The invention relates to an adjustment device for automatic adjustment of a spacing between a first and a second cutting blade (21, 22； 51, 52), the first and second cutting blades being mounted so that the first and second contact surfaces (31, 32) are in contact with and parallel to each other； wherein at least one of the first and second cutting blades (21, 22； 51, 52) is displaceable to produce a cutting action between first and second cutting edges (23, 24). The first and second cutting blades are connected by at least one adjustment device (28； 50； 90) arranged to maintain the spacing between the first and second planar contact surfaces. The adjustment device (28； 50； 90) comprises a connector (34； 64； 79； 84； 94) extending through a recess in the respective first and second cutting blades at right angles to the first and second contact surfaces and arranged to guide the movement of the displaceable cutting blades relative to the other cutting blade； a first stop (33； 63； 102) fixed to one end of the connector (34； 64； 79； 84； 94) and arranged in contact with the outer surface of the second cutting blade； and an adjustment means (40； 70； 80； 100) located at the opposite end of the connector and arranged to apply a pre-tensioning force on the outer surface of the first cutting blade (21； 51) and maintain the first and second contact surfaces (31, 32) in contact.
