H6 arbor tolerance to improve cutter chattering and overall cutting tool life. T.I.R. 0.0004" measured from NMTB 30 taper to face and arbor. Made from alloy steel harden to 54-60 HRc. Taper ground to AT3 or better accuracy standard for maximum contact with spindle. All critical surface are precision ground. Each holder comes with a lab certificate on its taper accuracy and runout measurement.