the domestic steel ball final processing technology used to mainly use cast iron disc to add abrasive for grinding, but due to the quality of cast iron disc, the addition of abrasive agent is not uniform, the abrasive agent is not easy to keep clean and other factors, resulting in unstable surface quality of the steel ball, especially the vibration value of the steel ball is not easy to reduce. To this end, in the late 1990s, China began to introduce resin grinding wheels from abroad, and began to process high-precision steel balls with grinding research and application.
1 Advantages of grinding steel balls with resin grinding wheel
The main components of the resin grinding wheel are abrasive, resin, wetting agent, etc., which is characterized by fine particle size, high hardness, uniform and dense internal organization, regular appearance and accurate size. Because the resin grinding wheel is elastic, the pressure on the steel ball is small in the grinding process. Under similar grinding conditions, the steel ball processed with the resin grinding wheel is higher in precision, better in surface quality, lower in vibration value, and better in silence effect after the bearing is closed. In addition, the resin grinding wheel has high grinding efficiency, and its production efficiency is more than 1 times that of cast iron disk. Although the price of resin grinding wheel is higher than that of cast iron disc, its processing efficiency is high, the durability of the grinding wheel is high, and its comprehensive cost is lower than the grinding cost of cast iron disc. At the same time, the special grinding fluid (oil) of the resin grinding wheel is more environmentally friendly, and the surface of the steel ball after processing is easy to clean.
2 Selection of grinding machine with resin grinding wheel
The domestic resin grinding wheel grinding process is initially carried out on the vertical ball lapping machine, because the grinding plate of the vertical machine tool is placed horizontally, resulting in the grinding process of resin grinding wheel off the grinding chips embedded in the grinding groove, it is difficult to remove by washing the grinding fluid. Therefore, the difficulty of removing chips becomes the difficulty of grinding resin grinding wheel for vertical machine tools.
For this reason, the horizontal machine tool has become the preferred machine tool for grinding steel balls with resin grinding wheel. 3M77 series steel ball lasing machine adopts horizontal layout, open overall design, the design of bed and head frame adopts the modular design of high and low combination (low center of gravity L-shaped structure) guide rail, improving the strength and rigidity of the machine tool. The machine bed adopts special technological means in the processing to eliminate potential deformation and fully ensure the coaxiality and parallelism of the machine tool. At the same time, the special spindle structure and technological means are adopted, and the imported bearings and optional installation are adopted, which greatly improves the rotating accuracy of the spindle and provides a prerequisite for the resin grinding wheel to process high-precision steel balls. The movement of the spindle head frame is positioned by rolling linear guide rail, which has high repeated positioning accuracy of the machine tool, and overcomes the change of the parallelism of the two disks caused by the positioning of the cylinder in different positions of the machine tool during the pressure process. The application of the separation of pressure and sliding mechanism of the machine tool greatly improves the rigidity and stability of the whole machine, and provides a guarantee for the batch processing of G5 or even G3 grade low noise steel balls. The electric and hydraulic system of this series of machine tools adopts advanced frequency conversion control technology and electro-hydraulic proportional regulation technology, which can realize the three-level program control conversion of initial pressure, medium pressure and fine pressure. According to the process route, the processing parameters such as pressure and time are set in advance to achieve automatic conversion in the work, which greatly improves the production efficiency.
3M77 series horizontal machine tool adopts resin grinding wheel of a Japanese company, size 800 mm×420mm, 660mm×420mm, thickness 30 mm and 50 mm, commonly used particle size 2000#, 3000#, 4000#, tissue number is 9, The binders are BZ(long life) and BZL3(good cutting performance) and are used well.
3 Selection of resin grinding wheel
Although the process of grinding steel ball with resin grinding wheel has been used, due to the lack of understanding of resin grinding wheel, there are many misunderstandings in the selection of resin grinding wheel grade and organization structure, so that the ideal steel ball grinding processing effect is not achieved.
