Evaluation and selection of hard coatings for micro milling of hardened tool steel

In conventional or macroscale milling, appropriate physical vapour deposition (PVD) coatings can be used to improve machining performance, promote the use of higher cutting speeds and facilitate dry machining or the use of minimum quantity lubrication (MQL). When micro tools (1–999 μm in diameter) are used in milling, the undeformed chip thickness is usually very small and comparable to the cutting edge radius. This condition determines the effective rake angle and hence plays a significant role in the mechanics of micro machining. The differences between macro and microscale machining influence process factors such as cutting interface temperatures, forces, strains and strain rates. The mechanisms through which coatings protect cutting tools are influenced by such process conditions. Additionally, due to the size of micro tools applying coatings evenly around the cutting edges is a technological challenge. Despite these new challenges, there is hardly any work reported in literature dedicated to the selection of hard coatings for micro machining of tool steels. The work reported in this paper identified coating wear mechanisms in micro milling of hardened tool steels. A wide range of PVD coatings were evaluated based on multiple criteria of tool wear, surface finish and burr size. The results clearly show that compared to other coatings or uncoated fine grain carbide tools, TiN coatings offer the best performance in terms of tool wear reduction and improvement in quality of machined surface.