Drilling characteristics optimization of multi-fiber (flax and steel) reinforced laminated epoxy composites

Drilling is one of the most widely practiced machining operations for creating holes; however, its application to composite materials is particularly challenging due to their heterogeneous and anisotropic nature. In this context, the present study focuses on flax-epoxy composites prepared in two variations, one without steel mesh reinforcement and the other with embedded steel meshes, which are then subjected to drilling experiments designed using the Taguchi L 25 orthogonal array. To evaluate drilling performance, the influence of spindle speed, feed rate, and drill diameter on thrust force, torque, and delamination is systematically examined using the Taguchi approach. For multi-response optimization, grey relational analysis is employed to identify the most suitable parameter combinations that enhance overall drilling quality. The results reveal that feed rate emerges as the most dominant factor to overall performance for both flax-epoxy and flax-steel-epoxy hybrid composites, contributing 62.16% and 57.76%, respectively, followed in significance by drill diameter and spindle speed. To further understand the observed outcomes, chip formation mechanisms and the surface morphology of drilled holes are investigated, providing insights into chip types and the underlying causes of delamination.