Looking back on the development of bearing industry at home and abroad, the process flow and method of stable ring grinding superfinishing from double-end grinding, centerless cylindrical grinding and raceway cutting to raceway superfinishing have been formed in the 1960s. Up to now, the technological process has not changed fundamentally, but the bearing manufacturing technology is developing with each passing day: in the 1960s, a series of cutting centerless grinders and superfinishing machines were born, with the processing accuracy of parts reaching 3 ~ 5um and the processing time of single parts reaching 65433. In 1970s, a lot of new technologies were adopted: 60m/s high-speed grinding, control force grinding, electronic control technology featuring integrated circuits and digital control technology. The pre-tightening method of rolling NSK bearing improves the machining accuracy of parts to 1 ~ 3um, and shortens the machining time to 10 ~ 12s. Since 1980s, on the premise of stable quality, we have been pursuing high precision, high efficiency and high stability of equipment, as well as numerical control, flexibility and factory automation of manufacturing system.
1. Grinding of bearing rings
In bearing production, the grinding labor accounts for about 60% of the total labor, the number of grinders accounts for about 60% of all metal cutting machines, and the grinding cost accounts for more than 15% of the total bearing cost. For high-precision bearings, the grinding ratio is larger. Grinding is the most complicated link in the whole machining process, and it is also the link with the least understanding. The ferrule requires many performance indexes and high precision, and its machining and forming mechanism is complex, and there are many factors affecting machining accuracy, so it is difficult to detect machining parameters online. Therefore, for the grinding process, which is one of the key processes in bearing production, how to complete the grinding process with high precision, high efficiency and low cost, the comparison table (80) between the old and new codes of domestic bearings and imported bearings is the main task of grinding. As a rising star in the development of bearing equipment industry, Shanghai Rifa actively explores the introduction and practice of new technologies and new processes, stands on the shoulders of predecessors, digests and absorbs the latest grinding technologies at home and abroad, walks at the forefront of the industry development, opens up a unique development path, and gradually establishes a high-precision and efficient image of Rifa in the industry.
(1) high-speed grinding technology
High-speed grinding can achieve two goals pursued by modern manufacturing technology: improving product quality and labor efficiency. Practice has proved that when the grinding speed is increased from 35m/s to 50 ~ 60 m/s, the production efficiency can generally be increased by 30 ~ 60%, the durability of grinding wheel can be increased by about 0.7 ~ 1 times, and the parameter value of workpiece surface roughness can be reduced by about 50%. Domestic high-speed grinding technology started late, and the grinding speed is above 45m/s, which is generally called high-speed grinding. However, foreign high-speed grinding has developed rapidly and is widely used, adopting high grinding ratio? The grinding speed of superhard abrasive with high durability, such as CBN grinding wheel, has reached 80 ~ 120 m/s, or even higher. For example, the centerless grinder of Mikrosa in Germany, the centerless grinder of KOYO in Japan, the bearing internal grinder of Toyo in Japan and so on. The linear speed of the grinding wheel on the outer surface is120m/s, and the linear speed of the grinding wheel on the inner surface is 60 ~ 80m/s. Increasing the power of the grinding wheel drive (transmission) system and improving the rigidity of the machine tool are important measures to realize high-speed grinding, and the high-speed spindle unit is the most critical component of the high-speed grinding machine. In high-speed grinding, the grinding wheel should not only have enough strength, but also ensure good grinding performance in order to obtain high-speed grinding effect. In addition, the cooling device is also one of the indispensable devices to realize high-speed grinding.
(2)CBN grinding wheel grinding technology
Cubic boron nitride abrasive is called CBN abrasive for short, and the grinding wheel made of it is called CBN grinding wheel. The comparison table (79) between the old and new specifications of domestic bearings and imported bearings has the following characteristics: (1) high hardness, high thermal conductivity and good thermal stability, and can withstand the high temperature of 1300 ~ 1500℃. ⑵ High durability and low wear, the grinding ratio can reach 4000 ~ 10000 (grinding ratio refers to the ratio of material removal of workpiece to wear of grinding wheel), while ordinary corundum grinding wheel is only 50 ~ 80. (3) The grinding force is small, the grinding heat is small, the stress of the processed workpiece is small, and the surface stress is thin or not. (4) The auxiliary time (dressing grinding wheel and replacing grinding wheel) is greatly reduced.
In foreign countries, the application of CBN grinding wheel technology is called "a great revolution of production and processing technology". Since 1982, CBN grinding wheels have been widely used in Japan and developed rapidly. For China's bearing industry, grinding rings with CBN is still a new processing technology, which started late and needs to be widely used. It is also necessary to study and solve a series of technical problems such as manufacturing technology, dressing technology, special bearing grinder and grinding coolant. Shanghai Rifa has actively explored this aspect and achieved some initial results.
(3) Automatic balancing technology of grinding wheel for external surface grinding.
For external surface grinding, because the grinding wheel is large and uneven, the center of gravity of the grinding wheel system always deviates from the center of the spindle, which causes the vibration of the grinding wheel system and the whole machine tool when it rotates at high speed, affects the service life and machining accuracy of the machine tool, and produces grinding vibration lines on the workpiece surface, which increases the waviness. Mechanical or other automatic balancing devices are installed on the grinding wheel spindle, and the system automatically and quickly approaches the optimal balance state after starting. The breakthrough of this technology promotes the development of grinding technology, and at the same time, it can greatly prolong the life of grinding wheel, dressing diamond and spindle bearing, reduce the vibration of machine tool and maintain the original accuracy of machine tool for a long time.
