1 Introduction
In recent years, the demand for a variety of micro-miniaturized products in fields such as civil and defense has been increasing, and the requirements for the functions, structural complexity, and reliability of micro-devices are also increasing. Therefore, it is of great significance to research and develop a microfabrication technology that is economically feasible, capable of processing three-dimensional geometric shapes and diversified materials, and features micro-scale to millimeter-level precision three-dimensional micro parts. At present, micro-cutting has become an important technology to overcome the limitations of MEMS technology, and micro-milling technology has become a very active research hotspot due to its features of high efficiency, high flexibility, ability to process complex three-dimensional shapes, and a variety of materials.
2 Micro-path milling cutter and its manufacturing technology
(1) 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, under the action of the grinding force, a sharp cutting edge is ground on an uneven tool material. The mouth is a very difficult thing. It has also become a technical bottleneck for the development of micro-path milling cutters. For this reason, from a theoretical and experimental point of view, a machining method that does not generate cutting force (such as laser processing, focused ion beam processing, etc.) can be selected.
The focused ion beam processing method is more suitable in principle for manufacturing micro-diameter milling cutters. Friedrich and Vasile et al. used a focused ion beam processing technique to produce a micro-diameter milling cutter with a minimum diameter of 22 mm. Using micro-milling cutters and custom high-precision milling machines, an 89.5° straight wall microgroove structure was fabricated on polymethyl methacrylate (PMMA) with a depth of 62 mm and an inter-groove rib thickness of 8 mm. Adams et al. used focused ion beam processing technology to produce micro-diameter milling cutters with a diameter of about 25 μm. The contour shapes are dihedron, tetrahedron and hexahedron. The cutting edge is divided into 2 blades, 4 blades and 6 blades. The tool material is High speed steel and carbide. These tools were used to perform micro-milling on aluminum, brass, 4340 steel, and PMMA four workpiece materials. However, due to the use of micro-path milling cutters, small feed rates must be used, and the tool wear is severe and the processing burr is large, and the processing effect has not been satisfactory.
The edge geometry of the end mill mainly consists of straight bodies, cone-shaped triangles (D-type), semi-circular (D-type) and commercially available spiral edge end mills. Fang et al. studied and compared the above four kinds of end mills by means of experiments and finite element analysis, starting from the tool stiffness and processing performance. The results show that the cone D-type end mill is more suitable for micro-cutting, and the micro-embossing mold with feature size smaller than 50 μm and micro-embossing mold with feature size smaller than 80 μm has been successfully fabricated with a cone-end milling cutter with a diameter of 0.1 mm.
However, from a practical point of view and application perspective, it is still preferable to choose a commercial spiral edge micro-end mill. Many studies have been conducted on such milling cutters. At present, carbide end mills with a diameter of 0.1 mm have been commercialized in foreign countries (in China, end mills with a diameter of 0.2 mm have also been commercialized), and end mills with a diameter of 50 μm are also on the market. The current manufacture of such milling cutters still relies on high-performance tool grinding machines.
In Europe, micro-end milling cutters (minimum diameter 50 μm) are used to machine injection moulds for micro plastic components. The mould hardness is 53 HRC, the milling accuracy is <5 μm and the surface roughness is Ra <0.2 μm. The United States has developed a new type of micro-diameter milling cutter specially designed for the machining of molds and hard molds, enabling high-speed cutting of high hardness materials such as graphite and steel (cutting speed 30m/min up to 150m/min). The Swiss researchers have done a high-speed cutting of hard materials. They cut a 316L stainless steel with a 0.5 mm diameter TiAlN coated micro-diameter cutter. The depth of cut is 0.1 mm, the cutting speed is 80 m/min, and the spindle speed is 50000 r/min. The feed rate 240mm/min. The experimental results show that the tool life is up to 8 hours (117m).
(2) Tool Material
As a tool material, diamond, cubic boron nitride, ceramics, etc. have their own advantages and limitations, and the most used is a carbide material, at present more than 90% of the world's turning tools and 55% of the milling cutter are used hard Alloys. In the field of micro-path milling cutters, the tool material is also mainly cemented carbide. Cemented carbide is a sintered body composed of many grains. The size of crystal grains determines the sharpness of the cutting edge. In order to obtain a sharp cutting edge, tungsten-cobalt-based ultra-fine grain cemented carbide is usually used. At present, the grain size of the ultra-fine grained cemented carbide is about 0.5 μm, and the arc radius of the cutting edge is several micrometers.
