Longitudinal position of the end of a drill bit (21) is determined in order to accurately control depth of a hole to be drilled. A drill bit (21) is rotated at a predetermined speed and caused to penetrate a light beam of known vertical position. The light beam, which is generated by a light emitting diode (50) and received by a photo transistor (54), is modulated by the flutes at the tip of the rotating bit to cause the photo transistor to provide an output signal that is modulated at a frequency that is a fixed multiple of the speed of the rotating bit. An operational amplifier (74) having a feedback circuit (80) that is resonant at the frequency of the modulated electrical signal from the photo transistor receives the photo transistor output signal as its input. The amplifier output experiences a sharp increase in magnitude upon receipt of the input signal at the resonant frequency of its feedback circuit. Thus a reference position of the bit is signalled and stored by the bit spindle control (18) that will move the bit longitudinally to a depth determined on the basis of the known bit reference position.
An optical device for establishing a spatial orientation for a drill bit includes at least three light sources and two cameras. The first light source is used to silhouette the drill bit; a second light source is used to generate a reflection from a primary facet of the drill bit; and a third light source is used to generate a reflection from the margin of the drill bit. In sequence, the first camera responds to the first light source to establish an axial position for the drill bit on the axis. The second camera then responds to the second light source to establish a gross rotational position for the drill bit on its axis. Finally, the first camera is again used. This time it establishes a precise rotational position for the drill bit on its axis to establish the spatial orientation for the drill bit.