(4) The technology of quickly eliminating the grinding clearance of the inner surface.
In all bearing grinding equipment, the level of internal grinder is symbolic. The comparison table (78) of the size parameters between the new and old specifications of domestic bearings and imported bearings is mainly because the grinding aperture limits the size of the grinding wheel and the setting parameters of the corresponding system mechanism, which fundamentally limits the rigidity of the process system and requires high machining accuracy. This requires us to conduct in-depth research on the process of internal surface grinding, not only to maximize the cutting ability of machine tools and grinding wheels, but also to reduce the auxiliary grinding time, which is also the key to improve grinding efficiency, because the grinding clearance accounts for about 10% of the whole grinding time.
At present, there are several technologies widely used at home and abroad to quickly eliminate grinding gap: control force grinding technology, constant power grinding technology, active measuring instrument technology and measuring electric spindle current technology.
(5) CNC technology and AC servo technology
Ac servo motor is connected with positioning module of PLC and servo amplifier to form servo system. The servo motor itself has an optical rotary encoder, and its output signal is fed back to the servo amplifier to form a semi-closed loop control system. Both high speed (3000 rpm) and low speed can ensure the positioning accuracy. The servo system can be used to complete fast jumping, fast feeding, dressing compensation and rough and fine grinding, which greatly simplifies the feeding mechanism of the machine tool and greatly improves the performance reliability.
(6) AC variable frequency speed regulation technology
When grinding, with the consumption of grinding wheel, the linear speed of grinding wheel decreases gradually. The ratio of head-to-tail linear velocity between domestic bearings and imported bearings (77) is about 3:2. At present, high linear speed grinding has been applied to the field of grinding wheels. In order to improve grinding efficiency and ensure the consistency of grinding quality, the grinding wheel radius after dressing the grinding wheel every time is calculated by using the calculation function of programmable controller, and then the input frequency of frequency converter to keep the linear speed of grinding wheel constant is calculated and transmitted to AC frequency converter, thus ensuring the linear speed of grinding wheel constant.
2. Ultra-precision grinding of bearing rings
Ultra-precision machining method was developed from the mid-1930s, and its basis is the machining of bearing rolling surface. This is an accurate and economical processing technology. With the continuous improvement of precision and surface quality requirements of machined parts, ultra-precision machining has been more and more widely used.
Ultra-precision machining, referred to as "ultra-precision machining" for short, generally means that under good lubrication conditions, the workpiece rotates at a certain speed, the oilstone elastically presses the surface of the workpiece under a certain pressure, and swings back and forth perpendicular to the rotation direction of the workpiece according to a certain law, so that the finishing can be automatically completed. It has the technical advantages of effectively reducing roundness deviation (mainly waviness), improving straightness of raceway generatrix or machining it into required convex shape, removing grinding metamorphic layer, reducing surface roughness value and increasing surface residual compressive stress. By improving grinding wheel FAG bearing technology, tile line is cut and thinned, and an ideal crosshair is formed on the machined surface, which can improve bearing rotation accuracy, reduce bearing vibration and noise, improve bearing capacity and improve bearing lubrication effect. Therefore, superfinishing technology plays an important role in the finishing of bearing manufacturing (polishing, belt grinding, superfinishing and superfinishing).
Superfinishing technology
The whole ultra-precision grinding process is divided into two stages: coarse ultra-precision machining and fine ultra-precision machining. In the rough overtaking stage, the whetstone abrasive is sharper, the whetstone pressure is higher, the workpiece rotation speed is lower, and the swing frequency is higher, so the cutting ability is strong, which is the main stage to remove the machining ability of the workpiece. In the finishing stage, the oilstone abrasive is relatively passive, the oilstone pressure is low, the rotating speed of the workpiece is high, and the swinging frequency is low, so the cutting ability is weakened, the polishing effect on the surface of the workpiece is enhanced, and the surface roughness value is greatly reduced. Among them, one-step two-step method, one-step two-step method, oilstone automatic compensation technology, oilstone automatic supply technology, coarse and fine super oilstone automatic conversion technology, high-frequency small oscillation plus low-frequency large reciprocating technology can all be embodied in daily equipment.
At present, the commonly used workpiece positioning methods of raceway superfinishing machine include end-face roller mechanical pressing type centerless clamping, hydraulic centering end-face roller mechanical pressing type clamping, double-roller driven end-face pressing type centerless clamping and so on.
Lubrication cooling technology
Ultra-precision machining has strict requirements on the filtration accuracy of lubricating coolant. SKF bearing rollers should have proper viscosity, antirust function, low volatility and repeated use. At the same time, ultra-precision lubricating fluid also plays the role of cleaning, cooling, lubricating and forming oil film.
Although some achievements have been made in the research and application of new ultra-precision grinding technology for bearing rings, there is still a long way to go to truly digest and absorb these new technologies, which requires unremitting efforts and diligent pursuit. Address: http://www.nskfag.org/news/201104 _ 36437.html.