The development and application of fine-grained, ultrafine-grained cemented carbide materials is the development direction for further improving the reliability of tooling. Its characteristic is the continuous development of new grades of tool materials, making them more adaptable to the material being processed and cutting conditions, thereby achieving improved cutting. The purpose of efficiency. Toolmakers adopt the "situation for remedy" strategy and continue to develop new tools with targeted tools. For example, the new KN9110 steel processing steel, the KC9225 stainless steel, and the cast iron KY1310, KC5410 for heat-resistant alloys, KC5510 for hardened materials, and KY1615 for non-ferrous materials. Compared with the existing old brand, the new brand can improve the cutting efficiency by 15% to 20% on average. Secondly, in the development of new grades, more emphasis has been put on optimizing the combination of the matrix and the 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 geometry and geometry to better adapt to the characteristics of the materials being processed and the requirements for chip breaking in different processes, and to reduce the cutting force and reduce the vibration. Make cutting more light and efficient.
(3) Tool coating
The coating has high hardness, wear resistance and chemical stability, can prevent the interaction between the tool-chip-workpiece material, can play a role as a thermal barrier, reduce the adhesive wear, dissolution wear, surface peel wear, etc., and Can effectively delay the appearance of tool wear. Therefore, the application of the coating can greatly improve the tool performance.
Coatings can be divided into two categories according to their composition and role: one is a "hard" coating, characterized by high hardness and good wear resistance; the other is a "soft" coating, the main role is to reduce friction and reduce Cutting force and cutting temperature. According to the structure of the coating can be divided into single-layer coating, multi-layer coating, composite coating, gradient coating, nano-multi-layer coating, nano-composite structure coating. When selecting the coating, the thickness, smoothness of the coating, and compatibility with the substrate hard alloy should be considered.
The development of tool coating is characterized by diversification and serialization. The development and application of nano-coating, gradient structure coating, and new structure and material coating have played an important role in improving the performance of the tool. Among the endless new coating products, there are wear-resistant and heat-resistant coatings for high-speed cutting, dry cutting and hard cutting, as well as tough coatings for interrupted cutting, as well as for dry cutting and the need to reduce the friction coefficient. Lubrication coating. The diamond coating has also been further applied to improve the processing efficiency of non-ferrous metal and non-metal materials such as aluminum alloys. The practical application of a variety of nano-coatings (including nanocrystalline, nano-thickness, and nano-structured coatings) has resulted in greater improvements in coating performance. The latest result of nano-coating technology is the development of TiSiN and CrSiN coated end mills, both of which have a particle size of 5 nm. In addition, the coating tool's anti-friction and anti-adhesion capabilities can be improved by improving the surface finish of the coating.
3 Research on Micro Milling Technology
The traditional research and application of micro-milling technology mainly uses miniature end mills with diameters of tens of micrometers to 1 mm to perform micro-machining on conventional super-precision machine tools. Since these machines are mainly used for machining non-miniature geometric parts with high precision, they usually require expensive design and manufacturing processes to achieve the desired target accuracy, while for the processing of small parts, they lack the necessary flexibility and processing costs. High and low efficiency. The miniaturized processing equipment has the advantages of space saving, energy saving, easy reorganization, and low cost. In recent years, the use of micro-processing equipment to achieve micro-milling has attracted widespread attention, and the use of micro-tools in micro-machine tools on the micro-processing process. In the research of micro-milling technology, the research focuses mainly on the processing of surface quality, cutting force, wear and life of the tool, chip status, and processing capability of small parts.
(1) Surface quality and burr
Surface roughness has always been a matter of concern in the study of microfabricated surface quality. South Korea’s W. Wang et al. performed micro-milling experiments on brass and used statistical methods to analyze the effects of parameters such as tool diameter, cutting depth, spindle speed, and feed rate on surface roughness, and established a new Surface roughness mathematical model. The study shows that the feed rate plays a major role, and the surface roughness increases linearly with the tool diameter and spindle speed. However, the hardness of the tool and the vibration of the spindle have a greater effect than the feed rate. Finally, it is pointed out that increasing the hardness and stiffness of the structure and the tool and reducing the vibration of the spindle is the best way to improve the surface quality under this processing condition.
