Unsupported drilling of thin hybrid CFRP/Aluminium stacks is widely recognized as a critical industrial operation, yet the deformation behaviour of the stack during the process remains largely unexplored. This study investigates out-of-plane displacement and elastic strain of each layer during drilling by combining high-speed visual tracking and strain measurements. Four commercial tool geometries were analysed and interpreted using the Active Cutting Edge Area (ACEA) as a geometry–based descriptor of tool–material engagement. Drilling tests in standard supported conditions were also conducted as reference and a Global Performance Index (GPI) was proposed and used as a selection metric for the optimal process parameter. Results show that the CFRP and aluminium layers deform coherently during CFRP cutting, but diverge sharply at the material interface, marking the onset of the interlayer gap. The magnitude of this gap, ranging from 0.44 mm to 1.80 mm depending on tool geometry, is governed by the coupled effect of ACEA and thrust force. These deformation mechanisms directly affect material removal efficiency and correlate quality metrics in different way depending on the type of the tool used: unsupported drilling increases exit burr height, surface damage at the interface, and delamination. By clarifying the deformation mechanisms that occur specifically in thin unsupported stacks, this study fills a fundamental gap in the literature on drilling and establishes an experimental basis for selecting the most reliable processes and designing the most reliable tools in lightweight structures.
Panico et al. (Thu,) studied this question.