Abstract: An automatic indexing chuck comprises a dividing pivot separately and in succession, operated by a dividing shaft and a locking sleeve. The dividing shaft is slideably and sealingly accommodated in a cavity of the locking sleeve which has retention planes for the dividing pivot and is in turn sealingly accommodated in a mandrel body cavity defining two adjacent chambers; the dividing shaft and the locking sleeve defining independent pistons operated by a single hydraulic circuit with reversible inlet and outlet ports, the pistons moving in opposite directions to perform consecutively unlocking, rotation and locking of the dividing pivot.