Germany's J. Schmidt et al. conducted extensive research on micro-milling. In the cutting of hard steel (HRC52), it was found that in the cut-in section, the surface roughness was unstable due to the severe wear of the tool, the worst on the side of the cut-in side of the milling, the middle part was the best, and the side of the milling was centered (Rz 0.5 ~1.6μm). With the continued wear of the tool, the roughness on the side of the counter mill becomes better, the side on the side of the down-cut decreases, and the surface roughness tends to be stable. However, in the case of soft steel (HRC42), the above phenomenon did not occur, and the surface roughness was always the best in the middle (Rz 0.7 to 1.8 μm). In addition, a milling experiment with a feed amount of 7 μm per tooth was performed, and a good surface quality was obtained, and this feed amount was considered to be inappropriate when cutting a high hardness material (HRC52).
Glitches are the main factors affecting the quality of micro-milling machining. Lee et al. experimentally studied the burrs produced when micro-milling aluminum and copper. Five types of burrs were observed in the experiment: burrs cut along the side of the milling, side burrs at the sides of the grooves, cut burrs on the bottom of the groove, and burrs cut on the side of the side of the cut. The burr size increases with the amount of the back knife and feed. Increase. Germany's J. Schmidt et al. found that only a few millimeters long burrs occur when the feed per tooth is 0.5 μm. In most cases, the height of the burrs is between 5 and 60 μm. This does not apply to the actual application of the processing tool. The effect was satisfactory. In addition, it has been found that the burr on the side of the crushing mill is larger, and the burr of the hard material is larger than the burr of the soft material; as the tool wears, the burr becomes larger, especially on the side of the counter mill; the burr is slightly increased as the cutting speed increases. There is a decrease.
At present, many countries around the world have conducted a lot of research on surface roughness, but there are few reports on the work hardening and residual stress. These factors have a great impact on the performance of small parts, I believe there is a lot of research value. Will become one of the future research directions.
(2) Fine cutting force
In the milling process, the loaded state of the tool is extremely complicated, and it is constantly subjected to mechanical impact and thermal shock load of different sizes and positions. Since the feed per tooth in the micro-milling is less than (or equal to) the radius of the blunt edge of the cutting edge of the tool, the cutting process mainly changes from shear to friction, squeezing, or plough; and the cutting speed is higher. The impact load is large, making the micro-cutting force very different from the traditional milling force.
Bao and Tansel studied the cutting forces when using micro-end mills for micro-milling and proposed an improved cutting force model. The model calculates the change of chip thickness caused by the tool nose trajectory when the tool rotates and advances, and considers the difference between the feed rate per tooth and tool radius ratio, the amount of tool bounce and tool wear on the cutting force, and through The experiment verifies that the model is more accurate than the traditional end milling model.
Vogler et al. proposed a mechanical model for fine vertical milling, taking into account the different phases in the heterogeneous material, and found that multi-phases in the metal material caused high-frequency changes in the cutting forces, thereby explaining the cutting forces when micro-milling multi-phase materials. The high frequency signal that appears in.
At present, there is not much research on micro-cutting force, and it is necessary to further understand the characteristics of micro-cutting force, and can consider dynamically adjusting the cutting amount through real-time monitoring of cutting force to control the cutting force, improve the surface quality, and prolong the use of tools. life.
(3) Micro-tool wear, life, and chip condition
When using a small-diameter end mill for micromachining, it is very difficult to trim the machined surface. Therefore, it is desirable to use a single milling cutter to complete the final machining process. In addition, the cutting time required for high-precision machining often takes several hours. Therefore, higher demands are made on tool life and cutting performance.
