELID grinding technology to process electronic computer semiconductor microprocessors, and application research in the fields of national defense, aerospace and nuclear industry is also underway. Pei ZJ et al. conducted a systematic experimental study on the processing process of precision grinding silicon wafers by self-rotation grinding and the influence of processing parameters, grinding wheel particle size, coolant supply and other processing conditions on grinding force, silicon wafer surface accuracy, surface grinding texture and surface roughness.
Germany is one of the first countries to study ELID grinding technology. In 1991, a German machine tool manufacturer designed a series of ELID special machine tools. In addition, Britain, France and other countries have also conducted in-depth research on ELID grinding technology.
Ultra-precision composite processing has developed rapidly, such as fluid polishing, ultrasonic vibration grinding, electrochemical polishing, ultrasonic electrochemical polishing, discharge grinding, electrochemical discharge dressing grinding, dynamic suspension grinding, magnetic fluid grinding, magnetic abrasive polishing, dynamic magnetic abrasive polishing, soft particle grinding, mechanical chemical polishing, swing abrasive flow polishing and electrophoresis grinding technology. The grinding diameter of micro-carbide tools processed by ultrasonic vibration grinding can be reduced by 10% to 20% compared with those without ultrasonic vibration, and the aspect ratio can be increased by 50%, and cylindrical tools with a diameter of 11 to 23 μm and a length of 50 to 320 μm can be obtained. Electrochemical polishing can obtain a surface roughness of 50 nm, free abrasive polishing can reach 8 nm, and the combination of the two can reach 6 nm. If the abrasive particle size is increased from 21 μm is replaced by 0.51 μm, and the surface roughness can reach 2 nm. Vibration magnetic abrasive polishing with a grain size of 90 μm can achieve a surface roughness Ra of 8 nm. The surface roughness Ra of the zero thermal expansion glass ceramic specimen processed by fluid polishing is less than 0.1 nm, and the fracture strength is 546 MPa. The surface roughness of the inner wall of the stainless steel capillary processed by fluid polishing is better than Ra = 0.5 μm.
Research on precision grinding in my country is still in its infancy, mainly concentrated in colleges and universities. The ELID research group headed by Professor Yuan Zhejun of Harbin Institute of Technology has successfully developed pulse power supplies, grinding fluids and grinding wheels for ELID grinding, and developed plane, outer and inner ELID grinding devices on domestic machine tools, achieving precision mirror grinding of a variety of difficult-to-process materials. Currently, this technology is being actively promoted and popularized to achieve its commercialization [8, 15]. Researchers from the School of Mechanical Engineering of Donghua University used the finishing process of low-frequency vibration (frequency f is 0.5 to 20 Hz, amplitude is 0.5 to 3 mm) of fixed abrasive particles and pressure feeding to study the appropriate economic processing conditions and related parameters, and verified that the surface roughness of ceramic workpieces after grinding can be further reduced by 2 to 4 levels after superfinishing [16-17]. Tsinghua University has conducted in-depth research in integrated circuit ultra-precision processing equipment, disk processing, ultra-precision belt grinding and polishing, and ultra-precision grinding of diamond micro-powder grinding wheels, and has launched corresponding products.
4. Issues that should be paid attention to in future research
Precision and ultra-precision grinding technology has achieved rapid development in all aspects and has become one of the key technologies of advanced manufacturing technology. In today's research, we should focus on the following issues: 1) Basic theory and process research of ultra-precision grinding, focusing on the research of multi-particle grinding mechanism, grinding surface generation and influencing factors; 2) Development of high-precision, high-performance, and highly automated processing machinery and mobile guide mechanisms and bearings of testing devices; 3) The current problem of ELID mirror grinding technology is that it is very difficult to supply power to the high-speed rotating grinding wheel. Contact brush power supply equipment is usually used. This equipment is complex and expensive, which affects the promotion and application of ELID mirror grinding technology. Therefore, solving the problem of power supply to the high-speed rotating grinding wheel in ELID mirror grinding is also a problem that should be paid attention to; 4) Development of new materials suitable for ultra-precision machining and can obtain ultra-high precision and ultra-high surface quality, such as ultra-fine powder sintered metal, ultra-fine powder ceramics, non-crystalline semiconductor ceramics, new polymer materials, etc.