Modelling and optimising the passage of tangency discontinuities in NC linear paths
CAM programs can generate cutting tool paths to be used by machining centres. Experience shows that CAM programmed feed rates are rarely achieved in practice during machining, especially when finishing free-form surfaces. These slower feed rates are due to the machines’ kinematic capabilities and behaviour of the numerical control (NC). To improve control over the machining process, applications need to be developed to predict the kinematic behaviour of the machines, taking the mechanical characteristics of the axes and NC capacities into account. Various models to simulate tool paths in linear and circular interpolation have been developed and are available in the literature. The present publication will first focus on the use of the polynomial model to simulate the behaviour of the machine when passing through transitions between programmed blocks with tangency discontinuities. Additional features are proposed to ensure enhancement of the match between the model and the machine’s behaviour. Analysis of machine behaviour shows that NCs do not always allow the axes to reach maximum performance levels, with an attendant loss in productivity. The present article proposes an optimisation procedure allowing control laws to be defined to reduce time spent in the transition. The contributions made by these optimised control laws are then evaluated, while impediments to their implementation are also considered.
KeywordsMachine tool NC Linear interpolation Discontinuities between blocks Polynomial transition Constrained non-linear optimisation
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