Rahman et al. performed a micromilling experiment on pure copper using an end mill with a diameter of 1mm, and established a quadratic model of the tool life in pure copper micromilling using statistical response surface methodology to obtain the cutting speed and the backing knife. The amount of tool life has a significant effect, while the effect of feed rate is not significant. The sharpening of the cutting edge shows an increase in cutting force. At the same time, the diameter and edge size of the microtool should be taken into consideration. Zhou et al. used a 2 mm diameter end mill to high-speed mill graphite electrodes to indicate that tool wear was dominated by abrasive wear. The wear patterns were flank wear, rake face wear, micro-cracking and breakage, and the chip shape was massive. Columns, spheres and sheets; the life of coated tools is 1.5 times longer than uncoated tools; the use of air jet nozzles and vacuum cleaners has been proposed to effectively reduce tool wear and breakage. Miyaguchi et al. pointed out that the tool life can be extended by reducing the tool stiffness. Due to the low rigidity of the tool, the tool's bending balances the cutting force and adjusts the effect of the run-out, resulting in uniform wear of the two cutting edges.
In the micro-milling of micro-tools, the chip status is an important factor in achieving precision machining, controlling the machining process, and judging the machining capability. Kim et al. conducted an experimental study of the formation of chips during the micro-milling process. The sculpting of the brass workpiece at different feed rates, collecting the chips and measuring the SEM image of the bottom surface of the groove revealed that when the feed per tooth is less than the radius of the obtuse circle of the cutting edge, the actual chip volume is the nominal volume of the chip. Several times, the interval between feed marks is also greater than the feed per tooth. As the feed per tooth increases, the actual chip volume gradually approaches the nominal chip volume. From this, it can be seen that during the micro-milling, the chips are not always formed during the feeding with a small amount of feed per tooth, that is, the formation of chips is intermittent and intermittent.
In order to improve the quality of micro-milling machining, the wear and life of the tool must be studied. The wear and damage of the tool can be considered through the cutting force, surface roughness, and tool vibration.
(4) Processing capability for tiny parts
At present, most of the micro-milling researches are focused on the shape feature capabilities that can be achieved. The purpose is to realize the practical processing of complex micro-components (such as micro-molds, etc.) on micro-processing equipment. In order to improve the ability to process complex shapes and processing efficiency, the research of multi-axis linkage micro-small processing equipment has begun.
South Korea's Young et al. developed a five-axis micro vertical milling machine that uses carbide flat heads with diameters of 200 μm and 100 μm to machine a 25 μm thick and 650 μm high wall structure on a brass workpiece. And miniature square columns (30μm × 30μm × 320μm), micro-cylinders and micro-impeller (diameter 600μm) structure.
Germany's J.Schmidt and others in order to prove the ability to micro-milling micro-molds, processing micro-wheel, micro-gear mold (workpiece hardness HRC52), to obtain a better accuracy (0.01mm) and the appropriate surface roughness, and Finish processing within one hour.
In China, the Institute of Precision Engineering of Harbin Institute of Technology developed the first micro-miniature horizontal milling machine in China with a size of 300mm x 150mm x 165mm, a maximum spindle speed of 140,000r/min, and a drive system resolution of 0.1μm. A thin-walled structure with dimensions of 700μm×40μm and 500μm×20μm was milled on hard aluminum LY12. At the same time, the numerical control of the surface was performed on two plexiglass materials with dimensions of 12mm×8mm and 8mm×5mm, respectively.
At present, Harbin Institute of Technology has also developed a three-axis micro vertical milling machine with a size of 300mm×300mm×290mm, a maximum spindle speed of 160,000r/min, and a maximum radial runout of 1μm. The drive system repeats positioning accuracy of 0.25μm and a speed range of 1μm. ~250mm/s; Full closed loop control, resolution 0.1μm. Using a 0.2 mm micro-end mill, a micro-groove (residual thickness about 20 μm) was machined on a 70 μm thick small steel sheet (HRC50).
4 Conclusion
In order to achieve the ideal micro-milling results, not only high-performance machining tools are required, but also excellent cutting tools and strict process control are required. Micro-tools with excellent cutting performance will play an important role in the future of micro-milling.
At present, in the field of micro-milling, many researches have been carried out on the roughness of the machined surface. However, there are not many studies on the work hardening and residual stress, and the research on the cutting force is not mature enough. In order to improve the machining effect of micro-milling, a comprehensive study can be made on the effects of factors such as cutting force, machining quality, tool wear, and machining vibration; through the in-depth research and development of micro-milling technology, the machining capability of micro-miniature machine tools can be further improved. With the ever-increasing market demand for precision three-dimensional micro parts, micro-milling technology will surely be promising.
