Fundamental Investigation on the Micro-milling Process for Machining of Difficult-to-machine Materials

Micro-milling is a precision manufacturing process with wide applications across biomedical, electronics, aerospace and aeronautical industries due to the process versatility, capability, economy and efficiency for an extended range of materials. In particular, the micro-milling process lends itself well to very precise and accurate machining for prototyping of micro-moulds with high aspect ratio features, as well as for rapid micro-texturing and micro-patterning which would have great prominence in the near future of bio-implant manufacturing. This is especially true for machining of typical difficult-to-machine (DTM) materials commonly found in both mould and orthopaedic implant manufacturing, such as super alloys and ceramics. However, the limitations of significant tool wear and tool breakage of commercially available fluted micro-end mill tools as a result of being scaled down from macro-milling tool designs leads to very weak cross sections, poor rigidity, poor tool strength and weak cutting edges, as a result of the decrease in tool cross section volume. The development of application-specific micro-tools for machining of very hard and wear resistant materials will allow the micro-milling process to overcome these issues and be adopted further in the fields of biomedical and mould manufacturing. Therefore, a design process is first established to determine an optimal micro-end mill tool designs for machining typical DTM biomaterials, namely CoCrMo and Ti6Al4V. The design process focuses on achieving robust stiffness and mechanical strength to reduce tool wear, avoid tool chipping and tool breakage in order to efficiently machine DTM materials. Static stress and deflection finite element analysis (FEA) are carried out to identify stiffness and rigidity of the tool design, from which the novel micro-souble straight edge end-mill (DSEE) tool design is developed. An experimental study then validates the optimum tool design in regards to cutting forces, tool wear and surface quality. The DSEE tools proved to have far less tool wear, lower cutting forces and better surface quality, in comparison to commercially available tools.
FEA is then used to build prediction models of cutting force and chip formation as a result of the 0° rake angle of the developed DSEE tools using the Johnson-Cook (JC) constitutive material flow stress model, during micro-milling of CoCrMo and Ti6Al4V workpiece materials. However, as no JC parameters have yet been presented in research for CoCrMo alloys, material characterisation to determine these parameters was first necessary.
To further develop the DSEE tool for micro-milling of wear resistant ceramics, an investigation into tool materials was necessary. Single straight edge (SSEE) cubic boron nitride (cBN) and DSEE polycrystalline diamond (PCD) micro-milling tools were manufactured, and a systematic experimental study was conducted on micro-milling of pure alumina (Al2O3) workpiece in order to achieve effective and efficient machining. The results show that both tool materials are suitable for micro-milling of very hard and brittle ceramic materials, and that successful micro-milling with a large depth of cut of 120 µm of pure alumina was achieved.
Finally, a robust micro-ball nose end mill tool design (BNDSEE) is presented for fabrication of micro-dimples on the CoCrMo and Ti6Al4V biomaterials. The results show that the BNDSEE tool design produces micro-dimples with high geometrical accuracy, low surface roughness, low cutting forces and therefore low tool wear, in comparison to conventional helical micro-ball nose end mill tools. Furthermore, only minor wear and rounding of the cutting edge occurred for the developed tools, while the helical tools suffered from major tool wear and edge chipping which led to high cutting forces, poor surface roughness and poor geometrical accuracy, especially at high depth of cuts.