Material removal mechanisms in non-contact ultrasonic abrasive machining

In this paper, we propose a new non-contact ultrasonic abrasive machining (NUAM) method that is performed using loose abrasives excited by ultrasonic energy in a liquid, and discuss its suitability for application to ultra-precision machining. A horn attached to the ultrasonic generator, having a resonance frequency of 28 kHz and an amplitude of 20 μm, has been used as a processing tool. Machining experiments on aluminum alloy (JIS-2014), using machining fluids mixed with Al2O3 abrasive grains with a mean diameter of 1 μm, have been conducted to investigate the fundamental mechanisms of NUAM. In NUAM, the following three kinds of processing modes occur: (a) material removal based on the erosion due to liquid cavitation collapse pressure (impact force), namely, cavitation erosion (Mode-A); (b) removal based on colliding or sliding of the abrasive grains accelerated by the impact force due to cavitation collapse on the workpiece surface (Mode-B); and (c) small-scale material removal arising from the abrasive grains excited by ultrasonic energy (Mode-C). The scale of the material removal decreases in order of Modes-A–C. In fact, in Mode-C nanoscale processing marks with a width of just 40–60 nm and a depth of 3–6 nm are generated. By using the NUAM method in Mode-C, a finished surface having a nanoscale roughness can be created.