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At present, airframes are mainly composed of monolithic components, instead of small parts joined using welding or riveting. Ribs, stringers, spars, and bulkheads can be included in this category. After milling, they are assembled and joined to the aircraft skins, which have also been milled. The aim of these parts is to obtain a good strength-weight ratio, owing to their homogeneity. The milling of a monolithic structural part implies removing up to 95 per cent of the weight from the raw block material. Therefore, the main objective is to achieve the highest removal rate possible. However, conditions required to achieve this (high feed, large depth of cut) in milling imply high cutting forces, which in turn induce part deflection or vibrations in those zones (thin walls and floors) where stiffness is not sufficiently high. These static and dynamic problems often lead to inaccuracy of geometry, roughness, and possible damage to the machine spindle. This paper proposes a working methodology for efficient process planning, based on previous analysis of the static and dynamic phenomena that can occur during high-speed cutting. This methodology provides several steps that can be taken in order to minimize the bending and vibration effects; suggests optimal monitoring methods to detect process instability; and describes the best way to tune the cutting conditions and chip load, by means of simulation at different machining stages. In this way, the reliability of aeronautical production significantly increases. The global approach presented in this paper has been applied to two test pieces and two real parts, which were milled without suffering either static or dynamic problems.

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In this paper, a three-dimensional dynamic model for the prediction of the stability lobes of high speed milling is presented considering the combined flexibility of both tool and workpiece. The aim is to avoid chatter vibrations in the finish milling of aeronautical parts that include thin walls and thin floors, taking into account the variation of the dynamic properties of the workpiece during machining. Hence, the accurate selection of both axial depth of cut and spindle speed can be accomplished. The model has been validated by means of a test device that simulates the behaviour of a thin floor. The following methodology is applied: first, a modal analysis of the test device is performed, second, stability lobes are calculated, and finally, a milling test validate the approach. Vibration signals from machining have been analysed to detect the chatter vibrations. The lobes diagrams obtained considering only the machine or only the tool are not in touch with reality; only through consideration of the relative frequency response function, the real borderline of stability can be obtained.

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