Deep-hole bone drilling is critical in many surgical implantation procedures. Unlike most common bone-drilling processes, deep-hole bone drilling is performed using a high drilling depth to drill-bit diameter ratio, which can lead to undesirable mechanical and thermal damage during surgical procedures. The objective of this study was to investigate the thrust force and torque generated in deep-hole bone drilling. Drilling tests were performed on bovine cortical bones at a drilling hole depth of 36 mm using a 2.5 mm diameter twist drill bit with a spindle speed of 3000 rpm and feed rates of 0.05, 0.075, and 0.1 mm/rev. Bone chips were collected at different depths and examined using a fiber-optic microscope. Not only are drilling forces a good indicator to assess drilling performances but also chip formation and morphology are important aspects for understanding bone-drilling behaviors. The force signals revealed two distinct states, which were referred to as normal and abnormal states in this study. In the normal state, the force signals remained constant once the drill tip became fully engaged in bone cutting, whereas after a certain drilling depth, the forces considerably increased in the abnormal state. The results of this study indicate that the rapid increase in the force in the abnormal state is mainly attributed to chip clogging inside the flutes as the drilling depth increases. This study also demonstrated that the chip morphology varies with respect to drilling depth, where fragmented chips are produced at shallow drilling depths and powdery chips are produced at deeper drilling depths.