Panico, Martina (2025) Comprehensive Analysis of Drilling Process in Aerospace Manufacturing: One-Up Drilling Strategy with Automated and Semi-Automated Solutions. [Tesi di dottorato]

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Abstract

The increasing demand for lightweight and high-performance materials in aerospace manufacturing has driven the adoption of advanced hybrid stacks, particularly combinations of aluminium alloys and carbon fiber-reinforced polymers (CFRP). These materials offer high strength-to-weight ratios and contribute to fuel efficiency and structural resilience. However, their distinct mechanical properties introduce complex challenges in the drilling process, which is critical for the assembly of stacked materials. Thin stacks with low flexural stiffness are especially prone to burr formation, delamination, and interlayer gaps, issues that conventional drilling methods – using uniform parameters across all layers – fail to address. This research addresses these limitations by studying the one-up drilling (OUD) process for both homogeneous aluminium and hybrid CFRP/aluminium stacks, emphasizing process adaptability, boundary condition effects, and the specific challenges posed by thin-walled aerospace structures. Chapter 1 establishes the research context, outlining the role of drilling in aerospace assembly and the importance of achieving high hole quality to ensure structural integrity. The chapter highlight the industry’s shift towards hybrid material stacks, discussing their importance on drilling efficiency and defect formation. The gap of knowledge is identified, emphasizing the need for adaptive drilling strategies tailored to the mechanical characteristics of each material. In Chapter 2, a technical analysis of aerospace drilling is presented, detailing drill bit selection, cutting mechanics, and key process parameters such as feed rate, cutting speed, and thrust force. Existing analytical and numerical models used for drilling simulations are reviewed, exposing the limitations of conventional approaches in predicting forces, torque, and defect formation. Chapter 3 focuses on drilling challenges in aerospace materials, examining delamination in CFRP, burr formation in aluminium, and the effect of mechanical disparity in hybrid stacks. The formation of interlayer gaps—especially in thin stacks—is analysed, reinforcing the need for real-time process adaptation. Understanding the context of the research, Chapter 4 articulates the aims and objectives of the thesis, framing the research within the broader context of aerospace manufacturing requirements. In Chapter 5, the research delves into the one-up drilling process for thin CFRP/aluminium stacks, with a detailed investigation into the effects of process parameters such as rotational speed and feed rate on hole quality. This experimental phase examines cutting force behaviours across different material layers and identifies key challenges in managing transitions between materials within hybrid stacks. Appendix A complements Chapter 5 by presenting an innovative methodology specifically developed for drilling hybrid CFRP/aluminium stacks. This section details a dual-method strategy combining dynamic and static parameter-switching approaches tailored to each material’s properties. Through spindle power monitoring, real-time dynamic switching enables immediate parameter adjustments as the drill transitions from CFRP to aluminium, optimizing hole quality by reducing burr height and minimizing delamination at the interface. The static approach applies a pre-set switching point based on tool depth, further enhancing the process’s adaptability for hybrid stacks. The methodology provides a robust solution for achieving consistent, high-quality holes across varied material layers, meeting industrial demands for efficiency and precision in hybrid stack assembly. Chapter 6 explores the impact of boundary conditions on drilling performance, particularly in thin stacks drilled without backup support. Three different clamping configurations (40%, 60%, and 80%) are tested to evaluate their influence on thrust force, material deflection, and hole defects. The findings reveal that reduced clamping exacerbates burr formation and increases material deformation, confirming the necessity of process adaptation to boundary conditions. The study is further advanced in Chapter 7, where a theoretical and numerical model is developed to predict interlayer gap formation during drilling of stacked materials. This model, based on the elastic curve equation of a simply supported beam, allows for accurate predictions of material deflection and clamping force requirements in multi-layered stacks. The generalized model is applied to more complex configurations, such as fuselage panels with stringers, providing a versatile tool for predicting interlayer gap formation across a range of clamping conditions and material properties. Through experimental validation, the model demonstrates its applicability in guiding clamping strategies and parameter selection to minimize defects, particularly in multi-material assemblies where traditional models fall short. Chapter 8 concludes the thesis, summarizing the primary contributions of the research to the field of aerospace drilling • Boundary conditions play a crucial role in defect formation, particularly under reduced clamping. • Adaptive parameter switching significantly improves hole quality in hybrid stacks. • A predictive numerical model offers a valuable tool for process optimisation. • The proposed methodology provides a scalable solution for automated aerospace drilling applications.

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