A straightening press of the type wherein a workpiece is supported for deflection in a given direction by an anvil engageable with the workpiece. A reference spring is supported for deflection with the reference spring supports being serially interconnected with the anvil. In a preferred embodiment, a floating bridge carries the anvil and reference spring supports. Deflecting force applied to the reference spring, as by a ram, is transmitted to the workpiece via the floating bridge. A change in the differential displacement of the ram and floating bridge is detected to determine the onset of plastic deformation of the workpiece.