High-end manufacturing precision grinding technology
With the continuous improvement of the level of science and technology, grinding processing has been widely used in roughing and finishing of metals and other materials, and it is a very important cutting processing method. At present, grinding processing has become the most effective and most widely used basic process technology for precision and ultra-precision processing in the field of modern machinery manufacturing, providing people with the development and research of high-precision, high-quality, and highly automated technical equipment.
Precision grinding technology
Grinding refers to the processing method of removing materials with abrasives or abrasive tools. Generally speaking, it can be classified according to the linear speed of the grinding wheel. Grinding with a grinding wheel speed of less than 45 m/s is called ordinary grinding. Grinding with a wheel speed higher than 45 m/s is called high-speed grinding, and grinding with a wheel speed higher than 150m/s is called ultra-high-speed grinding.
According to the grinding efficiency, grinding is divided into ordinary grinding and high-efficiency grinding (high-speed grinding, ultra-high-speed grinding, slow feed grinding, high-efficiency deep-cutting grinding, belt grinding, fast short-stroke grinding and high-speed heavy-duty grinding). Load grinding).
The grinding precision that can be achieved by grinding has different precision ranges in different periods of production and development. At present, grinding is divided into ordinary grinding according to the grinding precision (processing precision> 1μm, surface roughness Ra0.16~1.25 μm), precision grinding (processing accuracy 1~0.5μm, surface roughness Ra0.04~1.25μm), ultra-precision grinding (processing accuracy ≤0.01μm, surface roughness Ra≤0.01μm).
Precision machining refers to a machining process with a high degree of machining accuracy and surface quality in a certain period of development. At present, it refers to machining with a machining accuracy of 1 to 0.1 µm and a surface roughness value of Ra0.2 to 0.01 µm. technology.
Precision grinding is currently one of the main methods for precision processing of ferrous metals such as iron and steel and brittle and hard materials such as semiconductors. It occupies a very important position in modern machinery and electronic equipment manufacturing technology.
Precision grinding generally uses high-hardness abrasive wheels such as diamond and cubic boron nitride, mainly relying on fine dressing of the grinding wheel, using diamond dressing tools with a very small and uniform micro-feed (10-15mm/min). A large number of micro Blades of the same height are obtained, and the grinding marks on the machined surface are fine, and finally no spark polishing is used.
Due to the combined effects of micro-cutting, slippage and friction, low surface roughness values and high precision requirements are achieved. The chips of high-precision grinding are very thin, and the abrasive grains of the grinding wheel are subjected to high stress. The surface of the abrasive grains is subjected to high temperature and high pressure. Generally, high-hardness abrasive wheels such as diamond and cubic boron nitride are used for grinding.
There are some differences between the mechanism of precision and ultra-precision grinding and ordinary grinding:
1) Ultra-micro excision. 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 granularity of the grinding wheel becomes finer, and the micro-cutting action of the micro-edge creates low roughness.
2) The contour cutting effect of the micro edge. The micro-edge is finely trimmed by the grinding wheel. There are many micro-edges distributed at the same depth on the surface of the grinding wheel, and the height is good, so the residual height of the machined surface is extremely small.
3) Single particle grinding process. The abrasive grain is an elastic body with elastic support and a large negative rake angle cutting edge. When a single abrasive grain is in contact with the workpiece during grinding, the first is the elastic zone, then the plastic zone, the cutting zone, the plastic zone, and finally It is the elastic zone, which is consistent with the shape of chip formation.
4) Continuous grinding process. The workpiece continues to rotate, the grinding wheel continues to cut, and the whole part of the grinding system is elastically deformed. The difference between the grinding cut (grinding depth) and the reduction of the actual workpiece size is the elastic tool allowance. After that, the grinding cut amount gradually became equal to the actual size reduction of the workpiece, and the grinding system was in a stable state. Finally, the grinding cut reaches the given value, but the elastic deformation of the grinding system gradually returns to the state of no-cut deep grinding.
History and development of precision grinding technology
Grinding is a processing method that uses abrasives to remove materials, and it is also the earliest production technique used by mankind.
In the mid-18th century, the world's first cylindrical grinder came out. It was chiseled from natural abrasives such as quartz stone and garnet into abrasive tools, and then sintered with natural abrasives and clay to form a grinding wheel. Later, a surface grinder was developed.
At the end of the 1940s, synthetic diamond appeared; cubic boron nitride was successfully developed in 1957; with the development of grinding technology, especially the application of super-abrasive synthetic diamond grinding wheels and cubic boron nitride grinding wheels, the range of grinding processing applications is increasing. Increased, grinding processing accuracy and processing efficiency also continue to improve.
