Adjustment device for automatic adjustment of a spacing between a first and a second cutting blade. The adjustment device includes a connector 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. The adjustment device further includes a first stop fixed to one end of the connector and arranged in contact with the outer surface of the second cutting blade, and an adjustment means 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 and maintain first and second contact surfaces of the first and second cutting blades in contact.