Development of models to demonstrate the influence exerted on form and dimensional accuracy in high-precision hard turning operations

Surface quality and form or dimensional accuracy are the most decisive factors with regard to the functional behaviour of high-precision parts. The technology of high-precision turning has enormous potential for the substitution of grinding and finish-grinding processes. it must be recognized, however, that the kinematics of turning are completely different from those of grinding. In turning processes, each point of the workpiece is generated by the cutting edge at one particular moment. Relative displacements between the cutting tool and the workpiece are the direct cause of form and dimensional error. Due to the kinematic characteristics of the turning process, these errors cannot be minimized by sparking-out as happens in grinding operations. Relative displacements can be caused by a number of influences such as forces, thermal effects, tool wear etc. Influences affecting workpiece accuracy have been examined at the Fraunhofer-Institut für Produktionstechnologle IPT. The principal p urpose of these investigations was to identify the correlation between thermal effects and machining forces and the levels of workpiece accuracy which can be achieved. Since the capacity to perform dry machining operations is one of the major advantages of the hard turning technique, it is particulary important to control thermal displacement of the workpiece. Workpiece and tool temperatures were measured during the machining operation, thereby enabling thermal distribution in the workpiece, chips and tool to be recognized. Thermal models, permitting calculation of the theoretical thermal extension of the workpiece in correlation with machining parameters have been developed. Additionally a model has been developed to estimate machining forces and their influence on workpiece accuracy. The tests and the derived models serve as a basis to optimize high-precision hard turning processes and to achieve higher qualities in form and dimensional accuracy.