In the realm of material processing, grinding plays a vital role in achieving precise and smooth surfaces. However, an issue that often arises during grinding is the generation of grinding cracks, represented by black fractured points scattered on the workpiece's surface. Detecting these cracks can be challenging, especially for novices. Fortunately, specialized treatment using grinding fluid can mitigate these cracks, limiting their depth to only 0.05-0.25mm.
Causes of Grinding Crack Generation:
1. Residual Stresses: One possible cause is the presence of surface-level residual stresses in the workpiece that exceed its fracture limit. These residual stresses result from previous machining or heat treatment, leading to an accumulation of mechanical and thermal stresses. During grinding, when these just-balanced stresses are removed, the remaining stress exceeds the workpiece's strength, giving rise to grinding cracks.
Key Factor: Grinding-Induced Heat Stress
Among various factors, grinding-induced heat stress stands out as the primary contributor to the problem. As grinding generates heat, the localized temperature on the workpiece surface increases rapidly, causing tempering or other heat treatment effects. Consequently, internal structural changes and surface shrinkage lead to the formation of cracks under tensile stress.
Examples Relating Feed Rate and Residual Stresses:
1. Tensile Stress and Feed Rate: Tensile stress gradually increases with the increase in the feed rate of the grinding wheel, approaching the workpiece material's tensile strength. Once this strength threshold is surpassed, cracks are formed.
2. Consistency of Residual Tensile Stress: The compressive stress does not vary significantly.However, when the feed depth is 0.05mm, the maximum residual tensile stress remains stable. Even if the feed depth is increased, the residual tensile stress does not rise significantly, likely due to the influence of abrasive detachment.
Examples Demonstrating Residual Stresses Altered by Feed Rate:
1. Greater Feed Rate, Deeper Residual Stress: A larger feed rate of the grinding wheel results in deeper residual stresses.
2. Residual Stress Distribution: Residual stresses, acting both tangentially and perpendicularly to the grinding direction, decrease sharply as they extend inward.
3. Evolution of Stresses: Stresses transform from compressive to tensile when acting along the grinding and perpendicular directions, reaching a maximum and then gradually reducing to minor compressive stresses.
Influence of Grinding Wheel Hardness and Speed:
The hardness of the grinding wheel(ranging from G, H, I, to J)correlates with the magnitude of residual tensile stress. Higher hardness corresponds to greater residual stresses.
Grinding wheel speed(circumferential velocity)also plays a significant role. Once the speed surpasses 1500m/min, residual stresses increase sharply.
Notably, material type can also influence the likelihood of grinding cracks. Some materials are more prone to such cracking than others.
Grinding cracks, while not immediate in their appearance, can sporadically emerge on the surface of the workpiece. Understanding the factors contributing to their generation is crucial for effectively preventing and mitigating these cracks during grinding operations. By controlling feed rate, wheel hardness, and speed, and implementing appropriate grinding fluid treatment, manufacturers can significantly reduce the occurrence of grinding cracks, ensuring the production of high-quality work-pieces with smooth, crack-free surfaces.
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