A cutter head for a wood chipper has a circular disc and is mounted on a spindle by which the cutter head is rapidly rotated. The disc has at least one knife detachably connected thereto with its cutting edge exposed on the face of the disc engaged by the lengths of wood to be chipped. The connecting means comprise an actuating member releaseably seated against the rear face of the disc in a manner such that it may be moved parallel to the slot through which the knife extends. The knife is releaseably connected to the disc to enable its cutting edge to be advanced or retracted relative to the face of the disc engaged by lengths of wood and is also connected to the actuating member in a manner such that movements of the actuating member in either direction parallel to the knife receiving slot are attended by corresponding movements of the knife in directions normal to movements of the actuating member.