1 Introduction
In ultra-precision machining, the main factors to ensure the quality of the machined surface are high-quality tools, in addition to high-precision machine tools and ultra-stable machining environments. Natural diamond has high hardness, good wear resistance, high strength, good thermal conductivity, low friction coefficient with non-ferrous metals, good anti-adhesion and excellent corrosion resistance and chemical stability. It can sharpen sharp edges and be sharpened. It is considered to be the most ideal tool material for ultra-precision cutting, and has an important position in the field of machining, especially in the field of ultra-precision machining.
2 Physical properties of single crystal diamond
Diamond is a crystal of a single carbon atom, and its crystal structure belongs to the equiaxed face-centered cubic system (a system with the highest atom density). Since the bond between carbon atoms in diamond is sp3 hybrid covalent bond, it has strong binding force, stability and directionality. It is the hardest substance known in nature at present, and its microhardness can reach 10000HV. Other physical properties are shown in the table.
Table 1 Physical properties of diamond
Physical properties - numerical values
Hardness -60,000 to 100,000 MPa, depending on crystal direction and temperature
Bending strength -210～490MPa
Compressive strength -1500～2500MPa
Elastic modulus - (9 ~ 10.5) × 10 12 power MPa
Thermal conductivity -8.4 ~ 16.7J / cm?s? °C
Mass heat capacity -0.156J/(g?°C) (normal temperature)
Start oxidation temperature -900~1000K
Start graphitization temperature -1800K (in inert gas)
Friction coefficient between aluminum alloy and brass -0.05 to 0.07 (at normal temperature)
In the late 1970s, in the research of laser nuclear fusion technology, it was necessary to process a large number of high-precision soft metal mirrors, which required the surface roughness and shape accuracy of soft metals to reach ultra-precision levels. If the traditional grinding and polishing methods are used, not only the processing time is long, the cost is high, the operation is difficult, and the required precision is not easily achieved. Therefore, there is an urgent need to develop new processing methods. Driven by real-world demand, single-crystal diamond ultra-precision cutting technology has been rapidly developed. Due to the physical properties of single crystal diamond, it is not easy to stick to the knife and produce built-up edge when cutting. The surface quality is good. When processing non-ferrous metals, the surface roughness can reach Rz0.1~0.05μm. Diamond can also effectively process non-ferrous metal materials and non-metallic materials, such as non-ferrous metals such as copper and aluminum and their alloys, ceramics, unsintered hard alloys, various fiber and particle reinforced composite materials, plastics, rubber, graphite, glass. And a variety of wear-resistant wood (especially solid wood and plywood, MDF and other composite materials).
3 Sharpening characteristics of natural single crystal diamond tools
In ultra-precision machining, the two basic accuracies of a single crystal diamond tool are the accuracy of the blade profile and the blunt radius of the cutting edge. It is required that the circular tool cutting edge for machining an aspherical lens has a roundness of 0.05 μm or less, and the blade straightness for processing a polyhedral mirror is 0.02 μm; the blunt radius of the cutting edge (Ï value) indicates the cutting edge of the tool. Sharpness, in order to adapt to various processing requirements, the blade edge radius ranges from 20nm to 1μm.
3.1 Crystal face selection of single crystal diamond tools
Diamond crystals belong to the cubic system. Due to the difference in atomic arrangement and atomic density on each crystal plane and the difference in the distance between crystal faces, the anisotropy of natural diamond crystals is caused. Therefore, the physical and mechanical properties of diamond not only on the crystal faces are exhibited. Different, its manufacturing difficulty and service life are different, and the microscopic damage strength of each crystal face is also significantly different. The microscopic strength of diamond crystals can be determined by the Hertz test method. Since diamond is a typical brittle material, its strength value generally deviates greatly, mainly depending on the shape and distribution range of the stress distribution, so it is suitable for analysis by probability theory. When the applied stress is the same, the (110) crystal plane has the highest damage probability, the (111) crystal plane is the second, and the (100) crystal plane has the smallest probability of damage. That is, under the action of external force, the (110) crystal plane is most likely to be broken, the (111) crystal plane is second, and (100) is the most difficult to break. Although the grinding rate of the (110) crystal plane is higher than that of the (100) crystal plane, the experimental results show that the (100) crystal plane has higher resistance to stress, corrosion and thermal degradation than other crystal planes. Combined with the micro-strength comprehensive consideration, using the (100) surface as the front and back knives of the tool makes it easy to sharpen the high-quality cutting edge and is less prone to micro-cracking.
