In micro-machining operations, a small amount of material is removed and minute cutting forces are created. Micro-end mills with less than a 2mm diameter have a short and unpredictable tool life. When these tools fail, their shafts are broken. An operator cannot detect tool breakage from the sound or by a visual inspection during the machining operation. In this paper, the tool-breakage mechanism of micro-end mills, wear-induced stress (WIS), is explained by evaluating the thrust direction cutting force characteristics, using scanning electron microscope (SEM) images to inspect tool tips and acoustic emission activity. The experimental data indicate that the tension between the tool and the workpiece suddenly releases when these tools break. A tool-breakage detector (TBD) is proposed to evaluate the impacts between the cutting edges and the workpieces. The proposed TBD identified tool breakage in all of the test cases without giving any false alarms when the cutting started and when the tool left the workpiece.
