Capability in machining systems

• The vehicle industry has derived a vision of simulatingmachining systems and their cutting processes with accuracy andcapability indices as a result. The accuracy of machined partsis the result of properties and characteristics of the totalmachining system and its environment. Natural deviation resultsin deviations between each machined part which in turn effectthe functionality of an assembled product.A machining system is defined as consisting of the fivemodules: machine tool, tool, cutting process, workpiece andfixture.The thoughts and discussions about the vision of simulatinga machining systems and its environment have resulted in aresearch question:“What is required to give a reliablesimulated value of capability and accuracy?"Being able to predict the accuracy of machined parts in aspecific machining system and its environment gives thepossibility of optimising the cutting process and detect errorsin the machining systems. It also gives the possibility todesign more robust machining systems and avoid mistakes duringthe designing of the machining systems.This study has shown that to be able to predict capabilityindices for both existing and future machining system it isnecessary to have thorough knowledge in four areas; capabilityas a statistical tool, accuracy in machining system,measurement tools and methods, and modelling andsimulation.To achieve a reliable capability index, the capability studyhas to be properly planned, performed and evaluated. Thestatistical method of capability indices needs to be well-knownwhen choosing index, comparing different indices and whenmaking comparisons between indices from several machiningsystems.The machining accuracy of a machining system is dependent onfour groups of characteristics: temperature influence, geometryand kinematics, static stiffness and, finally, dynamicstiffness.There are a great number of models available for cuttingprocesses, process state variables and different modules of amachining system. Usually, these models can only be useddiscretely since there is almost no relationship between thestructures, required data and assumptions of the models. It istherefore not yet possible to model and simulate a machiningsystem to achieve a reliable accuracy value.

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