Conventionally, in a case where a turbine rotor for a generator is manufactured, it is necessary to form a groove, into which a base portion of a turbine blade is to be inserted, in a shaft of the rotor. Normally, the groove has a fir-tree-shaped contour or a Christmas-tree-shaped contour and is provided in a plurality by undercutting. This sort of tree-shaped groove is generally bilaterally symmetrical with respect to a center of the groove, is machined such that the groove gradually becomes narrower in a groove depth direction, in an upside-down Christmas tree shape in which the groove width increases and decreases, and narrow portions where the groove width becomes less alternate with wide portions where the groove width becomes greater. In order to machine this sort of tree-shaped groove in a single pass, a Christmas cutter, for example, may be used (refer to Patent Document 1, for example). However, because a complex groove is formed in a single pass, a heavy load is imposed on the Christmas cutter and sufficient durability is required. Therefore, a method is known in which the tree-shaped groove is formed not in a single pass, but through a plurality of processes.
For example, known tree-shaped groove forming processes shown in FIG. 12 include four processes of a “tapered groove process”, a “T-slot process”, a “semi-finishing process” and a “finishing process”. First, in the tapered groove process, a tapered groove is cut in a work material 1 by a taper tool 110. Next, in the T-slot process, a T-slot portion is cut in the bottom of the tapered groove by a T-slot tool 111. Then, the tree-shaped groove is cut by a semi-finishing tool 112 such that a specified cutting allowance is left in relation to a final planned groove shape. Finally, the cutting allowance which has been left is cut by a finishing tool 113 at one time, and machining is performed along the finished shape. In this manner, the tree-shaped groove is formed in the work material 1.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2001-71210
In the known tree-shaped groove cutting processes shown in FIG. 12, in the semi-finishing process, narrow portions in the finished shape are respectively formed by reduced diameter portions of the semi-finishing tool 112 that are narrower toward a radially inward direction, however, because the reduced diameter portions have a reduced diameter, a load may be imposed on the reduced diameter portions during cutting, and the reduced diameter portions may break. In order to prevent a load from being imposed on the reduced diameter portions, there has been a problem that the cutting feed cannot made faster, therefore, the tree-shaped groove cannot be rapidly cut. Further, there has been also a problem that the four processes must be performed to form one tree-shaped groove, which may be time-consuming and inefficient.