Abstract: A retainer device which is adapted to block temporarily in a predetermined relative position the hard-steel jaws of a chuck comprises a carrier ring on which there are slidably mounted and lockable in radial direction, a plurality of blocking slides, each slide carrying a double-hook element presenting two inwardly directed hook arms. Upon adaptation of the retainer device frontally against the chuck jaws, each double-hook element engages from both sides, with the said inwardly directed arms, the clamping head portion of a corresponding chuck jaw, thus blocking the jaws in the desired working condition at which their gripping surfaces can be ground true with respect to the workpiece that is to be supported therein.