At present, micro-cutting has become an important technology to overcome the limitations of MEMS technology. Micro-milling technology has become a very active research hotspot due to its high efficiency, high flexibility, and ability to process complex three-dimensional shapes and a variety of materials.
Micro Milling Cutter and Its Manufacturing Technology
Manufacturing Process and Tool Performance
Grinding is a traditional milling cutter manufacturing process, but for micro diameter milling cutters with a diameter of only a few tenths of a millimeter. It is very difficult to grind a sharp cutting edge on an inhomogeneous tool material under the action of grinding force. This has also become a technical bottleneck in the development of micro-diameter milling cutters. To this end, from a theoretical and experimental point of view, a processing method that does not generate cutting force can be selected (such as laser processing, focused ion beam processing, etc.).
The focused ion beam processing method is more suitable for manufacturing micro-diameter milling cutters in principle. Friedrich, Vasile and others used focused ion beam processing technology to make micro-diameter milling cutters with a minimum diameter of 22mm. Using a micro-diameter milling cutter and a customized high-precision milling machine, an 89.5° straight-wall micro-groove structure with a depth of 62mm and a rib thickness of 8mm between grooves was machined on polymethyl methacrylate (PMMA). Adams and others used focused ion beam processing technology to make some micro-diameter milling cutters with a diameter of about 25μm. Its contour shapes include dihedron, tetrahedron and hexahedron, and the cutting edges are divided into 2-edge, 4-edge and 6-edge. The tool materials are high-speed steel and cemented carbide. These tools were used to perform micro-milling on four workpiece materials: aluminum, brass, 4340 steel and PMMA. However, since the use of micro-diameter milling cutters for cutting must use a small feed rate, and the tool wear is severe, the processing burrs are large, and the processing effect is still unsatisfactory.
The blade geometry of the end mill mainly includes four types: straight body, cone triangle (D-type), semicircular (D-type) and commercialized spiral edge end mill. Fang et al. conducted a study and comparison of the above four end mills based on tool stiffness and processing performance through experiments and finite element analysis. The results show that the cone D-type end mill is more suitable for micro-cutting, and a cone end mill with a diameter of 0.1 mm was successfully used to produce biomedical parts with a feature size of less than 50μm and micro-embossing molds with a feature size of less than 80μm.
However, from a practical point of view and application prospects, commercialized spiral blade micro-diameter end mills should be given priority, and many studies are conducted on this type of milling cutter. At present, carbide end mills with a diameter of 0.1mm have been commercialized abroad (in China, end mills with a diameter of 0.2mm have also been commercialized), and end mills with a diameter of 50μm have also begun to be listed. At present, the manufacture of such milling cutters still depends on high-performance tool grinders.
In Europe, micro-diameter end mills (minimum diameter 50μm) are used to process injection molds for micro plastic components. The mold hardness reaches 53HRC, the milling accuracy is <5μm, and the surface roughness Ra is <0.2μm. The United States has developed a new type of micro-diameter milling cutter specifically for mold and hard mold processing, which can perform high-speed cutting processing on high-hardness materials such as graphite and steel (cutting speed 30m/min, up to 150m/min). Swiss researchers conducted an experiment on high-speed cutting of hard materials, using a 0.5mm diameter TiAlN coated micro-diameter milling cutter to cut 316L stainless steel, with a cutting depth of 0.1mm, a cutting speed of 80m/min, a spindle speed of 50000r/min, and a feed rate of 240mm/min. The experimental results showed that the tool life reached 8 hours (117m).
Micro Milling Cutter Tool Materials
As tool materials, diamond, cubic boron nitride, ceramics, etc. all have their own advantages and limitations. The most commonly used is cemented carbide. At present, more than 90% of turning tools and more than 55% of milling cutters abroad are made of cemented carbide. In the field of micro-diameter milling cutters, the tool material is also mainly cemented carbide. Cemented carbide is a sintered body composed of many grains. The size of the grain determines the microscopic sharpness of the blade. In order to obtain a sharp blade, ultrafine grain cemented carbide of tungsten-cobalt type is usually used. At present, the grain size of ultrafine grain cemented carbide is about 0.5mm, and the radius of the cutting edge arc is several microns.
The development and application of fine-grained and ultrafine-grained cemented carbide materials is the development direction to further improve the reliability of tool use. Its characteristic is to continuously develop new grades of tool materials to make them more suitable for the processed materials and cutting conditions, so as to achieve the purpose of improving cutting efficiency. Tool manufacturers adopt the strategy of “prescribing the right medicine for the right disease” and continuously develop new grades of tools with processing-specific features. For example, the new grades launched by Kennametal in the United States for turning processing include: KC9110 for processing steel, KC9225 for processing stainless steel, KY1310 for processing cast iron, KC5410 for processing heat-resistant alloys, KC5510 for processing hardened materials, KY1615 for processing non-ferrous materials, etc.
Compared with the original old grades, the new grades can improve the cutting efficiency by an average of 15% to 20%. Secondly, in the development of new grades, more attention is paid to the optimized combination of substrate and coating to better achieve the purpose of applicability development. In addition, the development of new grades usually also includes the improvement of the corresponding tool groove shape and geometric parameters. In order to better adapt to the characteristics of the processed materials and the requirements of different processes for chip breaking, and to reduce cutting force and vibration, so as to make cutting lighter and more efficient.
Micro End Mill Tool Coatings
The coating has high hardness, wear resistance and chemical stability. It can prevent the interaction between tool-chip-workpiece materials and act as a thermal barrier. It can reduce the adhesive wear, dissolution wear, surface peeling wear of the tool, etc. It can effectively delay the occurrence of tool wear. Therefore, the application of coating can greatly improve the performance of the tool.
Coatings can be divided into two categories according to their composition and function: one is “hard” coating, which is characterized by high hardness and good wear resistance. The other is “soft” coating, which mainly reduces friction, reduces cutting force and cutting temperature. Coatings can be divided into single-layer coatings, multi-layer coatings, composite coatings, gradient coatings, nano-multi-layer coatings, nano-composite structure coatings, etc. according to their structure. When selecting coatings, the thickness, smoothness and compatibility of the coating with the base carbide should be considered.
The development characteristics of tool coatings are diversification and serialization. The development and application of nano coatings, gradient structure coatings and new structure and material coatings have played an important role in improving the performance of tools. Among the endless new coating products, there are wear-resistant and heat-resistant coatings suitable for high-speed cutting, dry cutting and hard cutting. There are also tough coatings that are suitable for intermittent cutting. There are also lubricating coatings that are suitable for dry cutting and need to reduce the friction coefficient.
Diamond coatings have also been further applied to improve the processing efficiency of non-ferrous metals and non-metallic materials such as aluminum alloys. The practical application of various nano coatings (including nano crystallization, nano layer thickness and nano structure coatings) has greatly improved the performance of coatings. The latest achievement of nano coating technology is the development of TiSiN and CrSiN coated end mills, both of which have a particle size of 5nm. In addition, by improving the surface finish of the coating, the anti-friction and anti-adhesion capabilities of the coated tool can be improved.
At present, in the field of micro milling, many results have been achieved in the research on the roughness of the machining surface. However, there are not many studies on work hardening and residual stress, and the research on cutting force is not mature enough. In order to improve the processing effect of micro milling, the influence of factors such as cutting force, processing quality, tool wear and processing vibration can be comprehensively studied. Through in-depth research and development of micro milling technology, the processing capacity of micro machine tools can be further improved. With the increasing market demand for precision three-dimensional micro parts, micro-milling technology will surely have great potential.
