Grinding cracks, also known as black broken points, are not formed by sudden fractures but appear intermittently on the surface of the workpiece. These cracks can be difficult for beginners to distinguish from other surface defects. The depth of the grinding fluid treated with special chemicals is generally shallow, typically ranging between 0.05 to 0.25 mm.
There are several factors that contribute to the formation of grinding cracks. One primary cause is when the internal stress within the workpiece exceeds its fracture limit. This can happen due to residual mechanical or thermal stresses left in the surface layer from prior grinding or heat treatment processes. When these stresses are no longer balanced—often because a portion of the material is worn away during grinding—the remaining stress may surpass the material's strength, leading to the formation of cracks.
Among all the causes, "cracking caused by grinding" is the most critical issue. The main concern is the stress generated by the heat produced during grinding. As the grinding process occurs, the surface temperature rises rapidly, which can lead to localized tempering or other forms of heat treatment. This temperature change results in tensile stress due to structural changes and surface shrinkage, ultimately causing cracks.
For example, the relationship between the feed rate of the grinding wheel and the residual stress is significant. As the feed force increases, the tensile stress gradually rises and approaches the tensile strength of the material. Once this threshold is exceeded, cracks begin to form. In contrast, compressive stress remains relatively stable, although differences in scale and experimental conditions make direct comparisons challenging. However, it is commonly observed that when the depth of cut is 0.05 mm, the residual tensile stress is at its highest. Even with deeper cuts, the residual stress does not increase significantly, likely due to the effect of abrasive particles falling away.
Another example involves measuring residual stress after grinding by adjusting the feed rate. The higher the feed rate, the deeper the residual stress penetrates into the material. On the surface, the residual stress acts as tensile stress in the direction of grinding, and can also manifest as compressive stress perpendicular to the grinding direction. As depth increases, the stress decreases sharply.
When stress acts both along and perpendicular to the grinding direction, it initially becomes compressive and then suddenly shifts to tensile stress aligned with the grinding direction. It reaches a peak and then gradually decreases, eventually becoming a small compressive stress.
The hardness of the grinding wheel also influences residual stress. For wheels with hardness ratings G, H, I, and J, the higher the hardness, the greater the residual stress. Additionally, the rotational speed (circumferential speed) of the grinding wheel has a significant impact. Once the speed exceeds 1500 m/min, the residual stress increases sharply.
Moreover, the type of material being ground affects the likelihood and nature of grinding cracks. Different materials respond differently to grinding forces, heat, and stress, leading to variations in crack formation and propagation. Understanding these factors is essential for optimizing the grinding process and minimizing damage to the workpiece.
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