Optimisation of Machining Operations by means of Finite Element Method and Tailored Experiments

Licentiatavhandling, 2013

The experimental approach has long been the main method to investigate the responses associated with metal cutting process such as cutting forces and temperatures and also to optimise the machining operations to accomplish higher productivity. In recent years, with advances in computing power of computers along with development of robust numerical methods such as Finite Element Method (FEM), it has been possible to simulate different machining process under operational conditions. This method can therefore be applied more effectively to optimise the cutting process to achieve higher productivity, while the design requirements are fulfilled simultaneously. Nonetheless, there are some challenges in FE modelling of metal cutting process such as implementation of appropriate constitutive model and well-tuned parameters that need to be addressed first to attain reliable predictions. In the current study, a methodology is developed to determine the flow stress properties of the work material together with the optimum frictional condition at the tool-chip interface by means of inverse modelling of orthogonal cutting test. The calibrated constitutive model and the friction coefficients are then used to build three dimensional (3D) FE models to simulate the cutting process under operational conditions. The results of FE simulations are integrated with tailored tool life tests to reduce the number of experiments required for optimisation of machining process. The viability of the presented methodology is evaluated for two different processes; finishing operation of bi-metallic aluminium-grey cast iron component-like samples as well as transverse machining Alloy 718 in aged condition using uncoated cemented carbides. In order to optimise the finishing operation of the bi-metallic workpiece, a limited number of tool life tests are initially carried out under controlled conditions to determine the involved wear mechanism. Microscopic investigations revealed that the main wear mechanism constitutes thermal fatigue cracking. Hence, the milling operation is simulated by means of FEM and the thermally induced stresses at the cutting edge are calculated for certain combinations of cutting parameters including cutting speed, feed per tooth and depth of cut. Response Surface Methodology (RSM) is then employed to establish the relation between the cutting parameters and the amount of thermal stresses at the cutting edge. Subsequently, the optimum cutting condition is determined once the amount of thermally induced stresses on the cutting edge and the number of tool engagements per unit length of cut are both minimised. This approach leads to 32% reduction in cycle time compared to the reference tool life test, while the number of passes to failure for the carbide tool remained nearly at its maximum level. An FE based wear modelling approach is developed to simulate the flank wear evolution of uncoated cemented carbide when transverse machining Alloy 718 in aged condition. In this study, the relation between the volume loss per unit time and the rate of flank wear evolution is established based on Usui’s wear rate equation. The simulation cycle is then presented to integrate 3D FE simulation of cutting process with the established flank wear rate equation, to predict the wear evolution for predefined simulation cycles. The simulation results showed up to 70% error in prediction of flank wear evolution. The cause for this deviation and the guidelines for further implementation are discussed.