An electronic thread tapper includes a cam-operated feed mechanism which drives the tap carriage rigidly in the advance direction only. Separate force means are employed to move the carriage in the retract direction when the tap is being withdrawn from the work material. Spindle and feed motors operate separately, but are cycle-coordinated during a dwell period between tapping strokes. By measuring any position error developed between the spindle and feed positions, a fault condition, such as a worn tap, a workpiece material defect, or an obstruction in the machine can be sensitively detected.