Grinding technology is a crucial field in advanced manufacturing, and it is the most effective and widely used fundamental process technology in modern mechanical manufacturing for achieving high-speed, precision, and ultra-precision machining. Grinding accounts for 30% to 40% of the total machining volume.
High-speed grinding technology represents a revolutionary leap forward in grinding processes. It is a newly emerging comprehensive technology developed to meet the needs of modern high technology, integrating advanced technological achievements in modern mechanics, electronics, optics, computers, hydraulics, materials science, and metrology. With advancements in key technologies such as grinding wheel strength and machine tool manufacturing, grinding wheel speeds have significantly increased. Currently, grinding removal rates have surged to 3000 mm³ /mm·s or even higher, comparable to machining processes such as turning, milling, and planing. In recent years, the widespread application of various emerging hard and brittle materials, such as optical crystals, optical glass, ceramics, and single-crystal silicon, has driven the rapid development of high-speed grinding technology. The International Institute for Production Engineering (CIRP) has identified high-speed grinding technology as one of its central research directions for the 21st century.
Grinding with wheel speeds exceeding 45 m/s is termed high-speed grinding. Currently, high-speed grinding wheel speeds reach 60–250 m/s, with workpiece feed rates of 1,000–10,000 m/min. Within a wheel speed range of 60–120 m/s, using ordinary grinding wheels, the grinding removal rate can reach 500–1,000 mm³ /mm·s. Within a wheel speed range of 120–250 m/s, using CBN (cubic boron nitride) grinding wheels, the grinding removal rate can reach 2,000 mm³ /mm·s. The Technical University of Aachen in Germany conducted ultra-high-speed grinding experiments targeting wheel speeds of 500 m/s, comprehensively studying the relationship between grinding wheels and grinding technology. Previously, high-speed grinding was considered unsuitable for finishing large flat or cylindrical surfaces, primarily used for grinding grooves, notches, and plunge grinding. However, research in Japan and Germany has shown that increasing grinding speed can significantly improve workpiece grinding quality, reduce grinding force, achieve smaller dimensional and shape errors, and improve machining accuracy. A high-speed (wheel speed 160~260 m/s) cylindrical grinder developed in Japan, using CBN grinding wheels, can achieve a roundness error of 1 μm and a surface roughness value Rz = 1.2 μm.
I. Characteristics of Ultra-high Speed Grinding
Ultra-high speed grinding has the following significant advantages compared to conventional grinding:
(1) The grinding force is small and the machining accuracy of the parts is high. When the grinding efficiency is the same, the grinding force when the grinding speed reaches 200m/s is only 50% of that when the grinding speed is 80m/s. However, under the same single abrasive grain removal conditions, the grinding speed has a very small effect on the grinding force, thereby improving the machining accuracy.
(2) It can obtain a workpiece surface with high quality and low roughness value.
Under the condition of constant material removal rate, increasing the grinding speed can reduce the grinding depth of a single abrasive grain, thereby reducing the grinding force. This reduces the surface roughness of the workpiece and makes it easier to maintain machining accuracy when machining low-rigidity workpieces. If the original grinding force is maintained during high-speed grinding, the feed rate can be increased, machining time can be reduced, and production efficiency can be improved.
(3) It can significantly improve grinding efficiency and reduce the number of machines used. In the past, grinding was only suitable for finishing, and although the machining accuracy was high, the machining allowance was very small. Many roughing processes needed to be arranged before grinding, and different types of machine tools were required, forming a lengthy process chain. At present, the material removal rate of high-speed grinding is comparable to that of turning and milling. Therefore, grinding can be used for both finishing and roughing, which can greatly reduce the types of machine tools and simplify the process flow. For some products that use grinding as the final machining process, high-speed grinding can significantly reduce production costs and improve product quality.
