In a forage harvester having a motor driven sharpening stone driven back and forth along a path parallel to the axis of a rotating cutterhead to sharpen the knives of the cutterhead, and a door which normally prevents access to the knives, a control circuit is provided for controllig the opening of the door, the movement of the sharpening stone to sharpen the knives, and the closing of the door after the knives have been sharpened. The circuit is controlled by switches located at an operator's control panel and accessible to the operator while he is in the driver's seat of the harvester so that the operator does not have to leave his driving position to accomplish the knife sharpening operation. The circuit includes visual indicators lcoated on the control panel for indicating the status of the circuit during the entire operation. The circuit includes a sensor circuit for sensing the rate of rotation of the cutterhead, the sensor circuit preventing knife sharpening from taking place unless the cutterhead is rotating at a rate greater than the rate required for proper sharpening of the knives.