layers according to example 5 as compared with a comparative sample no 4 23 in samples nos 4 5 to 4 7 furthermore it was easier to discriminate used corners as compared with the comparative sample no 4 23 and the inserts were remarkably improved in wear resistance as compared with the comparative samples nos 4 23 and 4 24 samples nos 4 8 to 4 16 were remarkably improved in wear resistance as compared with the comparative samples nos 4 23 and 4 24 in samples nos 4 20 to 4 22 further it was easier to discriminate used corners and the inserts were remarkably improved in wear resistance as compared with the comparative samples nos 4 23 and 4 24 in cbn sintered body portions of a sample no 4 1 both the flanks and the cutting faces remained black after coating and it was impossible to discriminate used corners in a sample no 4 4 coating layers were chipped after 5 minutes from starting of cutting thus it is understood that proper thicknesses of the coating layers are 0 1 to 10 u03bcm in relation to the flanks a comparative sample no 4 25 caused de brazing after 30 minutes from starting of cutting a composite sintered body of a sandwich structure having cbn sintered bodies containing 85 percent by volume of cbn and a rest or remainder consisting of borides and carbides of w and co borides of tin al and ti aln and unavoidable impurities was formed.