The degrees of freedom for the selection of the OA
should exceed or at least equal to those for the design
parameters. For the needs of the present study, an L9 OA
was adequate since it holds eight degrees of freedom and
can handle three-level design parameters. L9 requests
nine cutting parameters combinations to be tested for the
studying of the entire parameter space. The parameters
to be examined and the levels of each parameter are
sorted out in Table 2.