CBN abrasive is a new abrasive developed in modern times. Because of its high hardness and micro-crushing structure, it can achieve good efficiency for grinding high hardness steel. However, CBN grinding wheel should pay attention to several problems in use.
Problems arising from thermal expansion of grinding wheel.
Most CBN grinding wheel inner discs are made of “metal”. When CBN grinding wheel is grinded, most of the thermal energy will be transmitted to the grinding wheel. Therefore, in the grinding process, heat energy will be continuously transferred to the grinding wheel. At this point, the metal inner disk will begin to expand due to the accumulation of heat energy.
Because the distance between each position of the grinding wheel and the grinding point of the heat source center is different, the expansion degree of each position is also different. The loss of the true roundness of the grinding wheel may cause “vibration” and other problems.
In addition, thermal expansion will also change the original set feed rate. For example, the original set feed rate is 0.01 mm, after thermal expansion, the feed rate may become 0.03 mm, and the dimensional accuracy is not easy to achieve.
Knife feeding problem caused by grain size difference
As mentioned before, CBN abrasives are harder than ordinary abrasives, and their shape is sharper. Although the cutting force is good, the roughness of the abrasive surface will become too rough (close to the cutting effect) if the same particle size is chosen as ordinary abrasives.
Therefore, in practice, the size selection of CBN will be 3-4 orders finer than that of general grinding wheel, such as the size of general grinding wheel is # 80, CBN will choose # 140 or # 170.
However, this increases the difficulty of tool feeding, because the appropriate amount of tool feeding is about 1/10 of the diameter of abrasive particles. When the selected size is small, the diameter of abrasive particles is small, and the range of tool feeding is reduced accordingly.
In addition to the difficulty of knife setting at the beginning, if the knife feeding is suddenly increased due to operation or machine problems.
It is easy to cause burns and other problems. It also results in the thermal expansion of the grinding wheel mentioned above.
In addition, CBN grinding wheel binder is stronger than ordinary grinding wheel, and its self-sharpening effect is slower, which will make this situation worse.
To avoid these problems, many operators will choose to “slow down the speed of grinding wheel” to avoid burns and other problems. However, CBN has a micro-crushing structure, too slow speed is not conducive to the development of CBN characteristics and advantages, will also reduce the grinding ratio, and reduce the life of grinding wheel.
Other solutions are:
(1) changing the metal inner disk to other materials that do not expand due to temperature: for example, ceramics. In order to solve the problem of thermal expansion of metal inner disk and loss of true roundness.
Or we can choose a new type of special binder BK with a large number of stomata: through the role of stomata, to achieve the effect of heat dissipation. At the same time, because the contact area decreases, the grinding resistance decreases and the grinding temperature decreases.