1" CAT 40 V-Flange Shell End Mill Arbor ~ for CNC machines having CAT 40 taper spindles. ~ Made pf 20CrMnTi alloy steel. ~ Electrolysis nickel coating. ~ Balanced to 8,000 RPM ~ Tolerance is .0002" Maximum ~ Material is 42 Chromo ~ Hardness is HRC 50-52 ~ Thread Tolerance Class is 4H-Class ~ Retention knob thread is 5/8"-11 ~ Hardened and precision ground.