A collet chuck for holding a mounting collar of an objectnulltool or workpiecenullto be rotated has a chuck body centered on and rotatable about an axis and formed with a forwardly open socket shaped to receive the collar. A plurality of jaws in the socket have front ends engageable inside the collar in the socket and rear ends. The jaws are angularly spaced and defining axially extending and radially inwardly open slots. A setting sleeve has a front cam surface engageable with inner faces of the front ends of the jaws so that the sleeve can cam out the front ends to lock the jaws to the collar in the socket. The sleeve is formed with radially outwardly projecting and axially extending ribs each received in a respective one of the slots.