FEM Approach to Predict Three Jaw Chuck Stiffness and Its Effect on Gripping Force for High Speed Turning and Experimental Verification

Higher safety norms and precision machining has pushed machine tool manufacturers to build high-speed machines with reliable work-holding devices. With advances in bearing manufacturing techniques and easy availability of precision roller bearing, hydrostatic bearings, active magnetic bearings and efforts are reduced to manufacture high speed and high precision spindle, leaving workpiece clamping subsystem as the weakest link. Popular work-holding device in lathe is a power-operated three-jaw chuck because of its self-centering properties. However, the problem with power-operated three jaw is the loss of gripping force at high speeds due to large centrifugal forces that act on three-jaw chuck. This loss in gripping force makes the machine operation detrimental in terms of safety of operator/machine as well as the accuracy of machined components due to loss of stiffness at the work side. It will be advantageous if the stiffness behavior of the work holding is known for the operating range. Though the supplier provides speed versus gripping force plot, this not sufficient as the measurement is done for ideal work holding diameter. The speed versus gripping force plots varies for different static gripping forces as well as for different holding diameters as the stiffness varies due to jaw positioning for different diameters. This paper proposes finite element method to predict the loss in gripping force/stiffness due to high spindle speed for various holding diameters. The finite element results are verified with experimental results.