Abstract
Increasing drilling depth in bone surgery introduces distinct mechanical and thermal challenges not typically encountered in shallow drilling, as force dynamics shift from a stable regime (Normal Zone) to abrupt escalations (Abnormal Zone). This study investigates the influence of key drilling parameters—drill-bit diameter, spindle speed, and feed rate—on force dynamics and Transitional Drilling Depth (TDD), defined as the depth at which mechanical instability emerges. Experiments were conducted on bovine cortical bones using a CNC-based drilling setup, reaching depths up to 36 mm while systematically varying parameters across clinically relevant ranges. Real-time measurements of thrust force and torque were segmented into Normal and Abnormal Zones and analyzed statistically to evaluate parameter influence. Results show that feed rate and drill-bit diameter are the primary determinants of thrust force and torque across both zones, supported by large η2 values. In contrast, spindle speed has a modest but statistically meaningful influence, particularly in modulating torque within the Abnormal Zone. Overall, smaller drill-bit diameters, lower feed rates, and higher spindle speeds reduce force levels in both zones, while lower feed rates and slower spindle speeds significantly increase TDD, prolonging stable drilling. No statistically significant three-way interactions were observed, and most two-way interactions with spindle speed were not significant. These findings reflect the limited role of spindle speed in deep-hole bone drilling, where benefits for chip evacuation are offset by heat buildup and flute clogging. Nevertheless, careful spindle speed selection remains essential to control force dynamics.
