Undercut is the distance between the plate or blanket cylinder and the pitch circle of the gears that drive the cylinders. In some cases, it is considered to be the distance between the outside diameter of the bearers on the plate or blanket cylinders, and the actual diameter of the plate or blanket cylinders. The exact undercut is specified by each press manufacturer based on the thickness of the plates and blankets they intend to use on the press they are designing.
Pitch line is the point at which two gears are in full contact with each other.
Circumference of a circle at a point where the pitch of a gear would fully contact another gear. The importance of undercut is to know exactly how much impression a press has to print with once the plates and blankets have been installed on the press. Normally, most press manufactures want a total plate thickness that is 0.002 inches to 0.004 inches thicker than the actual thickness of the plate cylinder undercut. The press manufactures also want the total thickness of the blanket materials, plus any packing, to be 0.004 inches to 0.006 inches thicker than the actual undercut of the blanket cylinder. The undercut of a blanket or plate cylinder can sometimes be found in either the press manual or the unit blueprints.