Abstract: Piezoelectric transducers are mounted on support members of a gag press for sensing pressure vibrations arising in a workpiece in part as reflections of an initial pulse produced by the striking of the workpiece by a press ram at the initiation of a straightening operation. An electrical circuit is connected to the transducers for analyzing the sensed pressure waves and for comparing the analyzed waves to pre-established sound-threshold data stored in a memory. Upon detecting pressure waves indicative of an internal structural flaw in the workpiece, a microprocessor in the electrical circuit signals a ram drive to raise the ram and thereby terminate straightening operations, the defect-bearing workpiece being transferred to a scrap store or to a heat treatment plant by a sorter according to signals from the microprocessor.