Grinding technology has developed rapidly, and grinding machines also account for a large proportion of processing machine tools. According to the 1997 European Machine Tool Fair (EMO) survey data, 25% of companies believe that grinding is the most important processing technology they use. Turning accounts for only 23%, drilling 22%, and other 8%; and grinding machines are among the companies. The proportion of machine tools is as high as 42%, lathes account for 23%, milling machines account for 22%, and drilling machines account for 14%.
From 1949 to 2007, my country developed and produced more than 1,800 general-purpose grinders and hundreds of special-purpose grinders. The ownership of grinders accounted for about 15% of the total ownership of metal cutting machine tools. It can be seen that grinding technology and grinders occupy an extremely important position in the machinery manufacturing industry.
In recent years, many achievements have been made in the development and research of precision and ultra-precision grinding technology abroad, which are mainly reflected in the research of ELID mirror grinding new technology and the application of processing silicon wafers and aspherical parts.
Professor Masaru Omori of the National Institute of Physics and Chemistry of Japan successfully developed a new ELID mirror grinding process for online dressing of grinding wheels in 1987.
ELID mirror grinding technology uses online electrolytic dressing to continuously dress the grinding wheel to obtain a constant blade height and good chip holding space. At the same time, a passivation film is gradually formed on the surface of the grinding wheel. When the abrasive grains on the surface of the grinding wheel are worn out, the grinding wheel becomes blunt. The film is scraped and removed by the wear debris on the surface of the workpiece. The electrolysis process continues, and the surface of the grinding wheel is trimmed. The surface roughness Ra of the machined surface reaches 0.02~0.01μm, and the surface is shiny like a mirror.
The United States has made breakthroughs in the application of ELID grinding technology in the processing of electronic computer semiconductor microprocessors. Application research in the fields of national defense, aerospace and nuclear industry is also underway. PeiZJ et al. The influence of silicon wafer processing process and processing parameters, grinding wheel particle size, coolant supply and other processing conditions on the grinding force, wafer surface accuracy, surface grinding lines and surface roughness have been systematically studied.
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.
my country's research on precision grinding is still in its infancy, mainly concentrated in universities. The ELID research group headed by Professor Yuan Zhejun of Harbin Institute of Technology has successfully developed a special pulse power supply, grinding fluid and grinding wheel for ELID grinding. Precision mirror grinding of a kind of difficult-to-machine materials. Currently, this technology is being actively promoted and commercialized. Researchers from the School of Mechanical Engineering of Donghua University used the low-frequency vibration of the consolidated abrasive grains (frequency f of 0.5-20Hz, amplitude of 0.5-3mm) pressure feed finishing, studied the appropriate economic processing conditions and related parameters, and verified them The surface roughness can be further reduced by super finishing the ceramic workpiece after grinding, which can be reduced by 2 to 4 grades. Tsinghua University has conducted in-depth research on 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 corresponding products have come out.
ELID grinding technology
ELID grinding technology is an ultra-precision mirror processing technology that uses electrochemical reaction non-traditional material removal technology to solve the problem of dressing metal-based super-abrasive grinding wheels. It has high efficiency, high precision, good surface quality, simple processing equipment and adaptability. It is widely used in the fields of electronics, machinery, optics, instrumentation, and automobiles.
Basic principles of ELID grinding
ELID (Electrolytic In-process Dressing) grinding is the use of nonlinear electrolytic dressing in the grinding process to achieve a dynamic balance between the continuous dressing of the surface oxide layer of the metal bond superabrasive grinding wheel and the effect of the passivation film to inhibit electrolysis. In order to obtain a stable thickness of the oxide layer, the grinding wheel abrasive grains can obtain a constant edge height and a good chip holding space, and realize a stable, controllable and optimal grinding process. It is suitable for ultra-precision mirror grinding of hard and brittle materials.
The necessary equipment for ELID grinding mainly includes five elements: grinder, power supply, electrolysis device, electrolyte and grinding wheel.