In recent years, the demand for a variety of micro-miniaturized products in fields such as civil and defense has been increasing, and the requirements for the functions, structural complexity, and reliability of micro-devices are also increasing. Therefore, it is of great significance to research and develop a microfabrication technology that is economically feasible, capable of processing three-dimensional geometric shapes and diversified materials, and features micro-scale to millimeter-level precision three-dimensional micro parts. At present, micro-cutting has become an important technology to overcome the limitations of MEMS technology, and micro-milling technology has become a very active research hotspot due to its features of high efficiency, high flexibility, ability to process complex three-dimensional shapes, and a variety of materials.
2 Micro-path milling cutter and its manufacturing technology
(1) 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, under the action of the grinding force, a sharp cutting edge is ground on an uneven tool material. The mouth is a very difficult thing. It has also become a technical bottleneck for the development of micro-path milling cutters. For this reason, from a theoretical and experimental point of view, a machining method that does not generate cutting force (such as laser processing, focused ion beam processing, etc.) can be selected.
The focused ion beam processing method is more suitable in principle for manufacturing micro-diameter milling cutters. Friedrich and Vasile et al. used a focused ion beam processing technique to produce a micro-diameter milling cutter with a minimum diameter of 22 mm. Using micro-milling cutters and custom high-precision milling machines, an 89.5° straight wall microgroove structure was fabricated on polymethyl methacrylate (PMMA) with a depth of 62 mm and an inter-groove rib thickness of 8 mm. Adams et al. used focused ion beam processing technology to produce micro-diameter milling cutters with a diameter of about 25 μm. The contour shapes are dihedron, tetrahedron and hexahedron. The cutting edge is divided into 2 blades, 4 blades and 6 blades. The tool material is High speed steel and carbide. These tools were used to perform micro-milling on aluminum, brass, 4340 steel, and PMMA four workpiece materials. However, due to the use of micro-path milling cutters, small feed rates must be used, and the tool wear is severe and the processing burr is large, and the processing effect has not been satisfactory.
The edge geometry of the end mill mainly consists of straight bodies, cone-shaped triangles (D-type), semi-circular (D-type) and commercially available spiral edge end mills. Fang et al. studied and compared the above four kinds of end mills by means of experiments and finite element analysis, starting from the tool stiffness and processing performance. The results show that the cone D-type end mill is more suitable for micro-cutting, and the micro-embossing mold with feature size smaller than 50 μm and micro-embossing mold with feature size smaller than 80 μm has been successfully fabricated with a cone-end milling cutter with a diameter of 0.1 mm.
However, from a practical point of view and application perspective, it is still preferable to choose a commercial spiral edge micro-end mill. Many studies have been conducted on such milling cutters. At present, carbide end mills with a diameter of 0.1 mm have been commercialized in foreign countries (in China, end mills with a diameter of 0.2 mm have also been commercialized), and end mills with a diameter of 50 μm are also on the market. The current manufacture of such milling cutters still relies on high-performance tool grinding machines.
In Europe, micro-end milling cutters (minimum diameter 50 μm) are used to machine injection moulds for micro plastic components. The mould hardness is 53 HRC, the milling accuracy is <5 μm and the surface roughness is Ra <0.2 μm. The United States has developed a new type of micro-diameter milling cutter specially designed for the machining of molds and hard molds, enabling high-speed cutting of high hardness materials such as graphite and steel (cutting speed 30m/min up to 150m/min). The Swiss researchers have done a high-speed cutting of hard materials. They cut a 316L stainless steel with a 0.5 mm diameter TiAlN coated micro-diameter cutter. The depth of cut is 0.1 mm, the cutting speed is 80 m/min, and the spindle speed is 50000 r/min. The feed rate 240mm/min. The experimental results show that the tool life is up to 8 hours (117m).
(2) Tool Material
As a tool material, diamond, cubic boron nitride, ceramics, etc. have their own advantages and limitations, and the most used is a carbide material, at present more than 90% of the world's turning tools and 55% of the milling cutter are used hard Alloys. In the field of micro-path milling cutters, the tool material is also mainly cemented carbide. Cemented carbide is a sintered body composed of many grains. The size of crystal grains determines the sharpness of the cutting edge. In order to obtain a sharp cutting edge, tungsten-cobalt-based ultra-fine grain cemented carbide is usually used. At present, the grain size of the ultra-fine grained cemented carbide is about 0.5 μm, and the arc radius of the cutting edge is several micrometers.
