The Pareto solutions and corresponding design variables for CF/PEKK and CF/epoxy are shown in Fig. It is worth noting that the spindle speed for CF/epoxy converges to a single value (5308 rpm), the upper limit within design range. This is because, with increasing spindle speed, two of the objectives (-MRR and thrust force) evolved to more optimized value (decreased) while Fda remained unchanged, see Eq. ( ), Fig. (d) and Fig. ). The highest spindle speed 5308 rpm within the design space can provide both satisfactory hole quality and high drilling efficiency. It is also worth mentioning that the range of feed rate for CF/epoxy is considrably lower than that for CF/PEKK. As indicated by Fig. , consider the low interlaminar fracture toughness of CF/epoxy (0.277 N/mm vs. 1.564 N/mm of CF/PEKK), the level of feed rate for CF/epoxy has to be restricted in order to maintain Fda < 1.4. On the other hand, ultilization of low feed rate should be avoided for CF/PEKK drilling due to the poetntial high temperature (> Tg) and severe delamination generated under such condition (see Fig. and Fig. ).