PURPOSE:To smooth the discharge of chips and promote cutting-off capability, by shaping the internal circumferential part of a grindstone as a polygon. CONSTITUTION:A diamond grindstone part 3 is provided along the internal circumference of an annular thin metal and shaped as a polygon. The nook 6 of the polygonal part 3 is placed out of contact with a surface to be cut off, so that a chip pocket is defined. For that reason, the inflow of grinding liquid is promoted, the cutting-off capacility is improved and the discharge of chips is facilitated to prevent loading. Although the central sections 7, 7', 7'' of the sides of the grindstone part 3 intermittently come into contact with the workpiece to grind it, the grindstone part receives no shock, because the cutting feed changes gradually. As a result, the workpiece is not chipped and the roughness of its cut- off surface is not high.
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