Ultra-high-speed grinding usually refers to grinding with a grinding wheel speed greater than 150m/s. Ultra-high-speed grinding has developed rapidly in developed countries such as Europe, Japan, and the United States, and is known as the "peak of modern grinding technology." The International Society for Production Engineering (CIRP) has identified it as a central research direction for the 21st century and has conducted some famous collaborative research. Ultra-high-speed grinding can achieve ductile zone grinding of hard and brittle materials, and also has good grinding performance on difficult-to-grind materials such as high plasticity. Compared with ordinary grinding, ultra-high-speed grinding shows great advantages:

Greatly improve grinding efficiency and reduce the number of equipment used. For example, if an electroplated CBN grinding wheel is used to grind a lawn mower crankshaft at a high speed of 123m/s, it originally required 6 turning and 3 grinding processes, but now only 3 grinding processes are required. The production time is reduced by 65%, and 180 millimeters can be processed per hour. pieces. Another example is when people use ordinary grinding wheels to efficiently grind hardened low carbon steel 42CrMo4 at a speed of 125m/s, with a removal rate of 167mm3/mms, which is 11 times greater than slow feed grinding.

The grinding force is small and the parts are processed with high precision. Tests at speeds below 360m/s show that within a narrow speed range (180-200 m/s), the friction state changes sharply from solid to liquid, accompanied by a sharp decrease in grinding force. In the high-speed grinding test of 45 steel and 20Cr steel with single abrasive grains, the author found that the friction coefficient decreased significantly with the increase of speed below the critical speed; after exceeding the critical speed, the friction coefficient decreased slightly with the increase of speed. There is an increase.

Reduce the surface roughness of the processed workpiece. When other conditions are the same, when grinding at speeds of 33m/s, 100m/s, and 200m/s, the surface roughness values are Ra2.0, Ra1.4 and Ra1.1μm respectively.

Grinding wheel life is extended. Under the same metal removal rate, the grinding wheel speed is increased from 80m/s to 200m/s, and the grinding wheel life is increased by 8.5 times. When grinding at a speed of 200m/s, the service life is still doubled with a removal rate of 2.5 times that at 80m/s.

1 Development of ultra-high speed grinding

Europe

In Europe, the development of high-speed grinding technology started early. The initial basic research on high-speed grinding was in the late 1960s, and the laboratory grinding speed has reached 210-230m/s. In the late 1970s, CBN grinding wheels were used for high-speed grinding. At the International Machine Tool Exhibition in Hannover, West Germany, in September 1973, the Italian company Famir exhibited the RFT-C120/50R high-speed applicable grinding machine with a grinding wheel circumferential speed of 120m/s for grinding the inner ring and outer groove of the bearing. . In the early 1990s, grinding experiments with a maximum speed of 350m/s had been achieved. At present, in practical applications, the maximum grinding speed of high-speed grinding and precision grinding is between 200-250 m/s.

In 1983, Germany's Guhring Automation Company manufactured a powerful grinder with a power of 60kW, a rotation speed of 10,000r/min, and a grinding wheel diameter of 400mm. The ultra-high-speed grinding machine of Achen University of Technology with a target of 500m/s is also manufactured by this company. A typical example of the application of high-speed grinding of German CBN grinding wheels is the processing of gear teeth. At a speed of 155m/s and a removal rate of 811mm3/mms, the efficient processing of 16MCr5 steel gears is achieved. Another example is the use of an electroplated CBN grinding wheel to achieve efficient machining of the narrow groove of the 100Cr6 high hardness (60HRC) rolling bearing steel water pump slewing wheel at a speed of 300m/s and a removal rate of 140mm3/mms. The Swiss Studer Company also used a modified S45 cylindrical grinder to conduct a 280m/s grinding test. The Swiss S40 high-speed CBN grinding wheel grinder has the best high-speed grinding performance at 125m/s, and it also works as usual at 500m/s. In addition, Kapp Company, Schandt Company, Naxa Union Company, Song Machinery Company, etc. have also launched various types of high-speed grinding machines.

