Abstract: A bearing for use in the main spindle of a machine tool is provide, in which at least the raceway of the bearing contains by mass, C: 0.6-1.3%, Si: 0.3—3.0%, Mn: 0.2-.5%, P: 0.03% or less, S: 0.03% or less, Cr: 0.3-5.0%, Ni: 0.1-3.0%, Al: 0.050% or less, Ti: 0.003% or less, O: 0.0015% or less, N: 0.015% or less; and the rest is composed of Fe and unavoidable impurities. The raceway is tempered after either quenching or carbonitriding and its surface hardness presents at least HRC 58 after tempering.

Abstract: A bearing for use in the main spindle of a machine tool is provide, in which at least the raceway of the bearing contains by mass, C: 0.6-1.3%, Si: 0.3-3.0%, Mn: 0.2-1.5%, P: 0.03% or less, S: 0.03% or less, Cr: 0.3-5.0%, Ni: 0.1-3.0%, Al: 0.050% or less, Ti: 0.003% or less, O: 0.0015% or less, N: 0.015% or less; and the rest is composed of Fe and unavoidable impurities. The raceway is tempered after either quenching or carbonitriding and its surface hardness presents at least HRC 58 after tempering.

Abstract: The invention relates to a casting core composition including refractory mullite grains and 1.0-2.2 wt % of a phenolic resin for binding the mullite grains. The mullite grains are substantially spherical in shape and have a special grain size distribution. That is, the mullite grains contain first to eighth fractions. These fractions respectively have first to eighth diameters which are respectively within ranges of larger than 420 .mu.m, 297-420 .mu.m 210-297 .mu.m, 149-210 .mu.m, 105-149 .mu.m, 74-105 .mu.m, 53-74 .mu.m, and smaller than 53 .mu.m.