HHIP Pro-Series AA Grade 5 Micron high accuracy ER spring collet is made of high grade SUJ2 steel. Hardened and precision ground 44 48HRC. Exceeds Federal specifications. Inside diameter ground to reach mirror finish increased grip force. Surface treated to reduce rust and improve hardness. Cryogenic treatment to significantly increase wear resistance. Collet must be assembled into collet chuck nut before inserting into collet chuck. Runout less than .0002. Superior Finish. Specification: Size/Range: 3/8" (.354-.393); Major Diameter: .830"; Overall Length: 1.230".