USA

In 1970, the American Bendix Company produced a 91m/s plunge-type high-speed grinder. In 1993, the American Edgetek Machine Company first launched an ultra-high-speed grinder that used a single-layer CBN grinding wheel with a circumferential speed of 203m/s. It was used to process hardened saw teeth and could achieve a high metal removal rate. The centerless cylindrical grinder from the Grinding Research and Development Center of Connectient University in the United States has a maximum grinding speed of 250m/s, a spindle power of 30kW, a dressing disc speed of 12,000r/min, automatic balancing of the grinding wheel, and automatic loading.

At present, high-efficiency grinding machines are very common in the United States, mainly using CBN grinding wheels. It can achieve high-efficiency grinding of high-temperature alloy Inconel718 at a speed of 160m/s and a removal rate of 75mm3/mms. After processing, it can reach Ra1~2μm and the dimensional tolerance is ±13μm. In addition, a ceramic CBN grinding wheel with a diameter of 400mm is used to grind at a speed of 150-200m/s, which can reach Ra0.8μm and the dimensional tolerance is ±2.5-5μm. An important research direction in high-speed grinding in the United States is low-damage grinding of advanced ceramics. The traditional method is to use multi-process grinding, while high-speed grinding attempts to use one-time roughing and finishing grinding to process high-quality silicon nitride ceramic parts with high material removal rate and low cost.

Japan

Japan's high-speed grinding technology has developed rapidly in the past 20 years. In 1976, CBN grinding wheels began to be used on cam grinders for high-speed grinding of 40m/s. Around 1985, on cam and crankshaft grinders, the grinding speed reached 80m/s. s. After 1990, ultra-high-speed grinding machines with speeds above 160m/s began to be developed. At present, the practical grinding speed has reached 200m/s. A 400m/s ultra-high-speed surface grinder has also been developed. The grinder has a maximum spindle speed of 3000r/min, a maximum power of 22kW, a grinding wheel with a diameter of 250mm, and a maximum peripheral speed of 395m/s. The effect of speed on the machinability of cast iron was studied in the speed range of 30-300m/s.

Japan's Toyota Machinery, Mitsubishi Heavy Industries, Okamoto Machine Tool Manufacturing Co., Ltd. and other companies can produce ultra-high-speed grinders using CBN grinding wheels. The CA32-U50A CNC ultra-high-speed grinder launched by Japan's Mitsubishi Heavy Industries uses ceramic bond CBN grinding wheels. The speed reached 200m/s.

China

my country's high-speed grinding started late. In 1974, the First Automobile Factory, the First Grinding Wheel Factory, Wafangdian Bearing Factory, Huazhong Institute of Technology, Zhengzhou No. 3 Grinding Institute, etc. successively conducted 50-60m/s grinding tests; Hunan University A high-speed grinding test of 60-80m/s was carried out. In October 1975, Nanyang Machine Tool Factory successfully trial-produced the MS132 80m/s high-speed cylindrical grinder. In 1976, Shanghai Machine Tool Factory, Shanghai Grinding Wheel Factory, Zhengzhou Third Grinding Institute, Huazhong Institute of Technology, Shanghai Jiao Tong University, Guangzhou Machine Tool Research Institute, Wuhan Material Protection Research Institute, etc. formed a high-speed grinding test team to test 80m/s, 100m/S The high-speed grinding process was experimentally studied. At the same time, Shanghai Machine Tool Factory designed and manufactured the MBSA1332 80m/s semi-automatic high-speed cylindrical grinder. The grinding efficiency has reached the productivity of turning and milling. In 1977, Hunan University successfully conducted 100m/s and 120m/s high-speed grinding tests in the laboratory. At the 2000 China CNC Machine Tool Exhibition (CCMT'2000), Hunan University launched a machine with a maximum linear speed of 120m/s. CNC camshaft grinder.

