Development of a hybrid model for the prediction of shape deviations in milling

Abstract

The productivity in machining of high precision components is limited among others by the achievable geometric accuracy. Shape deviations arise due to several mechanisms of which residual stresses and a varying material removal due to thermal expansion have been incorporated in a proposed hybrid model. The model is based on two initially separated sub models which require a set of experimentally obtained input parameters. Both sub models utilize the finite element method. Sub model 1 calculates the mechanic response of the workpiece loaded with a near surface residual stress field (source stresses) resulting from the interaction of tool and workpiece. Sub model 2 calculates the thermal expansion of the workpiece utilising a three-dimensional simulation of a moving surface heat source. The resulting inhomogeneous material removal is superimposed with the calculated shape deviations from sub model 1. This work focuses on distortion caused by machining induced residual stresses which are modelled as source stresses (sub model 1). These so called source stresses can be calculated directly from the bending and torsion of machined specimens and represent the distortion potential of the face milling process. The results demonstrate a pronounced correlation between the obtained source stresses and the width of cut a e . They further indicate that a separation of the effects resulting from the removal of material containing workpiece inherent residual stresses and from the newly generated residual stresses in the machined surface layer is required for the calculation of machining induced residual stresses.In der spanenden Bearbeitung von Hochpräzisionskomponenten ist die Produktivität u. a. durch die geforderten Maß-und Formtoleranzen begrenzt. Formabweichungen, welche die Toleranzen überschreiten können, entstehen dabei aufgrund unterschiedlicher Mechanismen, von denen zwei, die Entstehung von Eigenspannungen und der ungleichmäßige Materialabtrag durch thermische Dehnungen, in dem vorgeschlagenen Hybridmodell berücksichtigt werden. Das Hybridmodell basiert auf zweien zunächst getrennten Submodellen, welche mithilfe von Zerspanexperimenten parametriert werden. Die Berechnung erfolgt mit der Methode der Finiten Elemente. Im Submodell 1 wird die mechanische Antwort des Werkstücks auf bearbeitungsinduzierte Eigenspannungen (Quellspannungen) berechnet, welches aus randschichtnahen plastischen Verformungen des Bearbeitungsprozesses resultiert. Im Submodell 2 werden thermische Dehnungen des Werkstücks in einer dreidimensionalen FE-Simulation einer bewegten Wärmequelle berechnet. Der daraus resul-