The choice of resin grinding wheel for steel ball grinding should be determined according to the steel ball material, grinding mode, grinding parameters, processing accuracy and production efficiency. First of all, select the appropriate resin grinding wheel label, including abrasive, particle size, binding degree, adhesive, structure and size of the grinding wheel. When choosing resin grinding wheel, it is necessary to consider its grinding ability and workpiece grinding precision, resin grinding wheel should have moderate durability, but also to ensure a small amount of consumption, and always maintain good self-sharpness. Then according to the selected grinding wheel label to develop a reasonable processing technology to ensure the uniform grinding pressure.
In the processing of steel balls, the initial process generally uses ceramic grinding wheel (WA/C 180Z ~ 320Z), the intermediate process uses resin grinding wheel (WA500Z ~ 2000Z), and the final process uses resin grinding wheel (WA3000Z ~ 6000Z).
4 Selection of grinding fluid and filtration system
Selecting suitable grinding fluid, strictly controlling the temperature of grinding fluid (oil) and formulating suitable filtration system are very important in the grinding process of resin grinding wheel. Because of the fine particle size of the resin grinding wheel, the grinding chips, abrasive particles and bonds generated in the processing are often embedded in the gap between the grinding wheel grains, so that the grinding wheel loses the cutting ability. Under normal circumstances, the soft grinding ball is used to carry out the groove to eliminate the fatigue layer of the groove of the grinding wheel. Therefore, when selecting the grinding fluid, it is necessary to consider its machinability, as well as its cooling, lubrication and cleaning. Secondly, the filtration cleanliness of the grinding fluid is also particularly critical. The impurities that may arise in the processing of the steel ball grinding with the resin grinding wheel are: the grinding wheel abrasive falls off; Iron filings under the grinding of steel balls; Cutting fluid flows through the channel of debris, dust, paint and so on. At present, the resin grinding wheel is mainly imported from Japan, with 2000#, 3000#, 4000#, 6000#, which is equivalent to the micro-powder particle size of China's grinding wheel W10, W7, W5, W2.5. This requires the basic filtration accuracy to reach 10 ~ 2μm or so, the domestic filter system also needs to be improved, especially the resin grinding wheel sand drop caused by the system filtration problem, is still a key factor affecting the further improvement of the quality of the steel ball. Compared with cast iron disc grinding, the mechanical properties of cutting liquefaction have changed greatly. The content of physical impurities and the size of particles in the cutting fluid are different. Therefore, the impact on the surface and vibration value of the steel ball is relatively greater and more direct, so it is very important to choose an ideal filtration system. In addition, the temperature control of grinding fluid (oil) is also very important. According to a Japanese company, the temperature control of the grinding fluid (oil) in the precision steel ball grinding with resin grinding wheel is very strict, generally around 25 ℃.
5 Bonding, installation and dressing of resin grinding wheel
When the resin grinding wheel is bonded, the grinding wheel must first be cleaned and then bonded, otherwise it is difficult to bond firmly.
The dressing of the rotating grinding wheel is done by the grinding wheel dresser. The grinding wheel dresser is hydraulically driven, and the diamond dresser moves in a straight line to finish the dressing of the grinding wheel. Adjust the diamond dressing block to make it even to ensure that the finished grinding wheel surface is smooth. The dressing of the fixed grinding wheel disc needs to be removed from the machine tool.
6 Closing remarks
At present, the resin grinding wheel has been widely used in the 3M77 series horizontal steel ball lapping machine using resin grinding wheel "grinding instead of grinding" process successfully processed G10 ~ G5 low noise bearing steel balls.
Although the process of grinding steel ball with resin grinding wheel has achieved gratifying results, it is not perfect and there are still many problems. For example, how to achieve the dressing of the fixed grinding wheel disc and the automatic ditch of the disc on the machine tool; How to configure a suitable filtration system according to the granularity of the grinding wheel and the composition of the grinding fluid to improve the filtration accuracy of the grinding fluid; How to embody the design concept of humanization, health and environmental protection in the design of machine tools; How to realize vibration reduction, noise reduction and isolation of vibration source on machine tools will be the direction of future research.
