Abstract: A monitoring device is described for use with a rotary drive shaft of a machine tool, such as a power press. The monitoring device includes a light emitting means and a photo-electric receiving means for producing a discrete electrical signal for each rotation of the driveshaft through a selected fractional part of one rotation of the shaft, a data processor for receiving said electrical signals and converting said signals to rotational positions of said shaft in degrees and a digital display adapted to indicate the output of the data processor in degrees of a circle.