there are two inwardly extending support arms 98a 98b and a support pad 98c see fig8 connected to cross beam 100 the arms 98a 98b and pad 98c define a support shelf 98 the purpose of which will be described hereinafter at the front of the auxiliary frame there is a truss 102 connected between the side members of the frame with a fitting 104 extending inwardly therefrom the fitting 104 for reasons which will become clear hereinafter has four spaced fingers 105 to straddle a portion of truss 102 and also the vertical pin 103 the spacing of the fingers is such as to allow a small vertical movement between the fitting 104 and the truss 102 as shown best in fig8 the upper works 36 39 is received in the bearing 34 39 of the central base portion of the lower works 22 for rotation about the axis a 39 conventional power machinery not shown is provided to rotate the upper works with respect to the lower works the auxiliary frame 92 which surrounds the upper works is connected to the upper works at the front and rear of the upper works for rotation with the upper works the axis a 39 of rotation of the upper works passes through the center of the ring 70 and the rollers 96 of the frame 92 are equi spaced from the axis a 39 so that the auxiliary frame can rotate on the ring 70 through any angle the upper.