where 𝑓𝑓 𝐹𝐹 , 𝑓𝑓 𝐷𝐷 , 𝑓𝑓 𝑀𝑀𝑀𝑀𝑀𝑀 , 𝑓𝑓 𝑇𝑇 are functions for thrust force, Fda, MRR and Tmax, which can be found from Eq. ( ) and ( ) -( ). The cutting speed S and feed rate 𝐹𝐹 𝑓𝑓 are set to discrete values considering the practical drilling operation . The discrete step size of S is 1 rpm and that for 𝐹𝐹 𝑓𝑓 is 0.001 mm/rev.