CBN wheel grinding has the obvious advantages of high efficiency, high precision and low cost, which is the inevitable trend of camshaft grinding technology development. Based on the results of many years of experimental research and relevant technical literature, this paper points out the main restricting factors in the process of pushing CBN grinding technology of camshaft to the market in China, and puts forward some positive suggestions, in order to widely adopt CBN grinding technology in camshaft machining and improve the overall technical level of engine machining.
As one of the key parts of the engine, the machining quality of camshaft directly affects the dynamic characteristics of the engine. At the same time, camshaft is a kind of non-circular grinding workpiece, its machining allowance is large and the material is difficult to grind, the grinding precision and production efficiency requirements are very high, the processing difficulty is relatively large. Therefore, the grinding technology of camshaft has always been the focus of the industry. How to improve the grinding efficiency and machining quality is an urgent problem to be solved in camshaft grinding. The following influencing factors should be considered:
The characteristics of machine tools;
● CAM contour grinding forming method;
● Grinding wheel performance and coolant;
● Grinding process, including dressing tools and dressing process.
Development status of camshaft grinding technology at home and abroad
At present, CBN grinding technology has been introduced in camshaft production lines of most car engine factories and professional camshaft manufacturers in China, but there are still a lot of truck, diesel engine and motorcycle engine cams still use the traditional corundum wheel, by imitation shape grinding process. Domestic medium and low speed grinding machines (below 35m/s) are used in rough grinding process, while imported grinding machines are used in some manufacturers in fine grinding process, but their working speed is below 60m/s. Most dressing tools are single-point diamond pens, while imported grinding machines and a few domestic grinding machines use diamond roller dressing. The problems brought by this traditional technology to camshaft grinding are mainly reflected in the following aspects:
● CAM contour accuracy is low and difficult to improve
The CAM contour shape error can only be controlled within ±0.03mm by using dependent plate grinding, while it can be controlled within ±0.01mm by using full CNC non-dependent plate grinding. In addition, when ordinary abrasive grinding wheel is in use, the outside diameter varies widely (80mm ~ 100mm), and every 1mm change in the diameter of grinding wheel will make the CAM profile change 0.007mm, so it is difficult to further improve the accuracy of CAM profile.
● CAM surface is easy to produce burns, cracks and other defects, it is difficult to improve production efficiency
Because the CAM grinding allowance is large and the material is difficult to grind, the performance of ordinary abrasive wheel is difficult to adapt, grinding quality and production efficiency can not be taken into account.
● The comprehensive economic benefits are not high
Due to the low durability and service life of ordinary abrasive grinding wheels, frequent dressing or replacement is required, which accelerates the loss of dressing tools, increases the auxiliary time and labor intensity, which not only affects the production efficiency, but also increases the production cost. In addition, due to the large amount of grinding wheel, the quality fluctuation also affects the stability of grinding process, and because of the production of a large number of grinding residues, the amount of grinding fluid filtration and cleaning is increased, which causes certain pollution to the environment.
With the application of CBN grinding technology, the difficult problems in the traditional camshaft grinding process can be easily solved. More than ten years ago, the report of using CBN wheel to grind camshaft was only seen in limited foreign literature. In large-scale industrial production, the technology of grinding camshaft with CBN special CNC camshaft grinder has just entered the practical stage in some western developed countries. Keep up with the world development trend of advanced manufacturing technology, zhengzhou institute of abrasives grinding in the early 1990 s, in the domestic first independent research and development for car camshaft grinding of ceramic bond CBN grinding wheel, in domestic ordinary high speed (60 m/s) strong coarse grinding cold shock cast iron camshaft on the camshaft grinding machine, has obtained the good use effect. However, due to the limitations of machine tool conditions and grinding wheel manufacturing technology at that time, a more ideal effect was not achieved. By the mid-1990s, most automobile factories in industrialized countries had adopted CBN grinding wheels to grind camshafts, and the technology was introduced to major car production lines in China. It is introduced in the form of CBN special CNC camshaft grinding machine, high speed and high performance ceramic CBN grinding wheel, special grinding fluid, dressing roller and grinding process whole set of technology. The main characteristics of this technology are: high speed of grinding wheel (80m/s ~ 125m/s), high machining efficiency (the workpiece is finished by rough grinding and fine grinding at one time, and the efficiency is 2 ~ 3 times that of ordinary grinding wheel). With the continuous introduction of CBN grinding technology and special grinding machine for camshaft, the special high-speed CNC camshaft grinding machine for CBN studied in China will soon enter the market. In this regard, while improving the technology of coarse grinding camshaft with CBN grinding wheel for domestic grinder and actively studying the fine grinding of camshaft with CBN grinding wheel, Zhengzhou Abrasive Grinding Institute has started to develop high-speed and high-efficiency ceramic CBN grinding wheel for imported CBN special CNC camshaft grinding machine, and has achieved certain achievements. The service speed of the developed grinding wheel is 80m/s, and its durability and life are equivalent to 1/2 of the imported grinding wheel, while the price is only 1/3 of the imported grinding wheel. Compared with similar imported grinding wheels, the developed ceramic CBN grinding wheel has the main difference in durability, life and product quality stability, which can not completely replace the imported grinding wheel for continuous use in camshaft production line. In order to catch up with the rapid development of technology, in the past two years, zhengzhou institute of abrasives grinding was developed using the rate of 125 m/s camshaft ceramic CBN grinding wheel, its use speed, grinding efficiency, grinding quality have reached the level of imports of the same kind, has the very high cost performance, quality of product quality is stable, can completely replace the imported. All these indicated that the manufacturing and application technology of CBN wheel in our country had reached a new level.