The crystal face selection of single crystal diamond tools should normally be selected according to the requirements of the tool. In general, if the diamond tool is required to obtain the highest strength, the (100) crystal face should be selected as the front and back face of the tool; if the diamond tool is required to resist mechanical wear, the (110) crystal face is selected as the front and back of the tool. If the diamond tool is required to resist chemical wear, the (110) crystal face should be used as the rake face of the tool, the (100) crystal face should be used as the flank face, or the front and back facets should be (100) crystal faces. These requirements need to be achieved by means of crystal orientation technology.
3.2 Directional method of diamond cutter
Currently, there are three main methods for crystal orientation: artificial visual orientation, laser crystal orientation, and X-ray crystal orientation.
(1) Manual visual inspection of crystal orientation
The method is based on the external crystal geometry, surface growth, corrosion characteristics of the natural crystal and the geometric relationship between the crystal faces, and the rough crystal orientation made by observation and experiment by the operator's long-term work experience. The method is simple, easy, and does not require the use of equipment, but the accuracy of the orientation result is poor, the operator's experience is high, and the tool for processing and losing the characteristics of the natural single crystal crystal can no longer be manually determined.
(2) Laser crystal orientation
The laser crystal orientation is irradiated onto the surface of the diamond crystal by a laser with better coherence. The regular crystal lattice and microscopic pits formed in the growth process are reflected on the screen. Diffracted light image. However, in fact, due to external interference factors, the regular crystal lattice and microscopic pits formed naturally are not obvious or can not be observed at all. Therefore, the crystal is subjected to appropriate artificial etching before orientation to form a characteristic topography.
(3) X-ray crystal orientation
Since the wavelength of the X-ray is close to the lattice constant of the crystal, diffraction occurs when the X-ray passes through or is reflected back from the crystal surface. A dedicated X-ray crystal orientation instrument has been developed using this principle. This crystal orientation method has high precision, but because X-rays have certain harm to the human body, it is necessary to pay attention to the protection of operators when using.
3.3 Diamond tool crystal orientation selection
The diamond is anisotropic, so not only the hardness and wear resistance of each crystal face are different, but also the wear resistance of the same crystal face in different directions. If the crystal orientation is not properly selected, the sharpening efficiency will be greatly reduced even if the crystal face is selected correctly. At the same time, since the compressive strength of the diamond crystal is 5-7 times larger than the tensile strength, the easy-grinding direction of the crystal face should be selected during the sharpening process, and the cutting edge should face the positive direction of the sharpening wheel speed (ie, take Back grinding) to ensure sharpening efficiency and reduce the degree of microscopic cleavage of the cutting edge.
3.4 Grinding and damage of diamond cutters
The wear mechanism of diamond cutters is complex and can be divided into macroscopic wear and microscopic wear. The former is mainly mechanical wear, while the latter is mainly based on thermal chemical wear. Common diamond tool wear and tear forms are flank wear, flank wear and edge cracking. In the single crystal diamond tool sharpening process, it needs to be worn to sharpen the tool that meets the requirements, but if the unnecessary wear is generated, the sharpened front and back flank surfaces may be damaged. The edge cracking (ie, chipping) occurs when the stress on the cutting edge exceeds the local bearing capacity of the diamond tool, and is generally caused by microscopic cleavage damage of the diamond crystal along the (111) crystal plane. In ultra-precision machining, the cutting edge of the diamond cutter has a relatively small radius, which is itself a hard and brittle material. At the same time, due to its anisotropy and the (111) surface is prone to cleavage, along with the vibration and the grinding wheel to the cutting edge The impact of the mouth is often accompanied by a chipping phenomenon.
4 sharpening test
The test was carried out on an EWAGRS-12 sharpener. In the test, due to the lack of effective crystal orientation means, only through the structural analysis of the scrapped tool, the direction of the crystal plane of the tool is roughly determined, and then the contact force and contact sound of the tool and the surface of the grinding wheel during the sharpening process are taken into consideration, and the speed of the grinding wheel is taken into consideration. The parameters such as the reciprocating speed of the spindle and the swing amplitude are carefully searched for the appropriate sharpening angle of the tool. When the sound of the sharpening is more boring and the machine tool has a large vibration, the tool should be immediately withdrawn to avoid damage to the grinding wheel and re-adjust the angle. After the adjustment is appropriate, the sound of the sharpening is lighter and softer, the vibration of the hand-feeling machine is small, and the continuous knife is 0.05mm, and the machine does not have vibration fluctuations.
