An experimental investigation of hybrid fused filament fabrication with in-process machining

Fused filament fabrication (FFF) is the most widely used additive manufacturing process thanks to its low cost and easy setup, but it is limited by low accuracy, poor surface finish, slow build time, and inferior anisotropic mechanical properties. In this study, we experimentally investigate the integration of in-process machining with FFF using PLA filaments on a commercial multi-head printer setup. A hybrid FFF with in-process machining test platform with process monitoring capabilities was developed. The experimental platform development process identified that spindle rigidity and newly printed filament temperature control (e.g., quick cooling with compressed air nozzle) were two key considerations for a high-quality machined surface. To ensure the surface finish of the hybrid manufactured parts, especially on less accurate/repeatable FFF printer setup, it would be preferable to conduct finish cutting of FFF surfaces in one path to avoid misalignment error associated with multiple paths. With such hybrid FFF-machining integration strategies, a benchmark test showed that hybrid FFF using large layer thickness followed by a quick finish milling path yielded a surface finish of 5 times lower R_a value at 34% of the total cycle time compared to pure FFF with fine layers, which greatly enhances the efficiency and quality of FFF-based parts. In terms of hybrid part strength, Mode I fracture tests showed that the correlation between the machining depth of cut and the FFF print perimeter thickness was critical for the hybrid part fracture resistance. Partially cut filament could lead to weak bonding regions that were easier for crack initiation and propagation through a combination of inter-, intra-, and trans-laminar failures.