Modeling of cutting force and final thickness for low stiffness 2024-T3 aluminum alloy part milling considering its geometry and fixtures

The aeronautic industry is facing many challenges regarding the lifetime, weight and accuracy that aircraft skins must comply to meet stringent structural and aerodynamic requirements. Currently, mechanical milling of aircraft skin parts of 2024-T3 aluminum alloy is displacing the highly pollutant chemical milling. Consequently, flexible and reconfigurable vacuum holding fixtures are being increasingly employed, because they are adaptable to several part geometries, but, since their rigidity is extremely reduced, the low stiffness of parts limits severely their deployment. Aiming to harness the full potential of these holding systems for aluminum alloy skin parts, a complete analysis of final thickness achieved and cutting force is developed. Thin floor parts of different geometries are pocket milled, simply screwed at their corners, emulating a skin part supported by four vacuum cups. Process forces are continuously monitored, and final thickness is measured. It has been proven that the reduction of mass and stiffness during milling causes a corresponding reduction of the natural frequencies of the parts. Also, as long as natural frequencies are not excited, final thickness error is almost constant and not affected by the tool position, but only by the initial geometry and fixtures distribution of the part. Additionally, a new cutting force model for skin parts is empirically calculated. Unlike models designed for fully supported parts, this model is designed for skins held in flexible fixtures. It has a relative error of 5.6% and it allows to optimize the trajectory, geometry and support distribution, thus boosting the use of flexible fixtures.