When choosing the hardness of the abrasive tool, the most basic principle is to ensure that the abrasive tool has appropriate self-sharpening during the grinding process to avoid excessive wear of the abrasive tool; Ensure that no excessive grinding temperature is generated during grinding.
When choosing the hardness of the abrasive tool, the most basic method is: the hardness of the workpiece is high, and the hardness of the abrasive tool is low; The hardness of the workpiece is low, and the hardness of the grinding tool is high. Because the hardness of the workpiece is low, the pressure on the abrasive particle cut into the workpiece is correspondingly small, the abrasive particle is not easy to dull, in order to make the abrasive particle will not be broken or fall off before dull, so the choice of higher hardness of the abrasive tool is more appropriate; On the contrary, when the hardness of the workpiece is high, the pressure of the abrasive cut into the workpiece is correspondingly large and blunt, and the choice of abrasive tools with softer hardness can produce self-sharpening in time to maintain the grinding performance of the abrasive tools. However, when the workpiece material is softer and the toughness is large (such as soft bronze, brass, etc.), because the cut metal cutting is easy to block the abrasive tool, it is appropriate to choose the abrasive tool with coarser hardness.
The hardness of the grinding tool is also an important factor affecting the temperature of the grinding area. Grinding the workpiece with low thermal conductivity (such as alloy steel), due to the relatively high surface temperature of the workpiece, it is often easy to produce burns and cracks, at this time, the grinding wheel with lower hardness and looser organization should be selected, and the cooling should be strengthened, so as to effectively avoid the burn of the workpiece. Similarly, when grinding thin workpieces, it is also necessary to use a wheel with a looser structure and lower hardness. When grinding the outer circle of the thin-walled hollow workpiece, the hardness of the grinding wheel is lower than that of the solid workpiece, which is also to prevent the deformation of the workpiece caused by the rise of the grinding temperature.
When grinding discontinuous surfaces and castings to burr, hard or super-hard grinding wheels should be selected; When grinding billets with heavy load, a hard or super-hard grinding wheel should also be selected to avoid too fast wear of the grinding wheel.
(3) Substitute diamond abrasive tools for dressing (grinding wheel or whetstone), due to the large pressure during dressing, the need for higher hardness, so ultra-hard abrasive tools are often used.
④ Heavy-duty grinding machines and better rigid grinding machines, because they have small vibration during grinding, the abrasive particles are not easy to be destroyed, so the grinding wheel with lower hardness can be used.
The smaller the roughness value of the processing surface and the more accurate the workpiece size requirements, the lower the hardness of the grinding wheel should be selected to avoid excessive grinding heat and the deterioration of the workpiece surface tissue. For example, with ultra-soft mirror grinding resin bond grinding wheel, you can grind a surface with a roughness Rz of 0.05μm, but for general fine grinding wheel, the hardness is higher, otherwise it will affect the processing accuracy of the workpiece due to uneven wear on the working surface of the grinding wheel.
(9) The workpiece is easy to heat during dry grinding, and a soft 1 to 2 small grade grinding wheel should be used compared with wet grinding.
⑩ When the production efficiency requirements are high, soft grinding wheels can be used to facilitate the self-sharpening of the grinding wheel and reduce the number of dressing. However, the wear of grinding wheel will increase accordingly, so it is necessary to carry out comprehensive analysis and comparison in technical and economic indicators.
In high-speed grinding, when the feed speed is unchanged, the chips cut by the grinding grains become thinner, the cutting force borne by the grinding grains decreases correspondingly, and the wear of the grinding wheel is slower. In this case, in order to improve the self-sharpening of the grinding wheel, its hardness must be 1 to 2 degrees softer than that of ordinary grinding, which is the case in high-speed fine grinding. Similarly, for some unbalanced workpieces (such as crankshaft, etc.), because the workpiece speed can not be too high when grinding, the hardness of the grinding wheel should also be selected lower to avoid burning the workpiece. The main purpose of high-speed grinding is to improve the cutting efficiency, and the cutting feed should be increased, and the grinding force borne on the grinding grain will increase. In order to ensure that the abrasive particles do not fall off prematurely, the hardness of the grinding wheel should be 1 to 2 small levels higher than that of ordinary grinding.
When grinding carbide and high-speed steel tools, a grinding wheel with J~G hardness should be selected.
