Abstract: A digital control system is provided for controlling the indexing of the focussed beam of an ultrasonic transducer over a sloped or curved surface of a workpiece being inspected for flaws. To index the transducer to follow a sloped surface on the workpiece the angle of the sloped surface in half degree incrementation is setup and used to read out the sine and cosine functions from a memory. At the index time of a scan each of these functions is used to pass a portion of a preset number of clock pulses to provide vertical and horizontal clock pulses which respectively increment stepping motors to reposition the transducer along the sloped surface. To index the transducer to follow a curved surface on the workpiece, the radius of the circumferential path along which the transducer is to be moved is setup and used to help define when the transducer has moved along a chord of a half degree angle.