Careful monitoring of conditions during micro-drilling is important for insuring production of consistent high-quality holes. In this study, acoustic emission, ultrasonic sound, and electric spindle load were used to monitor micro-drilling performed using a micro-scale machine tool (mMT). Experiments were conducted where 0.508 mm diameter holes were drilled in polyetheretherketone (PEEK) polymer and 316 stainless steel. It was found that spindle load significantly increased when tool gumming occurred during drilling of PEEK. Increased spindle load when cutting 316 stainless steel was found to correspond to increased incidences of tool breakage, and a large reduction in spindle load was present during additional drilling operations following a breakage event. It was found that elevated acoustic emission levels were always present during drilling and that a lack of sufficient acoustic emission generation during retraction of a tool indicated tool breakage. The ultrasonic sound spectra were found to change in a manner that is a function of depth of cut and hence a function of depth of cut dependent tool dynamics.

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