PURPOSE: To improve machining accuracy in such a way that grinding stone pieces are fitted to be freely movable upward and slantly against springs onto the rear face of a flange which is installed to be freely movable vertically through a universal coupling so that contacting pressure of the grinding stone pieces to a work is kept constant.

CONSTITUTION: A housing 3 is pivotally supported to be freely movable slantly by a shaft 4 on a bracket 2. And, a grinding stone flange 13 is provided through a universal coupling 12 at the lower end of a piston rod 10 which is freely movable up and down, a pluraity of grinding stone pieces 14 are provided to be freely movable upward and slantly against each spring 15 at the rear face of the flange 13. Thus, the grinding stone pieces 14 are tured while being brought into contact with a work (a) under a proper pressure, and are moved along the work (a) while following up freely to the changes a curved surface of the work (a), thus the full area of the work (a) can be machined with high accuracy.