Deformation Control And Error Compensation In Precision Machining Of Thin-Walled Parts

Thin-walled complex parts, namely casing and blade made of difficult-to-machine materials, are widely used in the structure design of high performance aeroengine, which results in weight savings of up to 30% with consequent improvements in thrust-to-weight ratios. However, due to factors such as cutting force induced part/tool static deflection and dynamic vibration, precision machining of these low-rigidity complex parts has been providing a serious challenge for engineers. As a result, there is usually a significant deviation between the planned and machined part profiles while providing a poor surface quality. Hence, the main objectives of this research are to predict and compensate the tool-workpiece machining deformation errors, and to develop strategies to suppress chatter phenomenon and residual stresses induced thin-walled parts distortion.Firstly, an FE model is presented using ABAQUS/Explicit^TM to simulate continuous and saw-tooth chip formation when machining titanium alloy TCll. Modelling details, including the chip separation criterion, the sticking and sliding tool-chip frictional behavior, the adaptive meshing technique and the constitutive equation for the workpiece material, are discussed carefully. The model is also used to predict the cutting forces, stress, strain and temperature contours near the cutting zone. For the saw-tool chip, the magnitude of the cutting force is lower than with the continuous chip.Secondly, a general quasi-static error compensation methodology is proposed, which focuses on force-induced errors in machining thin-walled structures. The methodology is based on modelling and prediction of milling forces, finite element simulation of deflection of the part during machining and analysis of the resultant surface errors. An iterative procedure is used to determine the local equilibrium conditions between the cutting force and deflection at each cutter location. The results show that high machining accuracy could be achieved efficiently using multi-level error compensation scheme.Thirdly, a spiral milling process technique is presented to finishing thin-walled workpiece taking into account the residual stresses induced distortion. The residual stress variation in titanium alloy TC11 has been determined by a strain-gauge technique involving blind-hole drilling. The measured residual stresses are fed into an FEA model for simulation of the distortion behaviour of the part under two different tool path strategies. With the spiral tool paths, the allowances on both sides of parts are removed concurrently, therefore the residual stresses in the machined layers will be approximately maintained symmetrical balance status. As a result, the residual stresses induced distortion during finishing thin-walled workpiece is controlled successfully. The machining accuracy near blade tip region has been significantly improved by one order of magnitude.Finally, Based on dynamic cutting force model, a method for obtaining the instability or stability lobes is developed. In order to identify the modal parameters, the frequency response function of the machine-tool structure is determined by a standard impact test procedure. The predicted stability boundary has been used to determine the optimal cutting conditions to suppress chatter phenomenon while maximizing material removal rates. Furthermore, with respect to finishing sculptured surface blade, it is verified that the chatter vibration could be suppressed successfully through optimizing the allowances distribution during the semi-finishing process.