In 1976, Northeastern University cooperated with Fuxin No. 1 Machine Tool Factory to successfully develop the F1101 60m/s high-speed semi-automatic piston-specific cylindrical grinder. By the early 1980s, Northeastern University had conducted a large number of high-speed grinding experimental studies. The YLM-1 double-sided vertical semi-automatic grinding production line mainly developed by Northeastern University has a grinding speed of 80m/s and a grinding pressure of more than 2500-5000N. In the 1990s, Northeastern University began research on ultra-high-speed grinding technology and successfully developed my country's first ultra-high-speed test grinder with a circumferential speed of 200m/s and a rated power of 55kW, with a maximum speed of 250m/s.

2 Key technologies for ultra-high speed grinding

Ultra high speed spindle

Increasing the linear speed of the grinding wheel is mainly to increase the rotation speed of the grinding wheel spindle. Therefore, in order to achieve high-speed cutting, the grinding wheel drive and bearing speed are often required to be very high. The high speed of the spindle requires sufficient stiffness, high rotation accuracy, good thermal stability, reliability, low power consumption, long life, etc. In order to reduce the dynamic forces increased due to the increase in cutting speed, the grinding wheel spindle and spindle motor system are required to operate extremely accurately and with minimal vibration. At present, a large number of high-speed and ultra-high-speed machine tools produced abroad use electric spindles.

Foreign high-speed electric spindles are developing rapidly. For example, in Japan, at the 19th JIMTOF exhibition in October 1998, the ultra-high-speed spindles on display were basically between 10000-25000r/min. At present, the highest level electric spindle in the world is the product of Swiss Fisher Company (nmax=40000r/min, N=40kW). Practical high-speed electric spindles with speeds up to 200,000r/min and 250,000r/min are also being researched and developed. The ultra-high-speed turning and milling machine developed by Shenyang Institute of Technology uses an electric spindle with a speed adjustment range of 0-18000r/min and a maximum output power of 7.5kW. The GD-2 high-speed electric spindle with a rated speed of 1500r/min developed by Guangxi University of Technology uses Si3N4 ceramic ball bearings. The maximum speed can reach 18000r/min, and the main motor has a rated power of 13.5kW.

Spindle bearings can use ceramic rolling bearings, magnetic bearings, air static pressure bearings or liquid dynamic and static pressure bearings, etc. Ceramic ball bearings have the advantages of light weight, small thermal expansion coefficient, high hardness, high temperature resistance, dimensional stability at high temperatures, corrosion resistance, long life, and high elastic modulus. Its disadvantages are that it is difficult to manufacture, has high cost, and is sensitive to tensile stress and notch stress. The maximum surface speed of magnetic bearings can reach 200m/s, and it may become an option for ultra-high-speed spindle bearings in the future. At present, the main problems of magnetic bearings are low stiffness and load capacity, and the magnets used are too large and expensive compared to the size of the rotary body. Aerostatic bearings have the characteristics of high rotation angle, no vibration, small friction resistance, durability and high-speed rotation. Used in high speed, light load and ultra-precision applications. Hydrodynamic and static pressure bearings have too much power loss when there is no load and are mainly used for low-speed and heavy-load spindles.

Ultra-high speed grinding wheel

High-speed grinding wheels should have good wear resistance, high dynamic balance accuracy, crack resistance, good damping characteristics, high stiffness and good thermal conductivity, etc. They are usually composed of a matrix with high mechanical properties and a thin layer of abrasive particles. . The grinding wheel base should be free of residual stresses and should have minimum elongation during operation. By calculating the tangential and normal stresses of the grinding wheel, it is found that the maximum stress occurs in the tangential direction of the inner diameter of the grinding wheel base, and this stress should not exceed the strength limit of the grinding wheel base material. Most practical superabrasive grinding wheel bases are aluminum or steel. Japan and Europe have also developed CBN grinding wheels made of other materials such as CFRP composite materials. Although CFRP has a low elastic coefficient, the ratio of elastic coefficient to specific gravity is high, which can suppress the extension of the grinding wheel in the radial direction. Another advantage of CFRP is its lower linear elongation coefficient. Currently, a CBN grinding wheel with a diameter of 380mm based on CFRP can achieve a grinding speed of 200m/s and a feed speed of 2m/s. Japan has achieved a grinding test of 300 m/s using a CFRP-based ceramic bond CBN grinding wheel with a diameter of 250 mm on a 400 m/s ultra-high-speed grinder.

