Abstract: This application relates to a self-contained apparatus for mechanically turning individual workpieces travelling on a conveyor, such as lengths of lumber travelling into or exiting from a sawmill. The invention reduces the need for lumber graders to turn lumber manually and thereby helps avoid repetitive motion injuries. The apparatus includes an adjustable height frame which is positionable alongside the conveyor. The frame supports a workpiece holder defining an open-ended slot for temporarily receiving an end portion of a workpiece. When the end of a lumber board or other workpiece is received within the holder, a proximity sensor causes the holder to rotate, thereby turning the workpiece lengthwise 180 degrees. The workpiece is then deposited on to the conveyor in its inverted orientation for further travel thereon. The apparatus may also optionally permit workpieces to pass through the holder when a “skip” rather than an “invert” signal is received.