Technical elements of CBN grinding for camshaft
A notable feature of the development of CBN grinding technology in industrialized countries is that CBN abrasives, abrasives, grinders and grinding processes are carried out as a system engineering. Such as the United States in the early 1990s, in order to solve the camshaft CBN grinding technology, the united GE company (CBN abrasive), Norton company (abrasive) and Landis company (grinder) three top companies in the industry to jointly tackle key problems. They finally provide a complete set of technologies to the market with high-speed CBN grinding wheels, high-speed CNC CBN special grinding machines and CBN grinding process.
In THE grinding system of camshaft, IT is necessary to consider all the factors involved in the manufacturing and application technology of CBN wheel at the same time to get a satisfactory grinding result. The main influencing factors of camshaft CBN grinding technology are as follows:
Low CBN grinding wheel
The grinding wheel with superior performance must have the characteristics of sharp grinding, good self-sharpening and high durability. The bonding agent, abrasive material, concentration of grinding wheel and hardness of grinding tool are mainly considered when making and selecting grinding wheel.
1. Binder
Among the four kinds of binder (resin, ceramic, metal, electroplating) of CBN abrasive tools, CBN abrasive tools with ceramic binder develop the fastest. In the world, the proportion of ceramic CBN abrasive tools has risen from 4% in the 1980s to more than 50% now, the growth rate is rapid. Ceramic CBN grinding tool has become the first choice for high efficiency and high precision grinding because of its advantages such as high grinding efficiency, good shape retention, high durability, easy dressing, high abrasive utilization rate (more than 75%, other types of binder is 50%-60%) and long grinding wheel service life. At present, all CBN wheels used for camshaft grinding use ceramic binder.
2, abrasive selection
In grinding process, abrasive is the main body of abrasive tools, and its performance directly affects the grinding effect. CBN abrasives have higher hardness and strength than corundum abrasives (see Table 4), resulting in sharp cutting and abrasion resistance. In the case of high intensity grinding such as camshaft machining, CBN abrasive is the best choice. Different grades of CBN abrasives, due to different manufacturing processes, their crystal shape, particle shape is also different, they have different strength, thermal stability, chemical erosion resistance and crushing characteristics. Different grades of abrasives should be selected according to the type of binder, grinding workpiece and grinding method. It should be pointed out that the structure of CBN abrasive is easily damaged by chemical reaction with water and alkaline oxides at high temperature, which must be considered when choosing the type, pressure and flow of grinding fluid.
3 and the concentration of the
The grinding wheel concentration indicates the number of grinding grains per unit area of the grinding wheel working face during grinding. High concentration can lead to high grinding ratio. 200% concentration is 4 ~ 5 times longer than 100% concentration grinding wheel life. At present, high speed and high efficiency grinding use high concentration of grinding wheel, such as the imported camshaft grinding machine matching ceramic CBN grinding wheel concentration is generally 200%.
4, hardness
The hardness level of abrasive bonding agent on abrasive control force size, it is an important index of the manufacturer's process control, but also the user to choose the main parameters of abrasive performance. Uniform and stable hardness of grinding wheel and reasonable selection of hardness are important prerequisites to ensure grinding quality. Foreign CBN grinding wheels generally have 3 ~ 7 hardness levels to choose from. At present, there is no hardness test standard for CBN grinding wheels (including diamond grinding tools) in China, and the manufacturer only controls the hardness according to the formula.
Low grinding machine
Grinding wheel as an accessory in grinding system with grinding machine as the center, only through the excellent characteristics of grinding machine and optimization of various grinding parameters, can maximize the advantage, especially for CBN grinding.
