Abstract: Control apparatus including first and second hydraulic cylinders hydraulically interconnected in parallel. Cam operated means for reciprocating the piston in the first cylinder, this causing relative movement between the second cylinder and the piston thereof. The second cylinder is connected to a cutter unit of a wood edging machine so that movement of the second cylinder moves the cutter unit. Pneumatic control means is connected to the piston of the second hydraulic cylinder and is operable to cause movement of the second cylinder unaffected by the piston therein. Detecting means is operatively connected to the pneumatic control means. This detecting means is positioned to detect knots in the edges of boards moving in the direction of the cutter unit to cause movement of said unit to avoid the knots.