Abstract: Detection and gauging of a joint (18) in a rotating and axially moving drilling tool (16) subject to transverse axial displacement is achieved by positioning an ultrasonic sensor assembly (12) coaxially encircling the tool in a plane perpendicular to the tool. The sensor assembly (12) generates more than one reflected pulse to represent changes in tool (16) diameter by transmitting a beam from at least one ultrasonic sensor (32) toward the tool (16) as the tool axially advances or recedes, perpendicularly piercing the plane of the sensor assembly (12). Each reflected pulse is converted to an electrical output signal, all of which signals are summed to minimize measurement distortion due to noise and transverse axial displacement of the rotating tool and compared to a threshold detection voltage in a signal processor (14) to generate a detection output pulse.