Grinding cracks, also known as black broken points, are not formed through sudden fracture but appear sporadically on the surface of the workpiece. These cracks can be difficult for beginners to distinguish from other surface defects. The depth of such cracks is usually shallow, typically ranging between 0.05 to 0.25 mm, and they are often caused by the use of specially treated grinding fluids.
There are several factors that contribute to the formation of grinding cracks. One primary cause is the residual internal stress within the workpiece. This stress may result from prior grinding operations or heat treatment processes. When the stress in a certain area becomes unbalanced during grinding, it can exceed the material’s strength, leading to crack formation.
Among all contributing factors, "cracking due to grinding" is one of the most critical issues. The main problem stems from the heat generated during the grinding process. As the surface temperature rises rapidly, it can cause local tempering or other forms of heat treatment. This thermal effect leads to tensile stress due to structural changes and surface shrinkage, ultimately resulting in cracks.
For example, the feed rate of the grinding wheel has a direct impact on residual stress. As the feed force increases, the tensile stress gradually builds up and approaches the tensile strength of the material. Once this threshold is exceeded, cracks begin to form.
On the other hand, compressive stress remains relatively stable under different conditions. However, when the dressing depth is set at 0.05 mm, the residual tensile stress tends to be the highest. Even with deeper cuts, the residual stress does not increase significantly, which is generally attributed to the falling off of abrasive grains.
Another example involves measuring residual stress after grinding by varying 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 along the direction of grinding, while in the vertical direction, it may initially act as compressive stress before transitioning to tensile stress. As depth increases, the stress decreases sharply.
When stress acts both along and perpendicular to the grinding direction, it first manifests as compressive stress, then suddenly shifts to tensile stress aligned with the grinding direction. It reaches a peak and then gradually reduces, eventually becoming a small compressive stress.
The hardness of the grinding wheel also influences residual stress. For wheels with hardness grades G, H, I, and J, the higher the hardness, the greater the residual stress tends to be. Additionally, the rotational speed of the grinding wheel (circumferential speed) plays a significant role. Once the speed exceeds 1500 m/min, the residual stress increases sharply.
Moreover, the type of workpiece material also affects the likelihood of grinding cracks. Different materials exhibit varying levels of susceptibility to crack formation, depending on their mechanical and thermal properties. Understanding these factors is essential for minimizing damage and improving the quality of the finished product.
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