Tool holder

A tool holder comprising:

• a holder body (2) including a main grip portion comprising a cylindrical grip tube (2a) located at a front end of the holder body (2) and having a first tool holding hole (a1), a tapered outer peripheral surface (a2), and an axial slit (a3), said holder body (2) further including a trunk (2b) having a larger diameter than the grip tube (2a);
• a pressure ring (3) fitted around the tapered outer peripheral surface (a2) of the grip tube (2a) so as to be slidable in an axial direction of the tool holder;
• a nut (4) coupled to the pressure ring (3) so as to be rotatable relative to the pressure ring (3) and kept in threaded engagement with an outer periphery of the trunk (2b);
• a secondary grip portion comprising a sleeve (5) having an annular groove (5a) in an outer periphery thereof, a second tool holding hole (5c) in a center part thereof, the second tool holding hole (5c) having an inner diameter smaller than an inner diameter of the first tool holding hole (a1) in a free state, and a thin-walled portion (5b) defined between a bottom of the annular groove (5a) and the second tool holding hole (5c), the sleeve (5) being inserted in an axial hole (2e) at the center of the holder body at a location rearwardly of the grip tube (2a);
• wherein the annular groove (5a) is liquid-tightly isolated from the axial hole (2e) of the sleeve (5) such that the annular groove (5a) serves as a pressure chamber (9);
• at least one cylinder (10) provided in the trunk (2b) and having one end thereof open at a front end of the trunk;
• at least one piston (11) axially slidably inserted in the cylinder (10) with a front end portion of the piston (11) protruding from the cylinder;
• a communication channel (12) through which the cylinder (10) communicates with the pressure chamber (9);
• and
• hydraulic fluid sealed in the cylinder (10), the communication channel (12), and the pressure chamber (9),
• wherein the tool holder is configured such that when, with a tool inserted in the tool holder, the nut (4) is turned in one direction, the nut (4) simultaneously moves axially rearwardly, the pressure ring (3) moves axially rearwardly together with the nut (4), thereby radially compressing the grip tube (2a) while pushing the piston (11) such that hydraulic pressure is produced in the pressure chamber (9), and the thin-walled portion (5b) of the sleeve is radially compressed by the hydraulic pressure, whereby the tool is gripped by the first tool holding hole (a1) and the second tool holding hole (5c) at the same time.