The development and application of fine-grained, ultrafine-grained cemented carbide materials is the development direction for further improving the reliability of tooling. Its characteristic is the continuous development of new grades of tool materials, making them more adaptable to the material being processed and cutting conditions, thereby achieving improved cutting. The purpose of efficiency. Toolmakers adopt the "situation for remedy" strategy and continue to develop new tools with targeted tools. For example, the new KN9110 steel processing steel, the KC9225 stainless steel, and the cast iron KY1310, KC5410 for heat-resistant alloys, KC5510 for hardened materials, and KY1615 for non-ferrous materials. Compared with the existing old brand, the new brand can improve the cutting efficiency by 15% to 20% on average. Secondly, in the development of new grades, more emphasis has been put on optimizing the combination of the matrix and the 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 geometry and geometry to better adapt to the characteristics of the materials being processed and the requirements for chip breaking in different processes, and to reduce the cutting force and reduce the vibration. Make cutting more light and efficient.
(3) Tool coating
The coating has high hardness, wear resistance and chemical stability, can prevent the interaction between the tool-chip-workpiece material, can play a role as a thermal barrier, reduce the adhesive wear, dissolution wear, surface peel wear, etc., and Can effectively delay the appearance of tool wear. Therefore, the application of the coating can greatly improve the tool performance.
Coatings can be divided into two categories according to their composition and role: one is a "hard" coating, characterized by high hardness and good wear resistance; the other is a "soft" coating, the main role is to reduce friction and reduce Cutting force and cutting temperature. According to the structure of the coating can be divided into single-layer coating, multi-layer coating, composite coating, gradient coating, nano-multi-layer coating, nano-composite structure coating. When selecting the coating, the thickness, smoothness of the coating, and compatibility with the substrate hard alloy should be considered.
The development of tool coating is characterized by diversification and serialization. The development and application of nano-coating, gradient structure coating, and new structure and material coating have played an important role in improving the performance of the tool. Among the endless new coating products, there are wear-resistant and heat-resistant coatings for high-speed cutting, dry cutting and hard cutting, as well as tough coatings for interrupted cutting, as well as for dry cutting and the need to reduce the friction coefficient. Lubrication coating. The diamond coating has also been further applied to improve the processing efficiency of non-ferrous metal and non-metal materials such as aluminum alloys. The practical application of a variety of nano-coatings (including nanocrystalline, nano-thickness, and nano-structured coatings) has resulted in greater improvements in coating performance. The latest result of nano-coating technology is the development of TiSiN and CrSiN coated end mills, both of which have a particle size of 5 nm. In addition, the coating tool's anti-friction and anti-adhesion capabilities can be improved by improving the surface finish of the coating.
3 Research on Micro Milling Technology
The traditional research and application of micro-milling technology mainly uses miniature end mills with diameters of tens of micrometers to 1 mm to perform micro-machining on conventional super-precision machine tools. Since these machines are mainly used for machining non-miniature geometric parts with high precision, they usually require expensive design and manufacturing processes to achieve the desired target accuracy, while for the processing of small parts, they lack the necessary flexibility and processing costs. High and low efficiency. The miniaturized processing equipment has the advantages of space saving, energy saving, easy reorganization, and low cost. In recent years, the use of micro-processing equipment to achieve micro-milling has attracted widespread attention, and the use of micro-tools in micro-machine tools on the micro-processing process. In the research of micro-milling technology, the research focuses mainly on the processing of surface quality, cutting force, wear and life of the tool, chip status, and processing capability of small parts.
(1) Surface quality and burr
Surface roughness has always been a matter of concern in the study of microfabricated surface quality. South Korea’s W. Wang et al. performed micro-milling experiments on brass and used statistical methods to analyze the effects of parameters such as tool diameter, cutting depth, spindle speed, and feed rate on surface roughness, and established a new Surface roughness mathematical model. The study shows that the feed rate plays a major role, and the surface roughness increases linearly with the tool diameter and spindle speed. However, the hardness of the tool and the vibration of the spindle have a greater effect than the feed rate. Finally, it is pointed out that increasing the hardness and stiffness of the structure and the tool and reducing the vibration of the spindle is the best way to improve the surface quality under this processing condition.