(4) It can significantly extend the grinding wheel life, which is beneficial to the automation of grinding. Under the condition of constant grinding force, when grinding at a grinding speed of 200 m/s, the grinding wheel life is twice that when grinding at 80 m/s, and under the condition of constant grinding efficiency, the grinding wheel life can be increased by 7.8 times. The grinding wheel life increases logarithmically with the grinding speed. When using diamond grinding wheels to grind silicon nitride ceramics, the grinding speed increases from 30 m/s to 160 m/s, and the grinding wheel ratio increases from 900 to 5100, which is beneficial to the realization of automated grinding.
(5) Successfully overcome the influence of the grinding hot channel, the workpiece surface layer can obtain residual compressive force (which is beneficial to the stability of the workpiece).
II. Key Technologies of Ultra-High Speed Grinding
The diagram below lists the various technologies required for ultra-high speed grinding, among which the design and manufacture of high-speed bearings and high-speed grinding wheels are the most important factors affecting the application of ultra-high speed grinding technology.
1. Ultra-high speed grinding wheel
(1) Structure and manufacturing of ultra-high speed grinding wheels Ultra-high speed grinding wheels must meet the following requirements: The mechanical strength of the grinding wheel matrix must be able to withstand the cutting force during high-speed grinding; the appearance must be sharp, that is, the abrasive grains must protrude a large height so as to accommodate a large number of long chips; the bonding agent must have high wear resistance to reduce the wear of the grinding wheel.
Looking at the development trend of ultra-high speed grinding, diamond and CBN grinding wheels are playing an increasingly important role, and the bonding agents used can be resin, ceramics, and metals. With the further promotion and in-depth research of ultra-high speed grinding, new types of abrasives and bonding agents are constantly emerging.
Electroplated grinding wheels are widely used in ultra-high-speed grinding. Their abrasive grains have a large protrusion height, allowing them to hold a large amount of chips and reducing the likelihood of dulling the cutting edge, which is highly advantageous for ultra-high-speed cutting. Because electroplated grinding wheels have only one layer of abrasive grains, they do not require dressing during use, thus saving on expensive dressing equipment and time.
Recently, the Swiss company Winterthur developed a new type of CBN grinding wheel. The basic shape of the abrasive grains is tetrahedral, which splits when the grinding force increases to a certain level, thus forming new sharp cutting edges. When grinding alloy tool steel, it can effectively reduce cutting force and cutting temperature; while maintaining the same grinding wheel life, it can improve the material removal rate and workpiece accuracy. In recent years, due to the reduction of abrasive layer thickness and the improvement of corresponding manufacturing processes, Japanese ceramic-bonded grinding wheels can operate safely at 300 m/s, and the operating speed of single-layer electroplated CBN grinding wheels can be selected up to 250 m/s. Europe mainly uses single-layer electroplated CBN grinding wheels for high-speed and high-efficiency profile grinding and grooving grinding. The American company Norton has developed a metal single-layer grinding wheel using copper brazing technology, with an abrasive grain protrusion ratio of 70%~80%, greatly increasing the chip space, and the bonding tensile strength exceeding 1500 N/ mm² . Under the same grinding conditions, it can reduce the grinding force by 50%, further improving the grinding efficiency limit.
To ensure the grinding wheel remains sharp throughout its service life, its structure should facilitate abrasive grain splitting and maintain the self-excitation process. To achieve self-sharpening, besides minimizing the proportion of binder, the spatial distribution of abrasive grains must be optimized. This can be achieved by calculating the forces acting on individual abrasive grains during splitting using a computer, thus determining the appropriate binder ratio.
my country still lacks dedicated ultra-high-speed grinding wheels, and research on the design theory, manufacturing, and installation of ultra-high-speed grinding wheels has only just begun.
(2) Dressing of Ultra-high-speed Grinding Wheels Ultra-high-speed single-layer electroplated grinding wheels generally do not require dressing. In special cases, a coarse-grained, low-concentration electroplated cup-shaped diamond dressing tool is used to dress only a few high points at the micron level. Experiments show that when the dresser feed rate is 3~5μm, not only can the workpiece quality be guaranteed, but the grinding wheel life can also be extended.
