A hydraulic fluid actuated lathe tool mounting expander assembly incorporating a block having a base, an inner end, an outer end, a compression end, and a compression face; a first cylindrical void extending into the block, the first cylindrical void having a floor and opening at the block's compression face; a second cylindrical void extending into the block, the second cylindrical void having a floor and opening at the block's outer end; first and second pistons respectively slidably mounted within the first and second cylindrical voids; a conduit extending between the first and second cylindrical voids, the conduit being adapted for conveying the hydraulic fluid from the second cylindrical void to the first cylindrical void; and a jack screw actuator connected operatively to the block for driving the second piston against the hydraulic fluid for upwardly driving the first piston.

Provided is a holder device that uses a pressure medium suitable for gripping a long and thin grasped object, the device eliminating need for a small-diameter machining for a device body and allowing use of a material suitable for sleeve deformation at a portion where a pressure chamber is formed. A holder device capable of deforming in a radial direction for fastening a grasped object 7 through a function of a pressure medium, includes a device body 2, a sleeve 3 disposed at a leading end of the device body 2 for gripping the grasped object 7, a pressure chamber 8 formed circumferentially in the sleeve 3, and a support portion 9 provided continuously from the device body 2 for forming the pressure chamber 8 together with the sleeve 3.

A chuck assembly for holding a plate comprises a plate holding member configured to hold the plate, the plate holding member including a central opening, a rigid member configured to hold the plate holding member, a first channel formed by the plate holding member, a second channel formed by the rigid member, a first fluid conduit in communication with the second channel, and, a second fluid conduit at least partially disposed within the second channel and in communication with the first channel.