Ultrasonic vibration-assisted grinding of blind holes and internal threads in cemented carbides

Cemented carbides are widely used in aerospace and mechanical processing industries due to their excellent physical and chemical properties. However, cemented carbide products processed by conventional methods have such deficiencies as poor surface integrity, high surface roughness, and high consumables. Given this, a method for machining blind holes and internal threads in cemented carbide by ultrasonic vibration-assisted grinding is introduced in this paper. The methodology combines ultrasonic-assisted grinding with helical mill-grinding to machine cemented carbide via a diamond form grinding wheel. Based on the characteristics of ultrasonic longitudinal-torsional vibration, the motion trajectories of individual abrasive particles under conventional grinding (Con-G) and ultrasonic longitudinal-torsional vibration-assisted grinding (ULTVAG) were analyzed. Tests were conducted to investigate the influence of machining parameters on the grinding force, hole surface quality, thread profile accuracy, and wear conditions of the two diamond form grinding wheels. The results obtained strongly indicate that ULTVAG can reduce the grinding force, while the grinding force is mostly controlled by the feed rate and is less affected by the spindle speed and ultrasonic vibration amplitude (UVA). With an increase in UVA, the hole surface roughness exhibits first decreasing and then rising. The optimal hole surface quality is observed at the longitudinal ultrasonic vibration amplitude A = 4 μm. At too high values of UVA, the hole surface quality becomes worse than that acquired via the conventional grinding. The first few threads machined by Con-G and ULTVAG are shown to meet the requirements of 6H precision of thread. As the number of processed threads increases, the wheel of Con-G becomes severely worn, resulting in insufficient machining precision. The number of threads machined by a single grinding wheel under ULTVAG is higher than that of Con-G. It is proved that ULTVAG with an appropriate UVA can reduce the wear of grinding wheel and prolong its service life, as well as improve the thread precision and qualified rate.