The working principle is: the rotating shaft of the metal bond superabrasive grinding wheel is in contact with the brush, and the positive pole of the power supply is connected as the anode, and the copper electrode (tool electrode) is connected with the negative pole of the power supply as the cathode. There is a gap of 100~500μm between the grinding wheel and the negative electrode (the gap is adjustable), and the nozzle is used to spray the grinding fluid with electrolysis function to fill the gap. Under the action of high voltage (60~120v) and high pulse frequency power supply, the electrolysis of the grinding fluid produces an anodic dissolution effect, and the metal matrix on the surface of the grinding wheel is electrolytically removed. At the same time, a layer of insulating blunt is formed on the surface of the grinding wheel. The chemical film can effectively inhibit the excessive electrolysis of the metal matrix to reduce the excessive electrolysis loss of the grinding wheel matrix. Because the oxide film is very easy to wear, it is easy to expose the sharp edges and corners of the new abrasive particles to achieve the sharpening effect. During the whole machining process, the electrolysis effect and the inhibition effect of the passivation film reach a dynamic balance, which ensures a constant amount of abrasive grains, so that the grinding wheel always maintains the best grinding state with abrasive grains protruding during the machining process. This technology combines the on-line dressing of the grinding wheel with the grinding process to achieve continuous ultra-precision mirror grinding of the workpiece.
The ELID ultra-precision grinding process can be divided into four stages, which are summarized in the following figure.
As shown in the basic principle diagram (a) of ELID mirror grinding in the following figure, before the grinding wheel is corrected, the grinding wheel is composed of abrasive grains and metal bond, and the abrasive grains are evenly distributed. Before grinding, the grinding wheel needs to be electrolytically dressed separately (Figure b). A layer of metal bond is ionized and dissolved on the surface of the grinding wheel and replaced by a passivation film of a certain thickness. On the one hand, it can effectively reduce the shape error of the grinding wheel, and on the other hand, it can generate a passivation film with a certain elasticity in advance to prevent the grinding wheel from cutting in and causing hard scratches on the workpiece when grinding starts.
After the grinding starts, since the oxide film is very easy to wear, the abrasive grains fixed in the oxide film have sharp edges and corners, which can be regarded as the micro-cutting effect of countless large and small tools on the surface of the workpiece. After the grinding wheel passes through the contact area with the workpiece, due to the scraping effect of the workpiece material, the abrasive particles are worn and dull, the height of the blade is reduced, and the grinding effect becomes weak. Due to the thinning of the passivation film and the recovery of conductivity, when the grinding wheel is turned to the tool electrode position, under the action of the electrolyte, a new oxide film is formed on the surface of the grinding wheel, which increases the height of the abrasive edge (see Figure c). When the oxide film reaches the thickness before abrasion, the resistance is large enough to play an insulating role, and the electrolysis is terminated.
It can be seen that, as a result of this nonlinear electrolysis, the dressing process has a certain adaptive ability to the grinding process. The grinding wheel maintains the same thickness of oxide layer and the same height of the abrasive grains during each rotation cycle. During grinding, the electrolysis and wear of the oxide film on the surface of the grinding wheel reach a dynamic balance. Finally, the bonding agent matrix on the surface of the grinding wheel is continuously electrolyzed and new abrasives are continuously exposed to ensure the sharpness of the metal-based grinding wheel during the grinding process.
This kind of grinding method will not lose the cutting ability due to the wear and fall of the surface abrasive and cause the chip blockage, and will not cause the excessive consumption of the grinding wheel. It can give full play to the grinding ability of the super-hard abrasive, which is very conducive to the hardening. Brittle materials realize high-precision, high-efficiency ultra-precision mirror grinding.
Problems and development prospects of precision grinding
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. The following issues should be focused on in future research:
1) Research on the basic theory and technology of ultra-precision grinding, focusing on the study of multi-particle grinding mechanism, grinding surface generation and influencing factors, etc.;
2) Develop high-precision, high-performance, high-automation processing machinery and mobile guiding mechanisms and bearings for 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-type brush power supply equipment is usually used. This equipment is complicated and expensive, which affects the popularization and application of ELID mirror grinding technology;
4) Develop new materials that are suitable for ultra-precision processing and can obtain ultra-high precision and ultra-high surface quality, such as ultra-fine powder sintered metal, new polymer materials, etc.
Grinding technology has developed rapidly, and it plays a very important role in mechanical processing. At present, the development trend of grinding technology is to develop super-hard abrasives, study precision and ultra-precision grinding, high-speed and high-efficiency grinding mechanisms, develop new grinding processing technologies, and develop high-precision, high-rigidity automated grinding machines.
With the continuous improvement of the requirements for precision, reliability and life of mechanical products, the application of new materials with high hardness, high strength, high wear resistance and high functionality is increasing. Grinding processing technology is in the grinding mechanism, the development of abrasives, Precision ultra-precision grinding, high-speed ultra-high-speed grinding, grinding automation and intelligence, grinding process monitoring and detection technology, grinding processing software technology, etc. have developed rapidly, and they play a vital role in mechanical processing. Therefore, we should thoroughly study foreign advanced grinding technologies, systematically develop and promote various advanced and practical grinding technologies, and actively promote the progress of our machinery manufacturing technology.
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