FEM based Cutting Velocity Selection for Thin Walled Part Machining

Monolithic components are commonly used in the aeronautical industry due to their homogeneity and excellent strength-to- weight-ratio: ribs, stringers, spars, and bulkheads can be mentioned as examples. In order to assure enough stiffness to the whole component, monolithic parts are often made of thin walls, and webs, obtained usually starting from a raw block of material and removing up to 95% of the weight of the initial block. Therefore, increasing the removal rate as much as possible is the main condition to reach high productivity. The drawback is that, at high removal rate conditions (high feed, large depth of cut), the low stiffness of the thin walls causes dynamic problems, due to the forced vibrations induced by the milling process: even in case of a stable cutting process, vibrations appear as the result of the combination between the tool and workpiece natural frequencies, excited by the tooth-passing frequency harmonics. This paper presents an overview of a comprehensive milling process plan approach, based on finite element method (FEM), which by considering the effects of fixturing, tooltip dynamics, and material removal, allows to define the optimized cutting velocity in order to reduce deflections and vibrations during machining. The advantages of the proposed model over previous works are: (i) it provides a seamless interface to import the G-code generated by a CAM packages; (ii) it performs a computationally efficient modal superposition frequency response analysis of complex workpieces, considering the tooltip position provided by CAM; (iii) it predicts the workpiece non-linear behaviour during machining due to its changing geometry; (iv) it automatically tunes spindle speed continuously along the toolpath, taking into account both the tooltip dynamics and the local workpiece modal behaviour. The effectiveness of the proposed approach has been experimentally validated.