This paper studies the optimization of hole-making operations with multiple tools in which a hole may require various tools to its final size. The objective of this problem is to minimize the summation of tool airtime and tool switch time for the hole-making operations. As known, the problem is NP-hard. In this paper, we show a property that any infeasible permutation of hole-making operations can be transformed into a feasible one. Based upon this property, we apply a PSO based approach for solving a benchmark hole-making problem with the number of feasible operation sequences up to 5.41031. Numerical results indicate that the applied PSO approach can find superior solutions for the benchmark problem in the literature.