PURPOSE: To obtain a large cooling effect with cooling water sufficiently further surely supplied to a blade point by laminating two or more metal plates being spaced and providing a cooling water supply port, communicating with a space of the metal plates, in an axial center part and a cooling water discharge port in the peripheral blade point.

CONSTITUTION: A U-groove is provided in the peripheral direction with a fixed space in a disk 3 laminating a pair of steel plates 1 providing a space 2, and forming a segment 5, consisting of a diamond abrasive grain layer, between the U-grooves and a shaft hole in an axial center part, small holes 7 are formed with an equal space on the circumference in the periphery of the shaft hole. A drive shaft 11, whose small contour part 11b is fitted to a shaft hole of flanges 8, 8' and of the disk 3 with a bolt port threaded to a tap hole 13, holds the disk 3 to be fixed through the flanges 8, 8'. Cooling water is injected from a water injection port 23, reaching the shaft hole 14 from a water reservoir 21 via an escape hole 22 into the space 2 through a recessed groove 16 via a groove 15, recessed part 9 and the small hole 7, and the water is jetted in a diametric direction from a part of the U-groove, cooling a blade point.