Through the comparison of the various sharpening conditions, it is determined that the main cutting edge and the minor cutting edge are more reasonable. The direction of the grinding wheel should be directed to the direction of the blade pressing direction and form an angle of 15-30o. According to the machine tool data and considering the material removal rate and grinding ratio, the recommended grinding wheel speed is 8~65m/s. It is found through experiments that the grinding effect is best when the grinding wheel speed is 22~28m/s; the Rt value of the cutting edge is the smallest when the speed is 15m/s. Therefore, in the actual sharpening process, the cutter head is placed in the area around the grinding disc φ140, the grinding wheel speed is selected to be 2100 rev/min during rough grinding, and the grinding wheel speed is selected to be 1000 rev/min during fine grinding to ensure coarse grinding. The grinding wheel speed is about 23m/s, and it is about 15m/s during fine grinding. The reciprocating swing amplitude of the main shaft should not be too large, generally it is slightly wider than the sharpening edge, and the swing frequency should not be too fast.
In order to obtain an economical sharpening effect, the grinding contact pressure needs to increase as the blade length increases. In rough grinding, as the contact pressure increases, a positive abrupt change in material removal rate occurs. In super-fine grinding, the material removal rate increases gradually with the increase of contact pressure. When the contact pressure increases to 180N, the material removal rate gradually decreases. When the contact pressure between the cutter and the grinding disc during fine polishing is 12 to 14N, it is most beneficial to ensure the surface finish of the sharpening surface. Therefore, the tool should have proper contact force with the surface of the grinding wheel during sharpening. When rough grinding, try to use the pressure control of the machine tool. After the tool is clamped, the tool should be applied as soon as possible, and press the machine position shifting lever (the lever is used to manipulate the table to convert between the working position and the measuring position) to ensure Large contact force is required to avoid causing machine tool vibration to cause chipping.
5 Conclusion
For the anisotropy of diamond crystals, accurate crystal orientation is required before sharpening. At the same time, the temperature, machine vibration, grinding wheel granularity, rotation speed and reciprocating speed should be strictly controlled during the sharpening process. Grinding equipment with high rotation precision and grinding disc with high plane precision should be selected to avoid the hard and brittleness and poor heat of diamond crystal. Unnecessary grinding and damage due to stability. In addition, the problem of manual pressure can not be solved to ensure the stability of the pressure, and the detection method and the detection instrument matched with the sharpening process are matched to ensure the quality stability of the sharpening.
In ultra-precision machining, the main factors to ensure the quality of the machined surface are high-quality tools, in addition to high-precision machine tools and ultra-stable machining environments. Natural diamond has high hardness, good wear resistance, high strength, good thermal conductivity, low friction coefficient with non-ferrous metals, good anti-adhesion and excellent corrosion resistance and chemical stability. It can sharpen sharp edges and be sharpened. It is considered to be the most ideal tool material for ultra-precision cutting, and has an important position in the field of machining, especially in the field of ultra-precision machining.
2 Physical properties of single crystal diamond
Diamond is a crystal of a single carbon atom, and its crystal structure belongs to the equiaxed face-centered cubic system (a system with the highest atom density). Since the bond between carbon atoms in diamond is sp3 hybrid covalent bond, it has strong binding force, stability and directionality. It is the hardest substance known in nature at present, and its microhardness can reach 10000HV. Other physical properties are shown in the table.
Table 1 Physical properties of diamond
Physical properties - numerical values
Hardness -60,000 to 100,000 MPa, depending on crystal direction and temperature
Bending strength -210～490MPa
Compressive strength -1500～2500MPa
Elastic modulus - (9 ~ 10.5) × 10 12 power MPa
Thermal conductivity -8.4 ~ 16.7J / cm?s? °C
Mass heat capacity -0.156J/(g?°C) (normal temperature)
Start oxidation temperature -900~1000K
Start graphitization temperature -1800K (in inert gas)
Friction coefficient between aluminum alloy and brass -0.05 to 0.07 (at normal temperature)
In the late 1970s, in the research of laser nuclear fusion technology, it was necessary to process a large number of high-precision soft metal mirrors, which required the surface roughness and shape accuracy of soft metals to reach ultra-precision levels. If the traditional grinding and polishing methods are used, not only the processing time is long, the cost is high, the operation is difficult, and the required precision is not easily achieved. Therefore, there is an urgent need to develop new processing methods. Driven by real-world demand, single-crystal diamond ultra-precision cutting technology has been rapidly developed. Due to the physical properties of single crystal diamond, it is not easy to stick to the knife and produce built-up edge when cutting. The surface quality is good. When processing non-ferrous metals, the surface roughness can reach Rz0.1~0.05μm. Diamond can also effectively process non-ferrous metal materials and non-metallic materials, such as non-ferrous metals such as copper and aluminum and their alloys, ceramics, unsintered hard alloys, various fiber and particle reinforced composite materials, plastics, rubber, graphite, glass. And a variety of wear-resistant wood (especially solid wood and plywood, MDF and other composite materials).
