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In experimental investigations of micro milling of hardened tool steel SAE H11, with a hardness of 56 HRC, with commercially available micro square endmills of Ø 0.5mm, it was observed that the endmills suffered from severe tool wear/failure. Because of these problems, the quality of the machined workpiece was not satisfying, in terms of burr formation and form accuracy. By analyzing the results, it is concluded that the current endmill geometries are not suitable for micromilling, which are mainly derived from macro endmills. Based on these observations, the design guidelines for micro endmills for micro hard milling application were generated. The tool geometries are improved by means of analytical and FEM analysis. Micro endmills with new designed geometries were manufactured and validated through experiments. The results showed that the new endmills had a tool life about 6 times longer than the endmills with original design; accordingly, the workpiece quality was also improved significantly.