Grit size effect on HydroFlex polishing dynamics and performance

Controllable, adaptable internal polishing is critical to metal additive manufactured (MAM) complex channels. Conventional fluid-based internal polishing methods are challenging to control the material removal, resulting in varied surface roughness (Sa) depending on channel length, aspect ratio (AR), and complexity. HydroFlex is an internal polishing method which drives a fixed-abrasive grinding wheel via a flexible spindle to navigate complex, high AR channels controllably and predictably removing material. Key to HydroFlex performance is the orbital motion of the grinding wheel governed by the hydrodynamic and cutting forces to achieve uniform polishing. Effect of grit size on orbital motion, and corresponding performance was experimentally evaluated. 46 µm, 76 µm, and 91 µm grit sizes were tested at a rotational speed of 50,000 rpm and a channel to wheel (C/W) ratio of 0.54 and orbital frequency and consistency, grinding force, Sa, material removal rate (MRR), and wheel wear were compared. Orbital frequency was found to directly correlate with increasing grit size and consistency of orbit shown to be highest at moderate grinding forces. Sa and MRR were inversely proportional with sub-micron roughness achieved at 0.15 g/min and 0.34 g/min corresponding to 2.2 µm Sa. Wheel wear was effected by grain pullout, attrition, and capping with all grit sizes experiencing similar wear. These findings suggest that orbital motion can be controlled through manipulation of wheel kinetics enabling precise control of grinding dynamics essential to adaptable performance in complex, non-uniform MAM channels.