3 Sharpening characteristics of natural single crystal diamond tools
In ultra-precision machining, the two basic accuracies of a single crystal diamond tool are the accuracy of the blade profile and the blunt radius of the cutting edge. It is required that the circular tool cutting edge for machining an aspherical lens has a roundness of 0.05 μm or less, and the blade straightness for processing a polyhedral mirror is 0.02 μm; the blunt radius of the cutting edge (Ï value) indicates the cutting edge of the tool. Sharpness, in order to adapt to various processing requirements, the blade edge radius ranges from 20nm to 1μm.
3.1 Crystal face selection of single crystal diamond tools
Diamond crystals belong to the cubic system. Due to the difference in atomic arrangement and atomic density on each crystal plane and the difference in the distance between crystal faces, the anisotropy of natural diamond crystals is caused. Therefore, the physical and mechanical properties of diamond not only on the crystal faces are exhibited. Different, its manufacturing difficulty and service life are different, and the microscopic damage strength of each crystal face is also significantly different. The microscopic strength of diamond crystals can be determined by the Hertz test method. Since diamond is a typical brittle material, its strength value generally deviates greatly, mainly depending on the shape and distribution range of the stress distribution, so it is suitable for analysis by probability theory. When the applied stress is the same, the (110) crystal plane has the highest damage probability, the (111) crystal plane is the second, and the (100) crystal plane has the smallest probability of damage. That is, under the action of external force, the (110) crystal plane is most likely to be broken, the (111) crystal plane is second, and (100) is the most difficult to break. Although the grinding rate of the (110) crystal plane is higher than that of the (100) crystal plane, the experimental results show that the (100) crystal plane has higher resistance to stress, corrosion and thermal degradation than other crystal planes. Combined with the micro-strength comprehensive consideration, using the (100) surface as the front and back knives of the tool makes it easy to sharpen the high-quality cutting edge and is less prone to micro-cracking.
The crystal face selection of single crystal diamond tools should normally be selected according to the requirements of the tool. In general, if the diamond tool is required to obtain the highest strength, the (100) crystal face should be selected as the front and back face of the tool; if the diamond tool is required to resist mechanical wear, the (110) crystal face is selected as the front and back of the tool. If the diamond tool is required to resist chemical wear, the (110) crystal face should be used as the rake face of the tool, the (100) crystal face should be used as the flank face, or the front and back facets should be (100) crystal faces. These requirements need to be achieved by means of crystal orientation technology.
3.2 Directional method of diamond cutter
Currently, there are three main methods for crystal orientation: artificial visual orientation, laser crystal orientation, and X-ray crystal orientation.
(1) Manual visual inspection of crystal orientation
The method is based on the external crystal geometry, surface growth, corrosion characteristics of the natural crystal and the geometric relationship between the crystal faces, and the rough crystal orientation made by observation and experiment by the operator's long-term work experience. The method is simple, easy, and does not require the use of equipment, but the accuracy of the orientation result is poor, the operator's experience is high, and the tool for processing and losing the characteristics of the natural single crystal crystal can no longer be manually determined.
(2) Laser crystal orientation
The laser crystal orientation is irradiated onto the surface of the diamond crystal by a laser with better coherence. The regular crystal lattice and microscopic pits formed in the growth process are reflected on the screen. Diffracted light image. However, in fact, due to external interference factors, the regular crystal lattice and microscopic pits formed naturally are not obvious or can not be observed at all. Therefore, the crystal is subjected to appropriate artificial etching before orientation to form a characteristic topography.
(3) X-ray crystal orientation
Since the wavelength of the X-ray is close to the lattice constant of the crystal, diffraction occurs when the X-ray passes through or is reflected back from the crystal surface. A dedicated X-ray crystal orientation instrument has been developed using this principle. This crystal orientation method has high precision, but because X-rays have certain harm to the human body, it is necessary to pay attention to the protection of operators when using.
3.3 Diamond tool crystal orientation selection
The diamond is anisotropic, so not only the hardness and wear resistance of each crystal face are different, but also the wear resistance of the same crystal face in different directions. If the crystal orientation is not properly selected, the sharpening efficiency will be greatly reduced even if the crystal face is selected correctly. At the same time, since the compressive strength of the diamond crystal is 5-7 times larger than the tensile strength, the easy-grinding direction of the crystal face should be selected during the sharpening process, and the cutting edge should face the positive direction of the sharpening wheel speed (ie, take Back grinding) to ensure sharpening efficiency and reduce the degree of microscopic cleavage of the cutting edge.