Ultra-high-speed grinding wheels can use corundum, silicon carbide, CBN, and diamond abrasives. The bonding agent can be ceramic, resin or metal bonding agent. Grinding wheels made of resin-bonded corundum, silicon carbide, and cubic boron nitride abrasives can be used at speeds up to 125m/s. The single-layer electroplated CBN grinding wheel can be used at a speed of 250m/s, and has reached 340m/s in the test. The grinding speed of ceramic bond grinding wheel can reach 200m/s. Compared with other types of grinding wheels, vitrified bond grinding wheels are easier to dry dress. Compared with high-density resin and metal bond grinding wheels, ceramic bond grinding wheels can achieve a wide range of porosity by changing the production process. The special structure has a porosity of 40%. Due to the structural characteristics of the vitrified bonded grinding wheel, the chip space after trimming is large, the sharpening is simple, and even no sharpening is required in many applications. The use of flaky sintered ceramic grinding wheels and reliable bonding solves the problem of easy breakage due to the large difference between the elastic coefficient of the ceramic bond and the matrix. Norton Company in the United States has developed a method of holding abrasive particles with the help of chemical bonding, which can make the abrasive particles protrude 80% of the height without falling off. The tensile strength of the bond exceeds 1553N/mm2 (the electroplated nickel-based bond is 345- 449N/mm2). Achen University of Technology uses spray technology on the aluminum base plate of its grinding wheel to achieve reliable bonding of the abrasive layer and the matrix.

In addition, the reliability of the connection between the grinding wheel and the spindle must be fully considered. When the spindle rotates at high speed, the cone connection between the grinding wheel and the spindle expands unevenly due to the centrifugal force, and the connection stiffness decreases. In the ultra-high-speed grinding test, the author once experienced vibration during the startup process due to insufficient clamping force. Germany has developed HSK (short taper hollow shank) connection force type and technology for grade balancing of tools and automatic spindle balancing, but there are no reports of its use in ultra-high-speed grinding. Therefore, it is necessary to develop a connection method between the grinding wheel and the spindle with high precision, high stiffness and good dynamic balance performance.

Feeding system

High-speed machining not only requires the machine tool to have high spindle speed and power, but also requires the machine tool table to have high feed speed and motion acceleration.

The linear motor eliminates the intermediate transmission link and achieves the so-called "zero transmission". The feed speed can reach more than 60-200 m/mv, and the acceleration can reach more than 10-100m/s2. The positioning accuracy is as high as 0.5-0.05μm or even higher. It has large thrust, high stiffness, fast dynamic response, and unlimited stroke length. The main problem is that it generates serious heat, has an adsorption effect on dust and chips around its magnetic field, and is expensive. The linear motor produced by Siemens of Germany has a maximum feed speed of up to 200m/min. This high-efficiency surface grinder developed in Japan uses a linear motor for table feed, with a maximum speed of 60m/min and a maximum acceleration of 10 m/s2.

Grinding fluid and its injection system

The quality of the grinding surface, the accuracy of the workpiece and the wear of the grinding wheel are greatly affected by the grinding heat. Although liquid nitrogen cooling, jet cooling, minimum quantity lubrication and dry cutting have been developed, grinding fluid is still a cooling lubricating medium that cannot be completely replaced. Grinding fluids are divided into two categories: oil-based grinding fluids and water-based grinding fluids (including emulsions). Oil-based grinding fluids have better lubricity than water-based grinding fluids. However, water-based grinding fluid has a good cooling effect.

