Reasonable selection of the hardness of the abrasive tool is the key to obtaining a good grinding effect.

When choosing the hardness of an abrasive tool, the most basic principle is: to ensure that the abrasive tool has proper self-sharpening during the grinding process, to avoid excessive wear of the abrasive tool, and to ensure that the grinding does not produce too high grinding temperature.

As mentioned above, the hardness of the abrasive tool is related to the amount of bonding agent. The higher the hardness of the abrasive tool, the more bonding agent is required. The thicker the bonding agent bridge, the greater the holding force of the bonding agent on the abrasive, which makes the grinding The pellets can withstand greater grinding force without breaking or falling off. On the contrary, when the hardness of the abrasive tool is low, the holding force of the bonding agent on the abrasive particles is small, and the abrasive particles are easily broken or fall off. Therefore, if the hardness of the abrasive tool is selected too high, not only will the blunt abrasive grains not easily break or fall off and lose the cutting ability, but also increase the friction between the abrasive tool and the workpiece, and the surface of the workpiece will easily heat up and cause burns. In order to remove the blunt abrasive grains in time, the abrasive tools must be trimmed frequently, causing a large amount of abrasive wear. If the hardness of the abrasive tool is too soft, the abrasive particles will fall off when they are still sharp, causing unnecessary wear. At the same time, the abrasive wear is too fast, and its working surface wears very unevenly, which will also affect the machining accuracy of the workpiece.

In summary, only the correct choice of the hardness of the abrasive tool can maintain its normal grinding state and meet the needs of processing. Especially when sharpening magic couple writing tools, even a small deviation in the hardness of the abrasive tool will affect the quality of the sharpening. It can be seen that the impact of the hardness of the abrasive tool is very important.

When choosing the hardness of the abrasive tool, the most basic method is: the hardness of the workpiece is high, the hardness of the abrasive tool is low; the hardness of the workpiece is low, the hardness of the abrasive tool is high. Because when the hardness of the workpiece is low, the pressure of the abrasive grains on the abrasive tool into the workpiece is correspondingly smaller, and the abrasive grains are not easy to blunt. In order to prevent the abrasive grains from breaking or falling off before being blunt, the hardness is selected. Some abrasive tools are more suitable; on the contrary, when the hardness of the workpiece is high, the pressure of the abrasive particles cutting into the workpiece is correspondingly larger and becomes blunt. The selection of the abrasive tool with softer hardness can promptly self-sharp and maintain the grinding performance of the abrasive tool. However, when the workpiece material is softer and tougher (such as soft bronze, brass, etc.), since the cut metal is easy to block the abrasive tool, it is advisable to use a coarser grain size and softer abrasive tool for processing.

The hardness of the abrasive tool is also an important factor affecting the temperature of the grinding area. When grinding workpieces with low thermal conductivity (such as alloy steel), because the surface temperature of the workpiece is relatively high, it is often prone to burns and cracks. At this time, you should choose a grinding wheel with lower hardness and loose structure, and at the same time strengthen Cooling, so as to effectively avoid the burn of the workpiece. Similarly, when grinding thin workpieces, a grinding wheel with looser structure and lower hardness is also required. When grinding the outer circle of a thin-walled hollow workpiece, the hardness of the grinding wheel is lower than that of a solid workpiece. This is also to prevent the workpiece from deforming due to the increase in grinding temperature.

When choosing the hardness of abrasive tools, the following conditions should generally be considered.

① When the contact area between the abrasive tool and the workpiece is large, the hardness of the abrasive tool should be lower to prevent the workpiece from overheating and affecting the grinding quality. For example, the hardness of the grinding tools used for vertical shaft flat grinding is lower; the hardness of the grinding wheels used for flat grinding and internal soft grinding is lower than the hardness of the grinding wheels used for cylindrical grinding. However, when grinding a thin and long inner hole, due to the low speed of the grinding wheel, the grinding wheel is easy to wear, which makes the work piece taper (flare), so the hardness of the grinding wheel is higher than that of general internal grinding. Similarly, a harder wheel can be used to grind a workpiece with a small aperture, and a softer wheel should be used to grind a workpiece with a large aperture.

② When grinding intermittent surfaces and casting burrs, you should choose hard or super-hard grinding wheels; when grinding steel billets with heavy loads, you should also choose hard or super-hard grinding wheels to prevent the grinding wheels from wearing too fast.

③ Diamond abrasive tools (grinding wheels or whetstones) used for dressing, due to the high pressure during dressing and higher hardness, so super-hard abrasives are often used.

④ Heavy-duty grinders and grinders with better rigidity, because they have less vibration during grinding, and the abrasive grains are not easily destroyed, so the grinding wheel with lower hardness can be used.

⑤ In order to avoid burns of the workpiece during external plunge grinding, the hardness of the grinding wheel should be lower than that during axial feed.

⑥ The grinding machine with automatic cutting can use a softer grinding wheel than the grinding machine with manual cutting.

⑦ The smaller the roughness value required for the machined surface and the more precise the size of the workpiece, a lower hardness grinding wheel should be selected to avoid excessive grinding heat and deterioration of the surface structure of the workpiece. For example, with super soft mirror grinding resin bond grinding wheel, a surface with a roughness Rz of 0.05μm can be ground. However, for general fine grinding wheels, the hardness should be higher, otherwise the working surface of the grinding wheel will be uneven. Abrasion affects the machining accuracy of the workpiece.

⑧ Scratches on the surface of the workpiece are often related to improper selection of the hardness of the abrasive tool. When the hardness of the abrasive tool is too low, the abrasive particles are easy to fall off, so due to the effect of squeezing or friction, the fallen abrasive particles will scratch the surface of the workpiece, so the hardness of the abrasive tool should be appropriately increased at this time.

⑨ The workpiece is prone to heat during dry grinding, and a wheel with a softer grade of 1 to 2 should be used for wet grinding.

⑩ When the production efficiency is high, a softer grinding wheel can be used to facilitate the self-sharpening of the grinding wheel and reduce the number of dressings. However, the wear of the grinding wheel will increase accordingly, so a comprehensive analysis and comparison should be carried out on the technical and economic indicators.

⑪ In high-speed grinding, when the feed rate remains the same, the chips cut by the abrasive grains will become thinner, and the cutting force of the abrasive grains will decrease accordingly, and the wear of the grinding wheel will be slower. At this time, in order to improve the self-sharpening of the grinding wheel , Its hardness is 1~2 smaller than that of ordinary grinding, which is the situation of high-speed fine grinding. Similarly, for some unbalanced workpieces (such as crankshafts, etc.), since the workpiece speed during grinding cannot be too high, the hardness of the grinding wheel should also be selected lower to avoid burns to the workpiece. For high-speed grinding whose main purpose is to improve cutting efficiency, the cutting feed must be increased, and the grinding force on the abrasive particles 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 levels higher than that of ordinary grinding.

⑫ When grinding steel balls (balls), a super-hard grinding wheel should be selected; generally, the hardness of the grinding wheel should be selected to medium-hard when cutting the workpiece.

⑬ When sharpening hard alloy and high-speed steel tools, choose a grinding wheel with J~G hardness.

⑭ In order to maintain the correct geometry of the workpiece during forming and grinding, the wear of the grinding wheel should not be too large, so the hardness of the grinding wheel should be higher.