Deep-hole bone drilling is performed in many surgical implantation procedures. Unlike bone drilling of shallow holes (commonly 5 to 7 mm in depth), deep-hole bone drilling requires a larger hole depth (i.e., up to a depth of approximately 35 mm in cochlear implantation) using a high ratio of the length to diameter of the drill bit. This paper presents an experimental investigation of forces in deep-hole bone drilling. The hypothesis of this study was that the forces signals show a sudden transition with a considerable amount of increases during deep-hole bone drilling process. The objective of this study was to understand bone drilling behaviors as drilling depth increases. Drilling tests were performed on bovine cortical bones using a 2.5 mm diameter drill bit with a spindle speed of 3000 rpm and feed rates of 0.05 and 0.1 mm/rev at a drilling hole depth of 36 mm. The force signals showed two distinct states, which were referred to as normal and abnormal states in this study. After showing a constant force signals in the normal state once the drill tip became fully engaged in bone cutting, from a certain drilling depth, the force signals considerably increased, which is referred to abnormal state. This study observed that the chip morphology varies with respect to drilling depth, showing fragmented chips in the normal state and powdery chips in the abnormal state. The results of this study indicate that the abrupt increase in the force signals in the abnormal state is mainly attributed to chip clogging inside the flutes as the drilling depth increases.