The good lubrication effect of oil-based grinding fluid can effectively reduce the friction between chips, workpieces, abrasive cutting edges and grinding wheel bond. This reduces the generation of grinding heat and wear of the grinding wheel, and improves the integrity of the workpiece surface. However, oil-based grinding fluid will produce oil mist during operation, seriously polluting the environment; it can easily cause smoke, fire, and is unsafe; and it can cause serious waste of energy. Because water-based grinding fluid has good cooling effect, good fire resistance, and easy to solve environmental pollution problems, it contains various surfactants, oily agents, extreme pressure additives, corrosion inhibitors and anti-corrosion bactericides with superior performance. Water-based grinding fluid is an important development direction in recent years. In addition to the usual grinding fluid, gaseous or solid grinding agents can also be supplemented.

The combined application of mixed grinding oil and synthetic water-based grinding fluid is particularly effective for grinding difficult-to-machine materials. Wet the grinding wheel with a small amount of oil to improve the lubrication effect, and inject water-based grinding fluid into the grinding arc to improve the cooling effect. Alternatively, oil is added before the grinding zone, and the water is only used to cool the workpiece surface. By combining water and oil, surface roughness and metal removal rates were obtained that were comparable to those of emulsions. Compared with simply using emulsion, it can reduce the wear of the grinding wheel. Its disadvantage is that it requires subsequent oil-water separation.

During high-speed grinding, the air flow barrier prevents the factory grinding fluid from effectively entering the grinding area, and there may also be the effect of film boiling. Therefore, it is extremely important to use appropriate injection methods to increase the effective part of the grinding fluid into the grinding zone and improve the cooling and lubrication effects to improve the quality of the workpiece and reduce the wear of the grinding wheel. Commonly used grinding fluid injection methods include: manual liquid supply method and pouring method; high-pressure injection method; air baffle-assisted interception of air flow method; grinding wheel internal cooling method; use of slotted grinding wheel method, etc. In order to improve the cooling and lubrication effect, a variety of methods are usually used in combination. For example, using a shoe-shaped nozzle can directly lubricate the grinding wheel in a larger area in front of the grinding wheel contact area, and the nozzle itself acts as an airflow baffle. The graphite tube floating nozzle combines grinding fluid and solid grinding agent. The graphite tube itself is equivalent to the air flow baffle for internal cooling of the jet. It combines the jet with the internal cooling of the grinding wheel and uses radial jet impact to achieve enhanced heat exchange. Effectively, it can break through the obstacles of film boiling. The combination of high and low pressure nozzles is used. The high pressure nozzle and air baffle are used to supply liquid to the grinding wheel and grinding area, and the low pressure nozzle cools the workpiece. Annular nozzles are also used to cool the workpiece, and the lubricating nozzles supply fluid to the grinding wheel and grinding area to reduce the overall temperature of the workpiece and improve the dimensional accuracy of the workpiece.

The position and geometry of the nozzle also have a great influence on the cooling and lubrication effects. Increasing the distance between the nozzle and the grinding zone reduces the cooling effect. Therefore, the nozzle should be as close as possible to the grinding arc zone to increase the effective flow and pressure into the grinding arc zone. Optimize the nozzle and use a nozzle with a concave inner cavity. The inner wall of the mesh is smooth and the outlet has a sharp edge, which can homogenize the liquid flow and produce a long high-polymer jet to improve the cooling and lubrication effect.

High-speed grinding fluid must be purified. The choice of filtration system is related to the length, thickness and type of chips, and also depends on the cutting depth of the abrasive particles. Commonly used filtration methods include: physical methods, such as gravity sedimentation, vortex filtration, magnetic filtration, filter filtration, filter belt (paper) filtration; chemical methods, such as using diatomaceous earth as a filter aid. In the filtration system, multiple filter units are used for composite filtration at the same time to achieve better results. Ultra-high-speed grinding systems also need to take measures to reduce the temperature of the grinding fluid. The main cooling methods include natural volatilization, convection heat dissipation, strong volatilization and the use of refrigeration systems for cooling.

