Key success factors in micro-milling of hardened steels
Author: Paul Glendenning, Micro Systems (UK) Ltd, Warrington, UK
The differences between milling using large cutters (macro-scale) and milling using cutters of less than 1mm in diameter (micro-scale) influence the quality of parts that can be produced. Research and development has shown that factors such as chip formation, cutting interface temperatures, forces, strains, and strain rates are all influenced by the 'size effect'. An understanding of these factors is important for all those involved in the micro-milling industry because they are significant for tool design and selection, types of tool and coating to be used, machining strategies and workpiece materials. This knowledge is particularly important in micro machining of difficult to cut materials such as hardened tool steels, because in such applications, extending tool life is major challenge.
Feeds and speeds and the 'size effect'
Material removal rates in micro-milling are considerably lower than in conventional macro-scale machining. Unlike in conventional macro-scale machining, in micro-machining the undeformed chip thickness (material to be removed in the cut) can be comparable to the cutting edge radius of the milling cutter. An understanding of the principle illustrated in Fig 1 is important in micro-milling because once the undeformed chip thickness becomes less than the cutting radius of the tool, the rake angle effectively becomes negative, and the tool is 'ploughing' the metal rather than cutting it. In the ploughing situation, the metal is compressed by the cutting tool and recovers back, as opposed to being sheared by a sharp cutting edge to form a chip.
When the cutting edge radius is equal to the underformed chip thickness in the milling of hardened tool steel, there is a trade off between the ploughing and conventional shearing effects, resulting in better surface finish. In some cases, optimum finish occurs in 'ploughing' mode. The size of burrs produced is also related to the size effect, as a larger ratio of undeformed chip thickness to cutting edge radius results in smaller burrs (1). When machining in 'ploughing' mode, the material ahead of the tool is pushed, bent, and moved in the axial direction of the tool to form a burr.
The dominance of the cutting edge radius on the mechanics of machining implies that manufacturers should ideally use the sharpest micro tools provided this does not significantly compromise the strength of the tool. Unfortunately information on the nature and size of the cutting edge is seldom provided by suppliers of micro-tools. Manufacturers have to be pro-active in requesting this information from tool suppliers. It is also important that there should be consistency and high quality, from one micro tool to another so that the process inputs are of sufficient quality to enable output of the process to be predictable and controllable.
The use of oil mist for cooling and lubrication when machining slots in 45HRC H13 steel, has been shown to provide significantly better surface finish and reduced wear, versus flood cooling or dry machining. The selection of spindle speed, is also very important with respect to burr formation. For example, in most cases, higher spindle speed results in smaller burrs.
Effects of workpiece material
Grain structure of the workpiece material is important for high quality micro-milling. An ideal workpiece material for promoting better machinability at the microscale should have a fine and regular microstructure grains, and the material should have high hardness. For example, H13 steel hardened to 45HRC has a homogenous microstructure as shown in Fig 2. This is relevant because, the cutting tool needs to fracture or dislodge the grain if the grain size in the workpiece is too large/coarse, which results in poorer quality of the surface finish and larger burrs.
Selection of cutting tools and coatings
To achieve removal of the smallest possible amount of undeformed chip thickness, a very sharp cutting edge is required. However, the cutting edge radius achievable at present is limited by tool grinding technology and carbide grain size. Tool wear is also critical, as the cutting edge radius of uncoated carbide tools deteriorates rapidly during the machining of hardened steel, reducing surface quality and influencing tolerances.
An optimum cutting tool design to reduce the formation of burrs, would have a very sharp edge radius, such as that of a diamond cutting tool used to cut non-ferrous material. These tools can have an edge radius <600nm. However, with carbide tools used to machine steels, the grain structure of an ultra-fine carbide tool is comparable to the edge radius of the diamond tool, so it is difficult to achieve very small edge radius with carbide tools. At present most ultra fine grain carbide tools have edge radius in the region of 0.8 to 2 microns.
A typical two-flute micro-milling tool geometry is shown in Fig 3. Wear mechanisms that occur are shown in Fig 4. Wear mechanisms vary depending on whether the tool is being used for slot milling or the machining of closed pockets. In slot milling the wear is typically more uniform. Fig 5 (a) and (b) shows a cavity for a micro-mould for a microfluidic part and the cutter detail after final milling.
Titanium nitride (TiN) coating is one of the best coatings for hardened steel machining in terms of flank wear, chipping, edge radius wear, surface finish, and burr size (2). Other commercially available coatings can produce results exceeding those of TiN. These coatings typically have a proprietary composition, and may include elements such as chromium (Cr), aluminium (Al) and others. Other research has shown that under some cutting conditions, TiN produces favourable tool-chip contact phenomenon (reduced contact area) and hence its appropriateness as top layer coating.
The surface finish of the coating on the tool is also important to achieve a good surface finish on the steel, and the amount of cutting prior to the tool being discarded is also relevant. For example, a chromium nitride (CrN) coated tool can provide a higher quality surface finish initially, but a TiN coated tool will wear better and result in better overall finish after a longer cutting time.
The quality of the coating on the tool is very important to achieve a good surface finish on the steel. It is recommended that micro milling and drilling of hardened moulds and dies should be done using appropriate coatings, because the benefits of using coated tools are so significant.
Fig 6 shows a tool life comparison for coatings developed during research. This figure clearly shows that significant contribution of coatings in extending the life of cutting tools. It shows that in micro milling of hardened tool steels, TiN can extend the life of the cutting tool by over 5 fold (from 8.7 to 50.3 minutes) compared to using uncoated micro grain carbide tools.
The ELMACT project
The information above is part of the output from the ELMACT (Extended Life Micro-Tooling by Advanced Coating Technology) research project, funded in the UK by the Technology Strategy Board (TSB) under grant TP/6/MAT/6/K1028G. The industrial partners in the project were Teer Coatings Ltd, Agie Charmilles, Rainford Precision, Rolls Royce Plc, WLR Precision Ltd, and Micro Systems (UK) Ltd. Academic work in the project was carried out by the University of Manchester (research group headed by Dr Paul Mativenga) through the Engineering and Physical Science Research Council (EPSRC) grant DT/E010512/1.
2. A. Aramchararoen et. al., 'Evaluation and selection of hard coatings for micro milling of hardened tool steel', International Journal of Machine Tools & Manufacture 48, P1578-1584, (2008).
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