Abstract: A radially adjustable drive bracket (126) is affixed to, and rotatable with the drive gear (32), and a radially adjustable support bracket (128) is affixed to, and rotatable with the driven gear (42). A pair of parallel slide rails (30) are coupled at opposite ends to the drive bracket (126) and support bracket (128). A cutting head assembly (28) is slidably mounted on the slide rails (30), wherein a threaded lead screw (124) carrying a traveling nut (134) thereon causes longitudinal displacement of the cutting head assembly (28) along the slide rails (30). A cam arm (140) coupled through a one-way clutch to the lead screw (124) and adapted to be actuated by a tripper assembly (136) mounted so as to engage the cam arm (140) during orbiting movement of the drive and driven gear (32, 42).