way that the cross sectional configuration thereof becomes rectangular when they are sintered in addition to this method they may be produced using a hollow blade material eo having a wall of given thickness in the manner shown in fig9 in other words in fig9 each of the cutting parts is formed in such a way that each of the rake faces ab a 39 b 39 which each form the respective cutting parts io iio are disposed within axial planes of the shaft center o 39 a opposite to the rake faces do not eat into the rake faces a 39 b 39 of their adjacent cutting parts iio with such a method it is inevitable that waste portions f f 39 are created between each of the cutting parts on the blade material eo in addition as shown in fig9 the configuration of the cutting parts io iio iiio ivo and iivo is made reverse to that of the cutting parts vio viio viiio ixo and xo this causes a remarkable reduction in the number of cutting parts that can actually be used in contrast with the above and shown in fig4 each of the cutting parts in the present invention is formed along the lines or axial planes extending radially outwardly from the shaft center o of the hollow blade material e in this construction the waste portions f f 39 inherent to the conventional blade manufacturing method shown in fig9 can be avoided and it is also possible to avoid the situation wherein the configuration of a group of cutting parts becomes.