Germany's J. Schmidt et al. conducted extensive research on micro-milling. In the cutting of hard steel (HRC52), it was found that in the cut-in section, the surface roughness was unstable due to the severe wear of the tool, the worst on the side of the cut-in side of the milling, the middle part was the best, and the side of the milling was centered (Rz 0.5 ~1.6μm). With the continued wear of the tool, the roughness on the side of the counter mill becomes better, the side on the side of the down-cut decreases, and the surface roughness tends to be stable. However, in the case of soft steel (HRC42), the above phenomenon did not occur, and the surface roughness was always the best in the middle (Rz 0.7 to 1.8 μm). In addition, a milling experiment with a feed amount of 7 μm per tooth was performed, and a good surface quality was obtained, and this feed amount was considered to be inappropriate when cutting a high hardness material (HRC52).
Glitches are the main factors affecting the quality of micro-milling machining. Lee et al. experimentally studied the burrs produced when micro-milling aluminum and copper. Five types of burrs were observed in the experiment: burrs cut along the side of the milling, side burrs at the sides of the grooves, cut burrs on the bottom of the groove, and burrs cut on the side of the side of the cut. The burr size increases with the amount of the back knife and feed. Increase. Germany's J. Schmidt et al. found that only a few millimeters long burrs occur when the feed per tooth is 0.5 μm. In most cases, the height of the burrs is between 5 and 60 μm. This does not apply to the actual application of the processing tool. The effect was satisfactory. In addition, it has been found that the burr on the side of the crushing mill is larger, and the burr of the hard material is larger than the burr of the soft material; as the tool wears, the burr becomes larger, especially on the side of the counter mill; the burr is slightly increased as the cutting speed increases. There is a decrease.
At present, many countries around the world have conducted a lot of research on surface roughness, but there are few reports on the work hardening and residual stress. These factors have a great impact on the performance of small parts, I believe there is a lot of research value. Will become one of the future research directions.
(2) Fine cutting force
In the milling process, the loaded state of the tool is extremely complicated, and it is constantly subjected to mechanical impact and thermal shock load of different sizes and positions. Since the feed per tooth in the micro-milling is less than (or equal to) the radius of the blunt edge of the cutting edge of the tool, the cutting process mainly changes from shear to friction, squeezing, or plough; and the cutting speed is higher. The impact load is large, making the micro-cutting force very different from the traditional milling force.
Bao and Tansel studied the cutting forces when using micro-end mills for micro-milling and proposed an improved cutting force model. The model calculates the change of chip thickness caused by the tool nose trajectory when the tool rotates and advances, and considers the difference between the feed rate per tooth and tool radius ratio, the amount of tool bounce and tool wear on the cutting force, and through The experiment verifies that the model is more accurate than the traditional end milling model.
Vogler et al. proposed a mechanical model for fine vertical milling, taking into account the different phases in the heterogeneous material, and found that multi-phases in the metal material caused high-frequency changes in the cutting forces, thereby explaining the cutting forces when micro-milling multi-phase materials. The high frequency signal that appears in.
At present, there is not much research on micro-cutting force, and it is necessary to further understand the characteristics of micro-cutting force, and can consider dynamically adjusting the cutting amount through real-time monitoring of cutting force to control the cutting force, improve the surface quality, and prolong the use of tools. life.
(3) Micro-tool wear, life, and chip condition
When using a small-diameter end mill for micromachining, it is very difficult to trim the machined surface. Therefore, it is desirable to use a single milling cutter to complete the final machining process. In addition, the cutting time required for high-precision machining often takes several hours. Therefore, higher demands are made on tool life and cutting performance.
