The effects of tool geometry on surface integrity in hard turning of ball bearing inner raceways

Hard turning is increasingly adopted in the bearing industry as a cost-effective and environmentally friendly alternative to grinding. However, ensuring desired surface integrity in hardened materials remains a challenge. This study investigates the effects of tool geometry and cutting conditions on the surface integrity of 6208 deep groove ball bearing inner raceways, made from heat-treated bearing steel (58–62 HRC), commonly used in industrial applications. Three CBN inserts with different geometries—a round insert and two rhombic inserts—featuring effective rake angles of −15°, −28.5°, and −35°, were analyzed. Key metrics such as cutting forces, roundness error, surface roughness, residual stress, and white layer thickness were evaluated. It was observed that tool geometry significantly influences thermal and mechanical loads during cutting, affecting subsurface properties. The V-35 rhombic insert (35° rake angle) yielded the lowest surface roughness and roundness error, with maximum compressive residual stress of 770 MPa (hoop direction) extending to a depth of 20 µm. In contrast, the O-20 round insert produced a thicker white layer due to increased heat from larger tool–workpiece contact. Overall, the results demonstrate that insert geometry significantly affects surface integrity in hard turning, offering practical insights for optimizing bearing manufacturing processes.