Measuring And Modelling Thermal Distortion On CNC Machine Tools

Abstract

Heat sources internal and external to a machme tool cause temperature gradients to arise due to resistance to heat flow in and around the machine tool and workpiece. This causes linear expansion and distortion of the structural elements from which the machine tool is constructed. The connectivity of the structural elements sets the effect of the expansion and distortion on the relative positions of the tool and workpiece, leading to thermal errors on the workpiece. Measurement on a wide range of machine tools confirms that temperature gradients are significant in their effect on machining accuracy and that they move and change shape during the machining process. An analysis of machine tool structures and connectivity shows that both the position and magnitude of temperature gradients is important in its effect upon the relative movement of the tool and workpiece. A bending model that estimates the effects of thermal distortion using knowledge of the position of the temperature gradient is derived. The performance of the bending model is compared with a finite element analysis model and a model that has no knowledge of the position of temperature gradients. Results obtained from a vertical machining centre show that knowledge of the position and magnitude of temperature gradients is an essential part of predicting thermal distortion accurately.