Rahman et al. performed a micromilling experiment on pure copper using an end mill with a diameter of 1mm, and established a quadratic model of the tool life in pure copper micromilling using statistical response surface methodology to obtain the cutting speed and the backing knife. The amount of tool life has a significant effect, while the effect of feed rate is not significant. The sharpening of the cutting edge shows an increase in cutting force. At the same time, the diameter and edge size of the microtool should be taken into consideration. Zhou et al. used a 2 mm diameter end mill to high-speed mill graphite electrodes to indicate that tool wear was dominated by abrasive wear. The wear patterns were flank wear, rake face wear, micro-cracking and breakage, and the chip shape was massive. Columns, spheres and sheets; the life of coated tools is 1.5 times longer than uncoated tools; the use of air jet nozzles and vacuum cleaners has been proposed to effectively reduce tool wear and breakage. Miyaguchi et al. pointed out that the tool life can be extended by reducing the tool stiffness. Due to the low rigidity of the tool, the tool's bending balances the cutting force and adjusts the effect of the run-out, resulting in uniform wear of the two cutting edges.
In the micro-milling of micro-tools, the chip status is an important factor in achieving precision machining, controlling the machining process, and judging the machining capability. Kim et al. conducted an experimental study of the formation of chips during the micro-milling process. The sculpting of the brass workpiece at different feed rates, collecting the chips and measuring the SEM image of the bottom surface of the groove revealed that when the feed per tooth is less than the radius of the obtuse circle of the cutting edge, the actual chip volume is the nominal volume of the chip. Several times, the interval between feed marks is also greater than the feed per tooth. As the feed per tooth increases, the actual chip volume gradually approaches the nominal chip volume. From this, it can be seen that during the micro-milling, the chips are not always formed during the feeding with a small amount of feed per tooth, that is, the formation of chips is intermittent and intermittent.
In order to improve the quality of micro-milling machining, the wear and life of the tool must be studied. The wear and damage of the tool can be considered through the cutting force, surface roughness, and tool vibration.
(4) Processing capability for tiny parts
At present, most of the micro-milling researches are focused on the shape feature capabilities that can be achieved. The purpose is to realize the practical processing of complex micro-components (such as micro-molds, etc.) on micro-processing equipment. In order to improve the ability to process complex shapes and processing efficiency, the research of multi-axis linkage micro-small processing equipment has begun.
South Korea's Young et al. developed a five-axis micro vertical milling machine that uses carbide flat heads with diameters of 200 μm and 100 μm to machine a 25 μm thick and 650 μm high wall structure on a brass workpiece. And miniature square columns (30μm × 30μm × 320μm), micro-cylinders and micro-impeller (diameter 600μm) structure.
Germany's J.Schmidt and others in order to prove the ability to micro-milling micro-molds, processing micro-wheel, micro-gear mold (workpiece hardness HRC52), to obtain a better accuracy (0.01mm) and the appropriate surface roughness, and Finish processing within one hour.
In China, the Institute of Precision Engineering of Harbin Institute of Technology developed the first micro-miniature horizontal milling machine in China with a size of 300mm x 150mm x 165mm, a maximum spindle speed of 140,000r/min, and a drive system resolution of 0.1μm. A thin-walled structure with dimensions of 700μm×40μm and 500μm×20μm was milled on hard aluminum LY12. At the same time, the numerical control of the surface was performed on two plexiglass materials with dimensions of 12mm×8mm and 8mm×5mm, respectively.
At present, Harbin Institute of Technology has also developed a three-axis micro vertical milling machine with a size of 300mm×300mm×290mm, a maximum spindle speed of 160,000r/min, and a maximum radial runout of 1μm. The drive system repeats positioning accuracy of 0.25μm and a speed range of 1μm. ~250mm/s; Full closed loop control, resolution 0.1μm. Using a 0.2 mm micro-end mill, a micro-groove (residual thickness about 20 μm) was machined on a 70 μm thick small steel sheet (HRC50).
4 Conclusion
In order to achieve the ideal micro-milling results, not only high-performance machining tools are required, but also excellent cutting tools and strict process control are required. Micro-tools with excellent cutting performance will play an important role in the future of micro-milling.
At present, in the field of micro-milling, many researches have been carried out on the roughness of the machined surface. However, there are not many studies on the work hardening and residual stress, and the research on the cutting force is not mature enough. In order to improve the machining effect of micro-milling, a comprehensive study can be made on the effects of factors such as cutting force, machining quality, tool wear, and machining vibration; through the in-depth research and development of micro-milling technology, the machining capability of micro-miniature machine tools can be further improved. With the ever-increasing market demand for precision three-dimensional micro parts, micro-milling technology will surely be promising.