lower end thereof is a ring gear 47 and hydraulic motor 48 for driving the same the ring gear 48 is affixed to an undercutter frame 51 which includes a support arm 52 and a curvilinear portion 53 such that rotation of the ring gear 47 rotates the undercutter frame 51 about the vertical axis of the column 42 the undercutter frame 51 supports and connects three distinct portions of the undercutter assembly 10 the first portion is an undercutter guide bar 56 which lies in a generally horizontal plane in both the travel and work position undercutter guide bar 56 has a terminal sprocket 57 mounted at one end thereof and is of sufficient length to span the entire width of the track the second portion is an inclined guide bar 58 which lies in a plane intersecting the plane of undercutter guide bar 56 and forming an acute angle therebetween of about 50 u00b0 a drive sprocket 59 and motor 61 are located at the upper end of inclined guide bar 58 the third portion is intermediate the undercutter guide bar 56 and the inclined guide bar and comprises a plurality of guide rollers 62 which are supported for rotation on horizontal axles 63 mounted about the arcuate portion of curvilinear frame member 53 each guide roller 62 is in the form of a disc and each roller 62 is staggered relative to its adjacent roller 62 39 such.