3.4 Grinding and damage of diamond cutters
The wear mechanism of diamond cutters is complex and can be divided into macroscopic wear and microscopic wear. The former is mainly mechanical wear, while the latter is mainly based on thermal chemical wear. Common diamond tool wear and tear forms are flank wear, flank wear and edge cracking. In the single crystal diamond tool sharpening process, it needs to be worn to sharpen the tool that meets the requirements, but if the unnecessary wear is generated, the sharpened front and back flank surfaces may be damaged. The edge cracking (ie, chipping) occurs when the stress on the cutting edge exceeds the local bearing capacity of the diamond tool, and is generally caused by microscopic cleavage damage of the diamond crystal along the (111) crystal plane. In ultra-precision machining, the cutting edge of the diamond cutter has a relatively small radius, which is itself a hard and brittle material. At the same time, due to its anisotropy and the (111) surface is prone to cleavage, along with the vibration and the grinding wheel to the cutting edge The impact of the mouth is often accompanied by a chipping phenomenon.
4 sharpening test
The test was carried out on an EWAGRS-12 sharpener. In the test, due to the lack of effective crystal orientation means, only through the structural analysis of the scrapped tool, the direction of the crystal plane of the tool is roughly determined, and then the contact force and contact sound of the tool and the surface of the grinding wheel during the sharpening process are taken into consideration, and the speed of the grinding wheel is taken into consideration. The parameters such as the reciprocating speed of the spindle and the swing amplitude are carefully searched for the appropriate sharpening angle of the tool. When the sound of the sharpening is more boring and the machine tool has a large vibration, the tool should be immediately withdrawn to avoid damage to the grinding wheel and re-adjust the angle. After the adjustment is appropriate, the sound of the sharpening is lighter and softer, the vibration of the hand-feeling machine is small, and the continuous knife is 0.05mm, and the machine does not have vibration fluctuations.
Through the comparison of the various sharpening conditions, it is determined that the main cutting edge and the minor cutting edge are more reasonable. The direction of the grinding wheel should be directed to the direction of the blade pressing direction and form an angle of 15-30o. According to the machine tool data and considering the material removal rate and grinding ratio, the recommended grinding wheel speed is 8~65m/s. It is found through experiments that the grinding effect is best when the grinding wheel speed is 22~28m/s; the Rt value of the cutting edge is the smallest when the speed is 15m/s. Therefore, in the actual sharpening process, the cutter head is placed in the area around the grinding disc φ140, the grinding wheel speed is selected to be 2100 rev/min during rough grinding, and the grinding wheel speed is selected to be 1000 rev/min during fine grinding to ensure coarse grinding. The grinding wheel speed is about 23m/s, and it is about 15m/s during fine grinding. The reciprocating swing amplitude of the main shaft should not be too large, generally it is slightly wider than the sharpening edge, and the swing frequency should not be too fast.
In order to obtain an economical sharpening effect, the grinding contact pressure needs to increase as the blade length increases. In rough grinding, as the contact pressure increases, a positive abrupt change in material removal rate occurs. In super-fine grinding, the material removal rate increases gradually with the increase of contact pressure. When the contact pressure increases to 180N, the material removal rate gradually decreases. When the contact pressure between the cutter and the grinding disc during fine polishing is 12 to 14N, it is most beneficial to ensure the surface finish of the sharpening surface. Therefore, the tool should have proper contact force with the surface of the grinding wheel during sharpening. When rough grinding, try to use the pressure control of the machine tool. After the tool is clamped, the tool should be applied as soon as possible, and press the machine position shifting lever (the lever is used to manipulate the table to convert between the working position and the measuring position) to ensure Large contact force is required to avoid causing machine tool vibration to cause chipping.
5 Conclusion
For the anisotropy of diamond crystals, accurate crystal orientation is required before sharpening. At the same time, the temperature, machine vibration, grinding wheel granularity, rotation speed and reciprocating speed should be strictly controlled during the sharpening process. Grinding equipment with high rotation precision and grinding disc with high plane precision should be selected to avoid the hard and brittleness and poor heat of diamond crystal. Unnecessary grinding and damage due to stability. In addition, the problem of manual pressure can not be solved to ensure the stability of the pressure, and the detection method and the detection instrument matched with the sharpening process are matched to ensure the quality stability of the sharpening.
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