On a turret (30) which can be driven in rotation about a central axis (A), a plurality of expanding mandrels (42) are arranged all around the central axis (A). They are each essentially composed of a supporting member (44) which is secured to the turret (30) and defines an expanding-mandrel axis (B), a plurality of segmental bars (52) which are arranged all round the expanding-mandrel axis (B) and are guided on the supporting member (44) for radial adjustment, and a wedge (70) which is displaceable along the expanding-mandrel axis (B) to spread the segmental bars (52) apart. The wedges (70) are displaceable in succession, by a common actuating member, during one revolution of the turret (30). The actuating member is a central cam member (18) on which each of the wedges (70) is supported at least during a portion of each revolution of the turret (30) corresponding to the distance between two adjacent expanding mandrels (52).