movement of the output of the drive motor 74 to cause the grindstone carriage 43 to be moved between extreme right and left hand positions not shown where the grindstone 42 is out of contact with the chopper drum knives 38 the grinding arrangement for the chopper drum knives as thus far described is that disclosed in the above identified u s pat no 4 843 767 to which one should resort if further details of the structure or grinding procedure is desired suffice it to say that when the knives 38 need to be sharpened the chopping drum 22 is set in rotation usually in 1 the opposite rotating direction and or with a reduced rotational speed in comparison to the normal chopping operation the grindstone holding arrangement or carriage 43 with the grindstone 42 mounted thereon is displaced over the entire width of the chopping drum 22 from a not shown idle or parking position in which it is arranged laterally adjacent to the chopping drum 22 during this process the underside of the grindstone 42 is in contact with and sharpens the knives 38 during the grinding process the grindstone 42 is displaced or swept over the width of the chopping drum 22 several times by driving the motor 74 first in one.