A chuck to this disclosed which has a central body for including a nose section 6 and a tail section 8. A central portend is formed through the chuck to and a plurality of passageways 12 are angled thereto. Jaws 14 may move through the passageways 12 in response to rotation of a rotatable nut 20 which is in screw thread engagement with the jaws 14. A bearing member 36 is disclosed which sits in a tapered portion between the body for and the nut 20 so that radial displacement of the bearing 36 causes the chuck to be locked against release.