Beyond 3D printing: multi-axis CNC machining of TPMS geometries for sustainable water generation

Abstract `Atmospheric water generation (AWG) offers a sustainable solution to global freshwater scarcity by extracting moisture from the air. Transitioning from flat 2D plates to three-dimensional triply periodic minimal surface (TPMS) geometries substantially increases available surface area and, in turn, the potential water generation rate. This shift enhances condensation efficiency, as 3D structures expose more surface within the same volume, maximizing contact with humid air. While additive manufacturing (AM) enables the creation of these intricate TPMS structures, AM processes are often constrained by limited material selection, suboptimal surface roughness, and high unit costs that hinder economical scalability. To address these limitations, this study explores whether subtractive manufacturing can serve as a viable and scalable alternative for producing TPMS geometries with greater material flexibility and improved surface finish. To overcome these challenges, this study investigates the feasibility of employing multi-axis subtractive manufacturing—specifically computer numerical control (CNC) machining—to fabricate a range of common TPMS topologies (Gyroid, Schwarz, Diamond, Lidinoid, Neovius, and Split-P). Aluminum 6061-T6 is used for its thermal conductivity, machinability, and corrosion resistance. Two machining strategies, unit-cell and layer-by-layer approaches, are developed using CAD preprocessing and toolpath optimization to address issues of tool accessibility, collision avoidance, and surface fidelity. Experimental results demonstrate successful fabrication of Schwarz and Neovius single-layer structures with acceptable dimensional accuracy and surface finish, while more complex geometries encountered limitations due to tool reach and excessive machining time. A detailed cost comparison reveals that, for medium to high production volumes, subtractive methods can reduce per-part cost by up to 75% compared to direct metal laser sintering (DMLS)-based AM, making CNC machining a compelling alternative for scalable deployment of TPMS-enhanced AWG components. Finally, the paper outlines the key technical hurdles—such as fixture design and cutter selection—that must be addressed to broaden the applicability of subtractive manufacturing for future AWG system development. Graphical Abstract