It is not easy to use a micro-drill less than 1mm in diameter to drill a straight deep hole. When the material to be drilled is difficult to cut, as are the materials used in aircraft engine parts, the hurdles are particularly high and no established technology exists. A micro-drilling experiment was therefore performed to find optimum conditions for drilling difficult-to-cut materials. In the initial experiment, which used an L18 array, evaluation of dynamic characteristics was hindered by frequent tool breakage, so the evaluation was carried out by scoring four criteria on a smaller-is-better basis and calculating S/N ratios. In a follow-up L9 experiment centered on the optimum conditions that were obtained, no tool breakage occurred and the rpm flutter noted in the L18 experiment was eliminated. In the end, stable, breakage-free optimum machining conditions were found for a wide range of applications. In a subsequent life evaluation experiment, under the optimum conditions, drills 0.3mm in diameter lasted seven times as long as previously, each cutting 15 holes or more through plates 6mm thick.
