In a machine shop practice it is often necessary to repeat the same machine operating on a large number of workpieces, and powered workholders over hand-operated vices and the like has become widely accepted. These units require less physical effort by the machine tool operator, are faster, and are less likely to misalign or mis-grip a part. Examples of such holders are disclosed in U.S. Pat. No. 3,087,736, entitled "Collet Operator", issued to the present inventor, George N. Lukas, as well as in U.S. Pat. No. 3,338,573, entitled "Fluid Operated Vice" also issued to the present inventor, George N. Lukas.
Such pieceholders and like hydraulic force units are commonly driven by a booster unit which is a compressed air to hydraulic fluid transducer. By controlling the application of compressed air to the booster, hydraulic pressure in a line from the booster to the force unit is controlled, which, in turn, drives the workpiece holder or like force unit. Such a system using quick connectors has a great deal of flexibility and allows numerous connections and reconnections of the fluid and air hoses for numerous applications.
However, there are also some disadvantages to such a system which relate to the inherent nature of a hydraulic system to leak and the physical capacities of the units to displace hydraulic fluid. As long as the hydraulic fluid displaced in the power unit is moving from its retracted state to its maximum position is less than the effective volume of fluid in the hydraulic cylinder of the booster, the workpiece holder will, upon command, travel the required distance and exert great force against the workpiece. However, if that volume of fluid is just slightly under the needed volume, the unit travels to the workpiece, but the holding force drops off dramatically. The result is a loosely held piece which is undergoing machining. This can result in great damage to the workpiece, the machine tool, and, in certain circumstances, danger to the machine operator and others in the area.