For certain high-speed grinding processes, not only is high grinding efficiency required, but also high grinding quality (high machining accuracy and low surface roughness). Therefore, a complete dressing technique for grinding wheels is necessary.
Ultra-high speed metal bond grinding wheels are generally dressed by electrolytic dressing, while the dressing grit size of ultra-high speed ceramic bond grinding wheels has a significant impact on grinding quality.
The Toyota Koki GZ50 ultra-high-speed cylindrical grinding machine is equipped with a fully automatic dressing device at the rear of the spindle. The diamond roller rotates at a speed of 25,000 r/min, and the surface of the CBN grinding wheel is detected by an acoustic emission sensor. The ultra-high-speed grinding wheel is dressed with a feed accuracy of 0.1 μm.
Given the limitations of traditional dressing methods, various methods for dressing superhard abrasive wheels have been developed, including electrolytic in-line dressing (ELID), dual-electrode in-line dressing, elastic dressing, ultrasonic vibration dressing, and laser dressing. Among these, laser dressing is an ideal method for dressing superhard abrasive wheels. It offers advantages such as fast dressing speed, high efficiency, material savings, and ease of automation for in-line dressing. In particular, compared to dressing superhard wheels using conventional grinding methods, laser-dressed superhard abrasive wheels exhibit excellent grinding performance, reducing grinding force by 10%–15% when grinding ceramics under the same grinding conditions. Experiments have shown that the grinding force and surface roughness of diamond-dressed wheels are essentially close to those of laser-dressed wheels with a power density of 6.0 × 10¹⁰ W/ m² . However, as the grinding process continues, after several grinding strokes, laser-dressed wheels show an even better trend: the grinding force remains stable over a long period, and the surface roughness is lower.
(3) Dynamic balancing technology of ultra-high speed grinding wheels Due to manufacturing and adjustment errors, the grinding wheel spindle used in ultra-high speed grinding must be dynamically balanced after changing or dressing the grinding wheel, or even when restarting after stopping. For this reason, the high-speed grinding spindle must have a continuous automatic dynamic balancing system to minimize vibration during grinding, thereby obtaining higher machining accuracy and lower workpiece surface roughness value.
Based on the balancing principle and different structural forms of automatic dynamic balancing devices, automatic dynamic balancing technology for grinding wheels can be divided into the following types:
Electromechanical dynamic balancing technology: In the late 1980s, Schmitt Industries in the United States produced what was hailed as the world's most advanced online grinding wheel dynamic balancing system—the SBS computerized grinding wheel balancing system. This system uses a microcomputer-controlled micromotor to move tiny weights within the balancing device, thereby adjusting the balance of the grinding wheel. Japan developed a light-controlled balancing device that uses a microcomputer to control the transmission mechanism and drive components within the balancing device to move the balance weights and adjust the balance of the grinding wheel.
Liquid injection dynamic balancing technology: Hofmann GmbH of Germany (which specializes in dynamic balancing technology and devices) has proposed an automatic liquid dynamic balancing device for grinding wheels. This device has four water storage chambers of a certain capacity installed on the flange of the grinding wheel, evenly distributed in different quadrants. Each water inlet corresponds to a water spray nozzle controlled by a solenoid valve. By injecting a certain amount of liquid into different water storage chambers through different water spray nozzles, the mass of different quadrants of the grinding wheel can be changed, thereby achieving automatic dynamic balancing of the grinding wheel.
Liquid-vapor dynamic balancing technology: Balance Dynamics Corporation in the United States has successfully developed a liquid-vapor grinding wheel balancing device that uses Freon as the balancing medium. This device is characterized by its simple structure, the absence of openings, nozzles, valves, gears, and other moving parts, reliable performance, maintenance-free operation, and ease of use. The vaporized Freon cools and reverts to a liquid state, remaining within the chamber, thus maintaining balance even when the grinding wheel stops rotating.
