Saurabh Aggarwal
High speed milling (HSM) is the most known machining process due to its application in various industries. In milling, a rotating cutting tool removes a large amount of material along a predefined toolpath to manufacture the final part with a desired shape. Milling of prismatic parts1 is very important in automotive, aerospace, mold and die industries. Even complicated parts are machined from a blank first by 2.5D roughing followed by 3D-5D finishing. Modern production floors have adopted high speed CNC2 machine tools to execute part programs, developed by CAD/CAM3 systems, to manufacture the final workpiece. The overall productivity of the milling process depends on the choice of cutting conditions and the toolpath. Current CAD/CAM systems do not provide any guidance to select cutting conditions due to the unavailability of models of the complex physical and dynamic interaction of machine tool and workpiece systems. Moreover, toolpath generation by CAD/CAM packages is purely geometric in nature and results in engagement angle variation along the toolpath. The selection of cutting conditions and toolpath rely solely on the part programmer’s experi- ence, CAD/CAM systems, handbook guidelines or specifications provided in the catalogues of cutting tools and machine tools. Their poor selection often causes chatter, high fluctuation of cutting forces, and/or violation of the available limits of power and torque of the machine tool. These phenomena result in poor surface finish, workpiece damage, high cutting tool wear, violation of tolerance limits, additional cost, unwanted waste and significant reduction in machine tool working life. In order to avoid these problems, part programs need to be verified iteratively using trial and error experiments and often conservative cutting conditions are selected. These practices lead to long preparation time of part programs and lower machining performance, which in a nutshell significantly lower overall productivity. Moreover, machine tool capabilities are not fully utilized due to the conservative selection of cutting conditions. In order to address these challenges, a genetic algorithm (GA) based optimal milling (OptMill) system is developed for optimal selection of cutting conditions and/or toolpath for a given set of inputs of machine tool/spindle/tool holder/cutting tool and workpiece system. Operational constraints of the machine tool, such as spindle speed and feed limits, available spindle power and torque, chatter vibration4 limits due to the dynamic interaction between cutting tool and workpiece, permissible limits of bending stress and deflection of the cutting tool and clamping load limits of the workpiece system are embedded. The developed system is applied to different industrial use cases: (i) Minimization of pocket milling time considering one-way toolpath (ii) minimization of machining time for multi-feature prismatic parts with the imple- mentation of pre-processing modules: extraction of toolpath and workpiece boundary from APT5 and STEP6 files respectively and calculation of engagement angle along the toolpath (iii) optimal selection of cutting conditions and corresponding smooth and constant engagement toolpath for pocket milling. The selected cutting conditions and/or toolpath are also validated using dedicated experiments conducted during the course of the research work. The present research work is inspired from an ongoing CTI project7. Following enhanced methodologies the identification of important inputs to mathematical models for prediction of cutting forces and chatter free limits have also been developed to expand the scope of the developed OptMill system. • Tangential force coefficients, an important input for prediction of cutting forces and chatter free limits, are identified experimentally with the use of a cutting force dy- namometer. This experimental setup is quite costly and not practical for industrial implementation. An enhanced methodology is presented for the indirect identifica- tion of tangential force coefficients from the spindle motor current. The methodology includes the development of an empirical model for cutting torque prediction from spindle motor current with the implementation of a spindle power model that accounts for all mechanical and electrical power losses. The cutting torque predicted by the developed model is then used for tangential force coefficient identification, and is also validated experimentally with direct measurement using a cutting torque dynamometer. • Dynamicresponseofeachvariantofmachinetool/spindle/toolholder/cuttingtool,in terms of FRF8, is required to predict chatter free limits accurately. FRF is often measured with hammer testing experiments. In order to avoid these tedious tests, an enhanced procedure using the receptance coupling technique is implemented to predict the FRF of a machine tool/spindle/tool holder/cutting tool system for different cutting tools. The predicted FRFs via numerical simulation are also validated with experimental measurement. Though the existing mathematical models predict accurately the chatter free limits, their use in small production floors has not yet been achieved due to the absence of technical expertise and experimental resources. Moreover, even modern machine tools do not provide any guidance to the machine operator regarding the occurrence of chatter during machining. To meet industrial requirements, a computationally fast, easy to use and practical system is developed that detects chatter automatically during milling and thereafter proposes a control strategy to the machine operator. The developed online chatter detection and control system is also validated experimentally with an industrial end-user partner. Apart from the many challenges and the developments discussed above, milling of thin- walled workpieces is also a concern due to changing dynamics during machining. Thus, an enhanced numerical procedure is developed for the selection of chatter free cutting conditions while considering the change in workpiece dynamics along the toolpath using finite element analysis. In order to realize the developed system, MATLAB is used as a programming language. Ge- ometrical modeling and part programming of prismatic parts is done with CATIA. The data acquisition platform for the experimental validation is designed in LABVIEW. Finite element modeling and analysis is implemented with the ANSYS parametric design language (APDL). The developed system is very appealing for industrial application by direct integration with existing CAD/CAM systems and/or modern machine tools. Increase in overall productiv- ity is ensured by optimal selection of cutting conditions and/or toolpath and simultaneous avoidance of repercussions due to their wrong selection.
EPFL2012
