During the sharpening of the cutter blades of a crop chopping rotor of a farm implement the rotor is rotated in reverse from the input shaft of a reversible transmission having a number of output shafts operably connected with certain components of the implement. Normally, the rotor is driven from a power shaft through a releasable coupling and the input shaft is rotated through a drive from the rotor shaft. Upon release of the coupling, connection of the power shaft with one of the output shafts and reversal of the transmission, the drive between the input shaft and the rotor shaft effects rotor reversal.