A machine tool constructed in accordance with the present invention includes an improved coupling assembly which is operable between an engaged condition in which the coupling assembly holds a turret against rotation and a disengaged condition in which the turret can be rotated to index tools relative to a workpiece. The coupling assembly includes a first coupling member on which there is an annular array of teeth. A housing is fixedly connected to the first coupling member and forms a main chamber in which a second coupling member is disposed. This second coupling member is fixedly connected with a turret bar and is movable in the main chamber under the influence of fluid pressure to operate the coupling assembly between engaged and disengaged conditions. When the coupling assembly is to be disengaged, fluid pressure is applied against a surface on the second coupling member to move the second coupling member in the main chamber in a direction away from the first coupling member to disengage the two sets of teeth. When the coupling assemblies are to be reengaged, fluid pressure is applied against another surface on the second coupling member to move the second coupling member toward the first coupling member and move the two sets of teeth into meshing engagement.