2. Cooling and lubrication system for ultra-high speed grinding
In ultra-high-speed grinding, the performance of the cooling and lubrication system often determines the success or failure of the entire grinding process. The functions of cooling and lubrication are to improve the material removal rate, extend the service life of the grinding wheel, and reduce the surface roughness of the workpiece. During ultra-high-speed grinding, the cooling and lubrication system must perform four main tasks: lubrication, cooling, cleaning the grinding wheel, and chip removal. For high-precision, high-speed grinding, a temperature control system is also required to ensure a constant temperature of the cooling and lubricating fluid.
For ultra-high speed grinding, a fluid supply pressure of 7×106Pa or higher and a large flow rate are required, along with a suitable ultra-high speed grinding fluid supply method. This ensures that the chip space of the grinding wheel is thoroughly cleaned, preventing wheel clogging, which can lead to abrasive grain heating and wear, as well as increased grinding force.
In ultra-high-speed grinding, dry (green) grinding methods can be implemented to achieve the goal of eliminating grinding fluid. Examples include air-cooled grinding and liquid nitrogen-cooled grinding.
3. Ultra-high speed spindle and ultra-high speed bearing technology
(1) Ultra-high speed electric spindle technology. Currently, ultra-high speed grinding mainly adopts high-power ultra-high speed electric spindles. The advantages are small spindle inertial torque, low vibration and noise, good high-speed performance, and shortened acceleration and deceleration time. However, in terms of accuracy, how to reduce motor heat generation and heat dissipation will become a research topic in the future. At present, the German company Hofmann is conducting ultra-high speed grinding experiments, using a high-frequency spindle with a maximum power of 25kW to achieve a grinding wheel speed of 500m/s, and can work normally at speeds of 30,000r/min and 40,000r/min. A Japanese bearing factory has developed an ultra-high speed grinding head with an internally mounted servo motor, which can work stably at a high speed of 25×104r/min.
(2) Ultra-high speed bearing technology. High-speed precision bearings are the core components of ultra-high speed grinding spindle systems. They are divided into four categories: ball bearings, hydrostatic bearings, air hydrostatic bearings, and magnetic bearings. Due to the many advantages of rolling bearings, most ultra-high speed grinding machines abroad currently use rolling bearings, but steel ball bearings cannot be used. In order to improve their limiting speed, the following measures are often adopted: ① Improve the manufacturing precision level. ② Rationally select materials. By using a hybrid ball bearing with ceramic balls and steel bearing inner and outer diameters, its service life can be increased by 3 to 6 times, the limiting speed can be increased by 60%, and the temperature rise can be reduced by 30% to 60%, with its DN value reaching more than 3 million. ③ Improve the bearing structure. The new ultra-high speed spindle bearing developed by the German FAG bearing company has reduced the ball diameter to 70% and increased the number of balls, thereby improving the rigidity of the bearing. If the lubrication is sufficient and reasonable, its DN value during continuous operation can reach 2.5 million. Using hollow rolling elements can reduce the rolling mass, thereby reducing centrifugal force and gyroscopic torque.
The CNC ultra-high-speed surface grinder developed by Tohoku University in Japan uses ceramic ball bearings and has a spindle speed of 3×10⁴ r/min. Toyota Koki has equipped its G250 CNC ultra-high-speed cylindrical grinder with newly developed Toyoda State bearings, using ceramic-bonded CBN grinding wheels with a speed of 200 m/s, enabling efficient, high-precision, and highly flexible machining of rotating parts. Toshiba Machinery Corporation of Japan has achieved a spindle speed of 3×10⁴ r/min at high power using improved air bearings on its ASN40 machining center. Koyo Seiko Co., Ltd. of Japan and Kapp GmbH of Germany have successfully used magnetic levitation bearings in their high-speed grinders. Magnetic levitation bearings have low power consumption, low maintenance costs, and do not require complex and expensive sealing devices, but the bearings themselves are very expensive, and the control system is complex. Kapp GmbH's magnetic levitation bearing grinding wheel spindle can reach a speed of 6×10⁴ r/min. GMN GmbH's magnetic levitation bearing spindle can reach speeds exceeding 10⁴ r/min. In addition, hydrostatic bearings have been gradually applied to high-speed and high-efficiency grinding machines.
