Hydrodynamic Flexible Spindle Polishing of Complex Channels

Abstract

Complex channels, featuring small diameters, extended lengths, tortuosity, internal branching, thin walls, non-uniform cross-sections, and/or high length to diameter (L:Ø), are designed and fabricated for optimized thermal and fluid transfer efficiency in aerospace, energy, and tooling industries. Polishing these channels to improve internal surface finishing is critical to fatigue life, dimensional integrity, corrosion resistance, and fluid flow efficiency. Hydrodynamic flexible spindle (HydroFlex) polishing uses a high-speed fixed-abrasive grinding wheel driven by a flexible spindle to navigate through complex internal channels for fast, uniform, and controllable material removal and surface improvement. Critical to HydroFlex polishing is the presence and consistency of consistent grinding wheel orbital motion around the internal contour of the channel for controllable, consistent, and efficient surface improvement. The wheel orbital motion is a result of a dynamic equilibrium among the hydrodynamic force from the surrounding coolant, the grinding force for material removal, and the elastic force introduced by the flexible spindle. The influence of HydroFlex polishing parameters, including fluid viscosity, wheel rotational speed, and grinding wheel and spindle properties on these forces and the presence and consistency of the wheel orbital motion is challenging to understand due to complicated underlying physical phenomena. This dissertation establishes the fundamental physical relationships between wheel motion, hydrodynamic force generation, and polishing performance in HydroFlex polishing of both conventionally and additively manufactured metallic channels. First, a Taguchi-based sensitivity analysis was conducted to quantify the effects of wheel geometry, fluid viscosity, rotational speed, and workpiece material on orbital frequency and polishing outcome. The results showed that wheel orbit frequency and consistency are influenced by grinding speeds, channel material, and fluid media with down-grinding motion and stable orbital frequencies up to 588 Hz. Performance in deal conditions validated HydroFlex, producing rough surface reduction from 13.4 μm (as-built) to below 1.2 μm. Second, a computational fluid dynamics (CFD) model was developed to elucidate the hydrodynamic forces acting on the wheel under varying viscosity and rotational speed. The model revealed that rotationally induced asymmetric pressure gradients around the grinding zone are the dominant source of the tangential hydrodynamic force responsible for orbital motion in high viscosity fluids. A process map revealed viscosity and rotational speed regions, above which, this hydrodynamic component exceeds the grinding contact force, inverting the orbital direction to upgrinding. Simulated and experimental force comparisons confirmed that the transition between up- and down-grinding can be controlled through direct modification of grinding zone forces. Finally, HydroFlex was applied to curved and tapered channels representative of turbine cooling and fuel delivery systems. X-ray computed tomography and optical profilometry quantified the geometric and topographical evolution during polishing. For 10 mm and 25 mm radius channels, circularity error was reduced by over 60%, and surface roughness decreased from 10–12 μm to below 1.5 μm. In tapered geometries, HydroFlex maintained the originally specified dimension within ±1%, demonstrating its capacity for dimensional control even in non-uniform internal profiles. These findings collectively establish a model of the hydrodynamic and grinding forces governing orbital motion in HydroFlex polishing. By linking flow-field behavior, force dynamics, and material removal characteristics, this work provides a unified framework for predictive HydroFlex process control. The outcomes demonstrate that HydroFlex can achieve controllable, uniform surface finishing in complex channels while preserving dimensional fidelity, advancing its applicability for post-processing of additively manufactured and conventionally fabricated aerospace and energy components