The influence of tool coating on the length of the normal operating region (steady-state wear) for micro end mills

Increasing miniaturisation has significantly increased demand for highly accurate small parts to be machined. Micro milling presents a viable method for series machining parts such as miniature heat exchangers or fuel injectors. Micro end mill tool wear, however, is considerably more difficult to monitor than its macro counterpart, and the tools often fail due to chipping or shaft failure. High tool breakage rates make costs unpredictable and machining times inefficient. Conversely, macro end mill wear is well characterised, and the typical wear curve well understood.

The aim of this work is to elongate the steady state region (SSR) of the wear curve for micro end mills, as this is the practical life of the tool. Coatings can be applied to achieve this, improving wear rates and cutting performance.

This study examines the wear that took place during straight-slot machining with 500 μm micro end mills. This was then compared with observed wear mechanisms on the macro scale. The length of the SSR was used to evaluate the ability of various new coatings to extend the working tool life. The relative predictability in the SSR allows tool paths to be modified to account for changing tool geometry.

The results demonstrate that for micro-mills, the SSR could be elongated, in spite of less predictable wear mechanisms. Overall, this work presents a successful attempt to manipulate the wear curve for micro end mills and highlights the importance of developing an understanding of the wear mechanisms taking place for micro-mills as compared with macro-mills.