In addition, the power consumption of the grinding wheel spindle caused by the grinding fluid and the influence of the dynamic and static pressure of the grinding fluid on the grinding force in the grinding area should also be studied. Optimize the fluid supply pressure and speed of high-speed grinding. Effectively reduce power consumption and negative impact on the environment. Relevant research shows that there is a critical speed for a certain flow rate. When the grinding wheel speed is greater than the critical speed, as the grinding wheel speed increases, the normal grinding force decreases.

Grinding wheel dressing

During the grinding process, the grinding wheel becomes dull or loses its correct geometry due to wear and must be trimmed in time. Trimming is divided into two processes: shaping and sharpening. Shaping is to make the grinding wheel achieve the required geometric shape and accuracy. Dressing is to make the abrasive grains protrude from the bond to create the necessary chip space, so that the grinding wheel can achieve better grinding ability. Depending on the specific circumstances, these two processes can be carried out uniformly or simultaneously, or they can be carried out in two steps.

Commonly used shaping methods include turning, grinding, and diamond roller methods. New shaping methods such as electric spark and laser methods are also being studied. Commonly used sharpening methods include free abrasive grain sharpening method (such as gas sandblasting sharpening method, free abrasive grain extrusion sharpening method, hydraulic sandblasting sharpening method, etc.) and consolidated sharpening tool sharpening method (such as whetstone method, Corundum block cutting method, grinding wheel grinding method, etc.) are two major categories. In addition, there are electrolytic online dressing methods, electric spark sharpening methods, high-pressure water jet sharpening methods and laser sharpening methods.

For new trimming methods, practical research should be accelerated. The development of dressing systems should give priority to the research of universal and efficient dressing systems.

Simulation and intelligence of grinding

Experimental research on ultra-high-speed grinding requires a lot of manpower and material resources. Therefore, with the development of computer technology, the use of computers to simulate the grinding process is an important research topic. The CIRP Grinding Technology Committee has regarded the "virtual laboratory" as an important Cooperation project, virtual grinder can In order to establish a realistic virtual grinding environment, it can be used to evaluate, predict the grinding process and product quality, and train. Computer simulation can be used to simulate the grinding process and simulate the temperature field in the grinding area, grinding force changes, etc. Analyze and predict grinding accuracy and grinding surface quality under different conditions.

The grinding process is a complex process with multiple variables. With the development of artificial intelligence technology and sensor technology, intelligent grinding has also become an important research direction. The basic purpose of intelligent processing is to solve the many uncertainties in the processing process, which can only be solved by human intervention. Computers replace or extend part of human mental work in the processing process. The key to realizing the automation of decision-making, monitoring and control in the processing process is decision-making automation.

The basic framework of the machine tool intelligent grinding system consists of the following two parts: ① Process model and sensor integration module. Multi-sensor information fusion technology is used to process processing process information to provide more accurate and reliable information for decision-making and control. The implementation methods of multi-sensor information fusion include weighted average method, Kalman filter, Bayesian estimation, statistical decision theory, Shafer-Dempster evidence reasoning, production rules with confidence factors, fuzzy logic, neural network, etc.; ② Decision planning and The control module makes decisions and plans based on the processing process information provided by the sensor module, determines the appropriate control method, generates control information, and acts on the processing process through the NC controller to achieve optimal control and achieve the required processing tasks. ③Knowledge base and database, which store prior knowledge about the machining process, various prior models for improving machining accuracy, known factors that affect machining accuracy, and the relationship between machining accuracy and parameters related to the I-adding process, etc. In addition, it should be capable of automatic learning and automatic maintenance.