The grinding cracks (black broken spots) produced by surface grinding are not formed suddenly, but appear sporadically on the surface of the workpiece. Although there are grinding cracks, it is still difficult for novices to distinguish. The cracks of the grinding fluid treated with special chemicals are not deep, and the general depth is only 0.05~0.25mm.
The reasons for grinding cracks may be as follows: the internal stress of the workpiece exceeds the fracture limit, that is, the workpiece has residual mechanical stress and thermal stress on the surface due to previous grinding or heat treatment. Due to the grinding away of this part of the stress that can just maintain balance, the residual stress exceeds the strength of the workpiece, and grinding cracks are generated.
Among all the reasons, "cracks caused by grinding" is the crux of the problem. The biggest problem is the stress generated by the grinding heat. Because of the grinding heat, the local temperature of the workpiece surface rises rapidly, and this part is subjected to tempering or other heat treatment. Due to the change of the internal structure and the shrinkage of the surface, cracks are generated under the action of tensile stress.
1. Example of the relationship between grinding wheel feed and residual stress.
① The tensile stress will gradually increase with the increase of the feeding force of the grinding wheel, and gradually approach the tensile strength of the workpiece material. Once the tensile strength of the workpiece material is exceeded, cracks will occur.
② The compressive stress will not change too much, because the scale and the experimental conditions are different, so it cannot be compared, but what is almost unchanged is that when the back cut is 0.05mm, the residual tensile stress is the largest, even if the cut is deeper. It won't be too big. It is generally believed that this is due to the falling of abrasive particles.
2. An example of measuring residual stress after grinding by varying the feed rate of the grinding wheel.
① The greater the feed rate of the grinding wheel, the deeper the residual stress exists.
② The residual stress on the surface acts on the grinding direction as tensile stress, and can also act on the vertical direction of the grinding direction in the form of pressure, and the deeper it goes to the inside, the stress will decrease sharply.
③ When acting along the grinding direction and vertical direction, it becomes compressive stress first and then suddenly becomes tensile stress consistent with the grinding direction. When it reaches the maximum value, it gradually decreases, and finally becomes a small compressive stress.
The relationship between the hardness of the grinding wheel and the residual tensile force, the hardness is between G, H, I, J, the higher the hardness, the greater the residual residual stress.
The effect of the speed of the grinding wheel (peripheral speed) on the residual stress. Once the rotational speed (peripheral speed) exceeds 1500m/min, the residual stress will increase sharply.
In addition, due to the different materials of the workpiece, there are also differences in the susceptibility to grinding cracks and the difficulty of grinding cracks.
