Subsurface damage depth and distribution in rotary ultrasonic machining and conventional grinding of glass BK7

The morphology, depth, and distribution of subsurface damage (SSD) on glass BK7 specimens produced in rotary ultrasonic machining (RUM) and conventional grinding (CG) processes with three diamond tools were investigated. The ultrasonic effects on the SSD characteristics were also explored with respect to the material removal mechanisms and the specific kinematics principles of the abrasives. The experimental results demonstrated that the increased cutting velocity would improve the subsurface qualities, while the increased feed speed and cutting depth would deteriorate SSD depth of the RUM/CG specimens. Superimposition with the ultrasonic vibration increased the maximum cutting depth of each abrasive by an amplitude and also resulted in the cyclical variation in abrasive trajectories, hereby worsening the SSD depth of the final RUM surfaces. A large amount of incipient cracks which pulverized the material were just concentrated on the top RUM surface, thus increasing the subsurface crack distributions. With the abrasive moving ahead, these incipient cracks would propagate forward, hereby hindering the nucleation of the fresh cracks, and decreasing the subsurface crack obscurations of the RUM surface at a local level. Some rogue cracks were occasionally found to be located in deeper subsurface of the RUM/CG specimens, and the mutual interactions between the adjacent abrasive indentations would promote the propagations of the subsurface cracks, which gave rise to the nucleation of these rogue cracks.