The extension arbor (1) screwed onto the standard tool fixing element (30) is fitted with a sleeve (20). At its free end the sleeve (20) has a tool fixing or connection hole (3). The sleeve stands concentric to the extension arbor (1) and its free end has a contact surface and an axially protending driver pin (32) for the tool. The extension arbor engages the central mounting bore (32) of the fixing element by use of a threaded pin (2) and at the far end has a keying surface (5) for assembly or dismantling tools so arranged that the arbor can be twisted round to make the screw connection even when the sleeve has been fitted. The sleeve is positively joined to the fixing element (30) and is recessed (22) at this same end to be engaged here by the groove blocks used to pass rotative moment to its connected fixing element.
