Thrust force is considered to be the main reason causing delamination damage as well as hole sub-surface microstructural damage in composite drilling , therefore should be minimized during optimization for the improved hole quality. Fig. (a) shows the typical thrust force of CF/PEKK and CF/epoxy drilling and the average was taken for the 'stable cutting' phase (between P-I and P-II) for further analysis. The regression function of the thrust force 𝑓𝑓 𝐹𝐹 can be expressed as: