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In experiments of machining hardened tool steels (such as AISI H11, H13, and D2, up to 56 HRC) by commercial Ø 0.5 mm square endmills, it is observed that the tested micro endmills showed severe wear at an early stage of the process due to chipping off around cutting edge corners, resulting in unsatisfactory tool life and product appearance (burr formation). Detailed examination of current tool geometry shows that it is mainly inherited from that of macro endmills, making the cutting edge corners the weakest part on the tool. As the micromilling process is characterized by small values of machining parameters, the cutting edge corners of the micro endmill are the most loaded part of the cutting edges. New design rules are studied for improving the stiffness and strength of micro endmills used in micro hard milling applications. Analytical modelling and finite element method analysis are used to aid the design of tool geometry. By using a larger neck angle, optimizing tool core geometry, and choosing a negative rake angle, tool stiffness and cutting edge strength are improved. The new endmill designs, both two-flute and four-flute, are tested in experiments on hardened tool steels and showed considerable lower tool wear and increased tool life. Furthermore, the geometrical accuracy and appearance of the workpiece (burr formation) has been improved drastically.

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Micro features with high aspect ratios are one of the commonly encountered geometries found in micro products. In the literature, these structures are often used in demonstrator products machined by a micromilling process. In this paper, the challenges in micromilling thin ribs with high aspect ratios have been studied. Due to the scaling effect, micro-ribs have relatively low stiffness but high natural frequency. Therefore, on the one hand, average forces have to be controlled well to avoid structural bending or even damage, while on the other hand, micro features are unlikely to be excited by the dynamic forces. The characteristics of micromilling forces and their relationships to the machining parameters, namely, feed per tooth, depth of cut and width of cut, were studied theoretically by force models. In addition, the effects of different milling strategies (up-/down-milling) and tool paths on the quality of thin features have been investigated using FEM. The results allow measures to be taken to minimize the force effects and support the micro features during machining. The experimental results verify the theoretical studies. Thin ribs about 15 μm wide and with an aspect ratio of more than 50 were machined with good form and surface quality.