as large as 33 u03bcm and hence the life was only 10 6 cycles see fig8 in samples 8 and 9 foreign matter of a larger particle size than the particle size of abrasive grains of the compound being used was detected accordingly inclusions of the material of samples 8 and 9 were measured by a dissolving extraction method and larger inclusions than abrasive grains were observed that is the abrasive grains contain some larger than average particle size in the case of alumina or other oxide abrasive grains they are ground right after the start of grinding and become smaller than the average particle size but since abrasive grains of nitride and carbide are less likely to be ground abrasive grains larger than the average particle size are left over which are finally driven into the hoop surface the bulk specific gravity of the media is discussed hoops were fabricated in the same conditions as in embodiment 1 and marginal edges were removed by barrel polishing under various conditions in this barrel polishing using the resin having oxide abrasive grains bound by a binder various bulk specific gravities were set by varying the abrasive grain rate of the media the content of abrasive grains in the media in this barrel polishing the rotary barrel was set at a speed of 24 rpm and polishing was operated continuously for 4 hours table 3 shows other conditions of barrel polishing the maximum particle size of foreign matter extracted from the surface of the hoop.