A standard power actuator for a chuck, mandrel, or like machine tool adapted to hold a tool or a workpiece that is being machined typically has a hollow cylinder generally centered on an axis and normally fixed to, for instance, the chuck body, and a piston axially displaceable but nonrotatable in the cylinder, defining therein a pair of axially opposite pressurizable compartments, and normally fixed, for instance, to the jaw-actuating member of the chuck. A piston stem centered on the axis, fixed to the piston, and extending axially from the piston is formed with a pair of separate feed passages each having one end opening into a respective compartment and an opposite end opening axially on the stem. A distributor body rotatable about the axis on the stem at the opposite ends of the passages is provided with respective connections communicating with the opposite passage ends so that the compartments can be pressurized via the respective passages, thereby moving the jaws on the chuck body. Structure on the stem and distributor body permits the body to rotate about the axis on the stem but prevents the body from moving axially on the stem.
In order to use such an actuator in an automated machining system it is necessary to provide a position-monitoring unit that itself is connected to a controller that in turn operates the valves that themselves control the actuator. This is typically done by providing on the cylinder housing a bearing that is connected to an outrigger that carries a switch- or sensor-actuating member that can coact with a switch or sensor carried on the distributor body that itself is fixed axially but not rotationally on the piston stem. Thus this actuating member is axially but not rotationally fixed on the cylinder housing so that as the cylinder housing and piston move axially relative to each other the actuating member will move past the switch/sensor. The relative positions of the switch/sensor and the actuating member can be set such that, for instance, the switch/sensor is actuated when the chuck is fully opened or closed.
In U.S. Pat. No. 4,945,819 a threaded ring is provided on a threaded extension of the cylinder and stops carried on this ring extend through seals into the rear compartment where they act as abutments defining the rear end position of the piston. This is useful to limit the rearward stroke in automatic machining operations so that the chuck can be reloaded with limited wasted movement, that is without having to be opened all the way up if that is not necessary. The disadvantage of this system is, however, that on such readjustment it is necessary to also reset the element that trips the switch or valve that is actuated when the piston reaches its rear end position. Furthermore the abutment pins provide a ready route for leakage from the rear compartment.