A chuck has a chuck body at least generally centered on a body axis and a collet in the hole and having an external screwthread. A ring axially nondisplaceable in the body has an internal screwthread engaging the screwthread of the collet and external gear teeth. The body is formed with a radially outwardly open cavity seat in which some of the teeth of the ring are exposed. A gear fitted in the seat meshes with the ring teeth and can rotate about a drive axis not parallel to the body axis to rotate the ring about the body axis. According to the invention a sleeve on the chuck extends axially across and covers the seat to retain the gear therein and is formed with a through going aperture through which a tool can fit to engage and rotate the gear.