FIGS. 4A to 4D are diagrams useful in describing a modification of the radial bearing (radial bushing) shown in FIG. 2, in which FIG. 4A is a plan view illustrating the shaft-side split bearing member of the radial bearing, FIG. 4B a side view showing the shaft-side split bearing member illustrated in FIG. 4A, FIG. 4C a plan view showing the cutter-side split bearing member of the radial bearing,