1. High speed
Increasing the working line speed of grinding wheel can obviously improve the grinding efficiency and grinding ratio, reduce the grinding force, and thus reduce the grinding cost. For example, when the grinding wheel speed is increased from 80m/s to 160m/s, the grinding time is the same, the dressing interval (durability) will be increased by 2 times. When the grinding wheel speed increases from 35m/s to 60m/s, the normal grinding force (F 'n) decreases by about 1/3 at different unit metal removal rates (Q 'w). Because of its structure and CBN abrasive characteristics, CBN wheel provides the possibility for high speed and ultra-high speed grinding. The use speed of 80 ~ 125m/s has become one of the basic characteristics of CBN special camshaft grinder. Using the highest possible speed is an important prerequisite to improve the technical and economic performance of CBN grinding.
2. High rigidity and shock resistance of machine tool
The characteristics of high speed grinding and CBN grinding wheel require the spindle and the whole machine tool to have high rigidity and good shock resistance, so as to ensure the precision of grinding workpiece and surface quality, which is the basic requirements of grinding machine in CBN high speed grinding technology. Without these conditions, the geometric accuracy and surface quality of the workpiece will deteriorate in order to obtain higher metal removal amount when using CBN grinding wheel. Corrugation is a common surface quality defect, which is produced by vibration. There are many reasons for the vibration, including low rigidity of the machine tool, poor shock resistance, excessive grinding force caused by machine tool resonance or unreasonable design of grinding wheel parameters. Some data show that: the stiffness of the machine tool should not be less than 100N/0.001mm order of magnitude as well.
Low nap
Diamond wheel dressing can not only improve dressing efficiency, but also obtain better grinding wheel morphology. It is difficult to finish dressing the surface of high hardness grinding wheel with other dressing tools. The feed precision of dressing device should be high, and each feed should be at MM level. Excessive dressing will not only affect the grinding quality but also greatly reduce the service life of grinding wheel. The dressing speed ratio Vr /Vc is an important parameter in dressing process. Its change will make the surface morphology of grinding wheel change significantly, and eventually affect the surface quality of workpiece.
Low coolant
During grinding, more than 90% of the energy is converted to thermal energy, which must be maximally absorbed by the coolant or the workpiece will be burned. For CBN grinding wheel, the damage of abrasive caused by chemical reaction between CBN abrasive and water at high temperature should also be considered. The correct selection of coolant type and cooling process parameters will often get twice the result with half the effort. Different coolant will make the grinding ratio of the grinding wheel differ several times or even ten times. Table 5 shows the influence of coolant on CBN grinding wheel. The results show that the degree of mechanical wear, chemical erosion and thermal damage of grinding wheel are closely related to the cooling effect.
Questions and Suggestions
At present, in the process of promoting CBN grinding technology of camshaft to the market in China, the constraints are mainly reflected in the following aspects:
1. The imported grinding machine and the matching CBN grinding wheel are expensive. The price of the grinding machine is about 10 times that of the ordinary camshaft grinding machine in China, and 3 ~ 4 times that of the similar grinding machine developed in China. The price of the imported CBN grinding wheel is 3 ~ 5 times that of the domestic similar grinding wheel, which makes the further promotion in the domestic market restricted by the cost.
2. Most of the grinding machines used in domestic camshaft processing have low speed, and their accuracy, rigidity, vibration resistance, dressing, cooling and other conditions can not meet the requirements of using CBN grinding wheel. Therefore, when CBN grinding wheel is used, the desired effect is often not achieved.
3, domestic CBN grinding wheel grinding performance has reached the level of similar foreign products, only in the grinding wheel durability, service life and stable quality, adaptability are slightly lower, but the price is low, and therefore have higher cost performance, its technology and economy has been recognized by domestic and many car makers and production, but still need to further improve the level of CBN grinding wheel manufacturing technology.
4, the new technology publicity is not enough, some enterprises conformism, the demand for new technology is not strong, to improve production efficiency and processing quality do not pay attention to, is the external constraints of the promotion of this technology.
Therefore, the following suggestions are put forward:
1, speed up the development of domestic CBN special high-speed CNC CAM grinding machine, focus on solving the key technology of rigidity and vibration resistance, first make 80m/s grinding machine as soon as possible on the market, make its technical and economic performance to meet the needs of most small and medium-sized enterprises. On this basis, the development of higher speed, more perfect performance of the product.
2. The research of CBN grinding wheel should be based on improving performance, perfecting process, stabilizing quality and mass production. Should strengthen the basic theory research and special equipment research and development. Through the development of matching grinding wheel for imported grinding machine, the key technology of its manufacture is solved, and the needs of domestic production of imported grinding wheel are met. At the same time, considering the current technical status of domestic camshaft grinding machine, the design of CBN grinding wheel manufacturing technical parameters as far as possible to meet the requirements of this part of the grinding machine to use CBN grinding wheel, so that CBN grinding as far as possible to play the maximum effect.
3. Establish a close relationship between grinding machine, grinding tool manufacturing and users, strengthen communication and cooperation, and jointly promote CBN grinding technology