4. High-speed feed drive system
In the early 1990s, the slide drive system of ultra-high speed grinding machine tools mostly adopted large-lead ball screw transmission and increased servo feed motor speed, with a typical feed speed of 60 m/min. In order to achieve higher feed speeds, linear motor drive systems have emerged in recent years. Due to their backlash-free operation, low inertia, high rigidity, wide speed range, high repeatability, and no wear, high speed and high drive can be achieved through control circuits. In 1997, the feed speed reached 120 m/min.
Anorad Corporation of the United States is one of the world's most renowned manufacturers of linear motors. The company launched its brushless DC linear motor in 1988 and obtained a US patent. The company primarily produces brushless DC linear motors (permanent magnet synchronous linear motors). Its latest coreless LZ series linear motors have 48 models with a force ranging from 350 to 4000 N, forming a series of products with different structures and power ratings, widely used in various fields. Currently, besides Anorad, other well-known international linear motor manufacturers and suppliers include the American industrial giants Parker-Hannifin, Aerotech, and Kollmorgen; German companies Siemens and Indramat; Japanese companies Mitsubishi and FANUC; and the Swiss company ETEL.
The German company Indramat manufactures both induction linear motors and permanent magnet linear motors, offering over 50 different models. The permanent magnet models are characterized by high efficiency (up to 1.72 N/W) and high thrust density. Reportedly, their products can reach speeds of 600 m/min and thrust of 22 kN.
The permanent magnet linear motors from the Swiss company ETEL are well-suited for high-speed machine tool feed systems. Their main features include: a high motor constant Km; integration of the water cooling system into the motor windings, eliminating hot airflow from the motor to the machine tool; high force density, reaching 4.9~7.8 N/ cm² ; acceleration up to 300 m/ s² or even higher; and high servo stiffness, at 300 N/μm within the 30~60 Hz range.
III. Development Trends of Ultra-High Speed Grinding Technology
(1) Develop high-power high-speed spindles. We must vigorously develop high-power high-speed spindles. Only when the spindle can output a large amount of power at high speed can the economic benefits of high-speed grinding be fully realized.
(2) Developing new grinding wheels and intelligent processing technologies suitable for high-speed grinding. In addition to developing new abrasive grains, it is also necessary to conduct thorough research on bonding agents and the grooves of the bonding agent and abrasive grain system suitable for high-speed grinding. Furthermore, for expensive superhard abrasive grinding wheels, to fully utilize their advantages of high production efficiency and long service life, their consumption should be minimized and dressing efficiency and accuracy improved as much as possible. To this end, it is necessary to develop intelligent grinding processing technologies based on grinding databases and knowledge bases and detection systems.
(3) Improve the existing grinding machine structure and develop the third generation of high-speed machine tools. In order to minimize the vibration caused by the imbalance of the grinding wheel at high speeds, an online automatic dynamic balancing system should be configured to ensure that the machine tool is always in the best operating condition at different speeds. In order to improve production efficiency and machining accuracy, a high-speed, high-efficiency and high-precision linear feed drive system should be adopted.
The emergence of parallel kinematic machine tools represented a revolutionary change in machine tool design and manufacturing technology in the 1990s. Due to their unique advantages not found in conventional machine tools, they have attracted widespread attention. As a third-generation machine tool, parallel machine tools should be actively developed and gradually brought into application.
(4) Actively develop dry (green) cooling and lubrication technologies. In ultra-high speed grinding, the cooling and lubrication system should be further optimized. In addition, due to environmental protection needs, dry (green) cooling and lubrication technologies should be vigorously developed, such as air-cooled high-speed grinding and liquid nitrogen-cooled high-speed grinding.
(5) High-speed grinding is moving towards ultra-high speed. The next goal of research on ultra-high speed grinding applications is to break through the speed of sound barrier and increase the grinding speed to more than 350 m/s, thereby making a grinding speed of 500 m/s possible in industrial applications.
