Among the new materials widely used at present, such as various engineering ceramics, optical glass, monocrystalline silicon, etc., they are mostly hard and brittle materials, which are difficult to meet the requirements with general cutting methods. At present, mirror grinding method can be used to meet the requirements of precision mirror grinding. Then, how to grind the mirror of hard and brittle materials?
1、 Grinding mechanism
Hard and brittle materials are characterized by high hardness, poor toughness and plasticity, and small fracture toughness. During cutting, chips are mainly generated in the process of crack propagation fragmentation, and the material is removed in a brittle fracture way. Therefore, the machined surface quality is poor.
Mirror grinding of hard and brittle materials is mainly carried out by plastic grinding, and the chips are removed by plastic shearing. Experiments show that as long as a very small unit of chip is removed, the energy required to produce plastic deformation is less than that required for brittle fracture. The material is removed by plastic shear rather than brittle fracture. The key is to stably achieve a small enough removal unit cutting, which can achieve crack free, high-quality mirror grinding, and the surface roughness can reach 0.003-0.008 microns.
2、 Requirements for grinding wheel
The cutting edge of the grinding wheel should be extremely small, equal height and sharp to achieve fine chip removal. The grinding wheel should have the ability to keep sharp in the grinding process to ensure the stability of the cutting process. For this reason, ultra-fine ultra hard abrasive (diamond or CBN) wheels are used for precision mirror grinding. Compared with ordinary grinding wheel, its characteristics are as follows:
(1) The sharp angle of the abrasive tip is small, and the tip is sharp. When the cutting depth is reduced, the cutting layer can be cut off through small deformation.
(2) The micro edge of the grinding wheel has good contour, and the actual cutting depth of the abrasive particles is very small. It is in micro cutting, and the grooves formed are very shallow, so the surface roughness can be reduced.
(3) The micro blade is very small higher than the binder, and the chip space is often insufficient, and the grinding wheel is easy to be blocked.
(4) The fixed area of abrasive particles in the bond is small, the holding capacity of abrasive particles is low, and the abrasive particles are easy to fall off. Therefore, the emergence of metal bond, especially cast iron bond, greatly improves the holding capacity of superhard micro powder abrasive, makes the abrasive particles not easy to fall off, and improves the utilization rate of superhard abrasive wheel.
3、 Dressing grinding wheel by on-line electrolysis method
The use of metal bonded superhard abrasive wheels is more likely to cause blockage and make the dressing of the wheels difficult. In the 1980s, someone began to use online electrolysis to repair the grinding wheel. That is, in the grinding process, electrolysis is used to remove a small amount of metal bond on the surface of the grinding wheel, but it has no effect on the abrasive, so that the sharp grinding edge is gradually exposed, and at the same time, chip space is formed.
During grinding, the grinding fluid is used as the electrolyte at the same time, the cast iron based grinding wheel is used as the anode (the brush contacts the grinding wheel), and the electrode is used as the cathode. Under the action of electrolytic current, the grinding wheel is corroded, making the abrasive particles exposed to the surface of the grinding wheel. At the same time, an oxide passivation film is formed on the surface of the grinding wheel to prevent further excessive electrolysis on the surface of the grinding wheel. When the abrasive particles begin to blunt during grinding, the passive film begins to be destroyed, and the electrolysis continues, so the grinding edge always maintains the cutting effect. The passivation film can also prevent the cast iron bond from scratching the surface of the workpiece, and the passivation film has a certain friction and polishing effect, which can effectively improve the quality of the machined surface.
4、 Requirements of mirror grinding of hard and brittle materials on machine tools
The requirements for machine tools are the same as those for ordinary mirror grinding. But it is more emphasized that the machine tool spindle has extremely high dynamic stiffness, and the axial and radial runout is less than 0.001 micron; The feeding system has good rigidity, smooth and no creep; The spindle grinding wheel system requires precise balance and no vibration; After the grinding wheel is balanced, it should be trimmed and reshaped with high precision; To ensure that the rotation deviation is not greater than 0.001 mm.
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