The tool life was estimated from the tool wear to evaluate the
manufacturing cost. In conventional metal drilling, it is an established
practice to estimate the tool usage and wear from the cutting
parameters, excluding sudden breakage by external forces (Tansel
et al., 2000). The wear is defined as the distance between the
edge of the tool tip and the centerline, as shown in Fig. 7, and was observed via field emission scanning electron microscopy (FEe
SEM). Fig. 7 also shows the tool wear trend in terms of the number
of holes drilled under the reference conditions. The wear increased
significantly during the early stages of the tool life, and then
converged to a specific value, since wear first appears on the tip
surface rather than on the edge over a given tool life