the sprockets 26 mount the chains for reciprocal movement in a desired direction the chains are driven by hydraulic or other suitable means known in the art to the chains 24 is fixedly mounted a ram 30 so that as downward movement is applied to the chains 34 the ram 30 is carried downward thereby the operator of the rig 10 can by controlling the chain driving means impose a linear downward pressure upon the ram 30 the ram 30 transfers the linear force to a percussion assembly 32 extending downward therefrom it also imparts rotational motion to the percussion assembly 32 so that a bit element 34 disposed at the bottom of drill rod 22 extending from the percussion assembly 32 will effect the drilling function apparatus heretofore identified is known in the art for the sake of brevity therefore its detailed operation will not be described herein referring then to fig4 the percussion assembly 32 has a first axial end 36 formed in the first axial end 36 is a receptacle 38 from which in rigs known in the art a bit element 34 is suspended fig4 however illustrates a mandrel 40 in accordance with the present invention substituted for the bit element 34 the mandrel 40 includes a.