Pro Series by HHIP 3900-0697 Collet Chuck Nut, KM Bearing Type ER40, 18000 RPM Balanced to 18,000 RPM Tolerance is .0003" Maximum Material is 42 CrMo and the hardness is HRC 50-52 Thread Tolerance Class is 4H-Class Thread is M50 X 1.50 - D is 63 mm - L is 29 mm