Search

Thin floor machining is a challenging and demanding issue, due to vibrations that create poor surface quality. Several technologies have been developed to overcome this problem. Ad hoc fixtures for a given part geometry lead to meeting quality tolerances, but since they lack flexibility, they are expensive and not suitable for low manufacturing batches. On the contrary, flexible fixtures consisting of vacuum cups adaptable to a diversity of part geometries may not totally avoid vibrations, which greatly limits its use. The present study analyses the feasibility of thin floor milling in terms of vibration and roughness, in the cases where milling is conducted without back support, a usual situation when flexible fixtures are employed, so as to define the conditions for a stable milling in them and thus avoid the use of ad hoc fixtures. For that purpose, the change of modal parameters due to material removal and its influence on chatter appearance have been studied, by means of stability lobe diagrams and Fourier Transform analysis. Additionally, the relationship between surface roughness and chatter frequency, tooth passing frequency, and spindle frequency have been studied. Ploughing effect has also been observed during milling, and the factors that lead to the appearance of this undesirable effect have been analyzed, in order to avoid it. It has been proven that finish milling of thin floors without support in the axial direction of the mill can meet aeronautic tolerances and requirements, providing that proper cutting conditions and machining zones are selected.

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.

In the present scenario, citizens' concern about environment preservation creates a necessity to mature more ecological and energy-efficient manufacturing processes and materials. The usage of glass fiber reinforced polymer (GFRP) is one of the emerging materials to replace the traditional metallic alloys in the automotive and aircraft industries. However, it has been comprehended to arise a sustainable substitute to conventional emulsion-based coolants in machining processes for dropping the destructive effects on the ecosystem without degrading the machining performance. So, in this study, the comparison of the two sustainable cutting fluid approaches, i.e., dry and LCO2, has been presented based on machining performance indicators like temperature, modulus of cutting force, tool wear, surface roughness, power consumption, and life cycle assessment (LCA) analysis for end milling of GFRP. The cutting condition of LCO2 has been found to be superior in terms of machining performance by providing 80% of lower cutting zone temperature, tool wear, 5% lower modulus of cutting force, and reduced surface roughness with 9% lower power consumption that has been observed in the case of LCO2 in comparison with dry machining. However, to compress the CO2 for converting in liquid form, a higher amount of energy and natural resources is consumed resulting in a higher impact on the environment in comparison with dry machining. Considering the 18 impact categories of ReCiPe midpoint (H) 2016, 95% higher values of impacts have been observed in the case of LCO2 in comparison with dry machining.

Fixtures are a critical element in machining operations as they are the interface between the part and the machine. These components are responsible for the precise part location on the machine table and for the proper dynamic stability maintenance during the manufacturing operations. Although these two features are deeply related, they are usually studied separately. On the one hand, diverse adaptable solutions have been developed for the clamping of different variable geometries. Parallelly, the stability of the part has been long studied to reduce the forced vibration and the chatter effects, especially on thin parts machining operations typically performed in the aeronautic field, such as the skin panels milling. The present work proposes a commitment between both features by the presentation of an innovative vacuum fixture based on the use of a vulcanized rubber layer. This solution presents high flexibility as it can be adapted to different geometries while providing a proper damping capacity due to the viscoelastic and elastoplastic behaviour of these compounds. Moreover, the sealing properties of these elastomers provide the perfect combination to transform a rubber layer into a flexible vacuum table. Therefore, in order to validate the suitability of this fixture, a test bench is manufactured and tested under uniaxial compression loads and under real finish milling conditions over AA2024 part samples. Finally, a roughness model is proposed and analysed in order to characterize the part vibration sources.

In finishing processes, the quality of aluminum parts is mostly influenced by static and dynamic phenomena. Different solutions have been studied toward a stable milling process attainment. However, the improvements obtained with the tuning of process parameters are limited by the system stiffness and external dampers devices interfere with the machining process. To deal with this challenge, this work analyzes the suitability of elastomer layers as passive damping elements directly located under the part to be machined. Thus, exploiting the sealing properties of nitrile butadiene rubber (NBR), a suitable flexible vacuum fixture is developed, enabling a proper implementation in the manufacturing process. Two different compounds are characterized under axial compression and under finishing operations. The compression tests present the effect of the feed rate and the strain accumulative effect in the fixture compressive behavior. Despite the higher strain variability of the softer rubber, different milling process parameters, such as the tool feed rate, can lead to a similar compressive behavior of the fixture regardless the elastomer hardness. On the other hand, the characterization of these flexible fixtures is completed over AA2024 floor milling of rigid parts and compared with the use of a rigid part clamping. These results show that, as the cutting speed and the feed rate increases, due to the strain evolution of the rubber, the part quality obtained tend to equalize between the flexible and the rigid clamping of the workpiece. Due to the versatility of the NBR for clamping different part geometries without new fixture redesigns, this leads to a competitive advantage of these flexible solutions against the classic rigid vacuum fixtures. Finally, a model to predict the grooving forces with a bull-nose end mill regardless of the stiffness of the part support is proposed and validated for the working range.

215 p., Esta tesis doctoral presenta el desarrollo y caracterización de una solución flexible y de bajo coste que permita la fabricación de pieles aeronáuticas de aluminio mediante mecanizado convencional. Este concepto se basa, por un lado, en la adaptación e integración de láminas de goma de nitrilo butadieno (NBR) como utillajes flexibles de vacío. Por otro lado, de cara a compensar la diferencia de tolerancia de espesor de este tipo de elementos flexibles con los requisitos de espesor demandados en la industria, se plantea la integración de diferentes elementos de monitorización que permitan la adaptación de la trayectoria de la herramienta. Así, se pretende que este sistema sea capaz de fabricar este tipo de piezas aeronáuticas esbeltas sin tener que recurrir a tecnologías de bajo rendimiento, como el fresado químico, o incurrir en grandes inversiones, como las soluciones de fresado mecánico mediante máquina de doble cabezal.