Study on the grinding characteristics of the hotte

Research on the grinding characteristics of single crystal diamond tools

1 introduction

in ultra precision machining, in addition to high-precision machine tools and ultra stable machining environment, high-quality tools are also very important. How to maintain Jinan gold testing machine. Natural diamond has high hardness, good wear resistance, high strength, good thermal conductivity, low friction coefficient with non-ferrous metals, good adhesion resistance, excellent corrosion resistance and chemical stability. It can grind extremely sharp edges. It is considered to be the most ideal tool material for ultra precision cutting. It plays an important role in the field of machining, especially in the field of ultra precision machining, and has been widely used

2 physical properties of single crystal diamond

diamond is the crystal of a single carbon atom, and its crystal structure belongs to the equiaxed face centered cubic crystal system (a crystal system with the highest atomic density). Because the bond between carbon atoms in diamond is SP3 hybrid covalent bond, it has strong binding force, stability and directivity. It is the hardest substance known in nature at present, and its microhardness can reach 10000hv. See the table for other physical properties

Table 1 physical properties of diamond are not easy to absorb water

physical properties - numerical value

hardness - 60000 ~ 100000mpa, depending on crystal direction and temperature

flexural strength - 210 ~ 490mpa

compressive strength - 1500 ~ 2500mpa

modulus of elasticity - (9 ~ 10.5) × 10 to the 12th power MPa

thermal conductivity - 8.4 ~ 16.7j/cm · s ·℃

mass heat capacity - 0.156j/(g ·℃) (normal temperature)

starting oxidation temperature - 900 ~ 1000K

starting graphitization temperature - 1800K (in inert gas)

friction coefficient between aluminum alloy and brass - 0.05 ~ 0.07 (at normal temperature)

in the late 1970s, in the research of laser nuclear fusion technology, a large number of high-precision soft wares need to be processed.Metallic reflector, The surface roughness and shape accuracy of soft metal are required to reach the ultra precision level. If the traditional grinding and polishing processing methods are adopted, not only the processing time is long, the cost is high, the operation is difficult, but also it is not easy to achieve the required accuracy. Therefore, it is urgent to develop new processing methods. Driven by the actual demand, the ultra precision cutting technology of single crystal diamond has developed rapidly. Due to the physical characteristics of single crystal diamond itself, it is not easy to stick the knife and produce chip nodules during cutting, and the machined surface quality is good. When machining non-ferrous metals, the surface roughness can reach rz0.1 ~ 0.05 μ m。 Diamond can also effectively process non-ferrous materials and non-metallic materials, such as non-ferrous metals such as copper and aluminum and their alloys, ceramics, unsintered cemented carbides, various fiber and particle reinforced composites, plastics, rubber, graphite, glass and various wear-resistant wood (especially solid core wood, plywood, MDF and other composites)

3 grinding characteristics of natural single crystal diamond tools

in ultra precision machining, the two basic accuracy of single crystal diamond tools are the accuracy of the blade contour and the blunt radius of the blade. It is required that the roundness of the cutting edge of the arc cutter for Machining Aspheric lens is 0.05 μ M or less, the straightness of the blade used for processing polyhedral reflector is 0.02 μ m； Blunt circle radius of tool edge（ ρ Value) indicates the sharpness of the tool edge. In order to meet various processing requirements, the radius of the tool edge ranges from 20nm to 1 μ m。

3.1 crystal face selection of single crystal diamond tools

diamond crystal belongs to cubic crystal system. Due to the different atomic arrangement form and atomic density on each crystal face and the different distance between crystal faces, the anisotropy of natural diamond crystal is caused. Therefore, diamond not only has different physical and mechanical properties, but also its manufacturing difficulty and service life, The micro damage strength of each crystal plane also has obvious differences. The micro strength of diamond crystal can be measured by Hertz test method. Because diamond is a typical brittle material, its strength value generally has a large deviation, which mainly depends on the shape and distribution range of stress distribution, so it is suitable to be analyzed by probability theory. When the applied stress is the same, the failure probability of (110) crystal plane is the largest, (111) crystal plane is the second, and the failure probability of (100) crystal plane is the smallest. That is, under the action of external force, (110) crystal plane is the most vulnerable, (111) crystal plane is the second, and (100) is the least vulnerable. Although the grinding rate of (110) crystal surface is higher than that of (100), the experimental results show that (100) crystal surface has higher resistance to stress, corrosion and thermal degradation than other crystal surfaces. Considering the micro strength comprehensively, it is easy to grind high-quality cutting edges with (100) surface as the front and rear cutting surfaces of the tool, and it is not easy to produce micro edge collapse

generally, the crystal face of single crystal diamond tools should be selected according to the requirements of the tools. Generally speaking, if the diamond tool is required to obtain the highest strength, the (100) crystal surface should be selected as the front and rear cutting surface of the tool; If the diamond tool is required to be resistant to mechanical wear, the (110) crystal surface is selected as the front and rear cutting surface of the tool; If the diamond tool is required to be resistant to chemical wear, the (110) crystal surface should be used as the front cutting surface of the tool, (100) crystal surface as the back cutting surface, or both the front and back cutting surfaces should use (100) crystal surface. These requirements need the help of crystal orientation technology to achieve

3.2 orientation methods of diamond tools

at present, there are three main methods of crystal orientation: artificial visual crystal orientation, laser crystal orientation and X-ray crystal orientation

(1) artificial visual crystal orientation

this method is based on the external geometry of natural crystals, surface growth, corrosion characteristics and the geometric angle relationship between the crystal planes, relying on the operator's long-term work experience, through observation and test to make a rough crystal orientation. This method is simple, easy, and does not need equipment, but the accuracy of orientation results is poor, which requires high operator experience, and for the processed tools that have lost the characteristics of natural single crystal crystals, it is impossible to carry out artificial visual orientation

(2) laser crystal orientation

laser crystal orientation is to irradiate the surface of diamond crystal with a laser with good coherence. The regular crystal grains and micro pits formed in the growth process on the surface in different crystal directions are reflected on the screen to form a characteristic diffraction light image. But in fact, due to external interference factors, the naturally formed regular crystal planes, crystal lines and micro pits are often not obvious or can not be observed at all. Therefore, this kind of crystal should undergo appropriate artificial corrosion before orientation to form a characteristic morphology

(3) X-ray crystal orientation

as the wavelength of X-ray is close to the lattice constant of the crystal, diffraction will occur when X-ray passes through the crystal or is reflected from the crystal surface. Using this principle, a special X-ray crystal orientation instrument has been developed. This crystal orientation method has high accuracy, but because X-rays are harmful to human body, it is necessary to pay attention to the protection of operators when using

3.3 crystal direction selection of diamond tools

diamond anisotropy, so not only the hardness and wear resistance of each crystal surface are different, but also the wear resistance of the same crystal surface in different directions is different. If the crystal direction is selected improperly, even if the crystal surface is selected correctly, the grinding efficiency will be greatly reduced. At the same time, because the compressive strength of diamond crystal is 5 ~ 7 times greater than the tensile strength, the easy grinding direction of crystal surface should be selected in the grinding process, and the edge should face the positive direction of the linear speed of the grinding wheel (i.e. reverse grinding) to ensure the grinding efficiency and reduce the micro cleavage degree of the edge

3.4 grinding and damage of diamond tools

the wear mechanism of diamond tools is relatively complex, which can be divided into macro wear and micro wear. The former is mainly mechanical wear, and the latter is mainly thermochemical wear. The common wear patterns of diamond tools are front face wear, back face wear and edge cracking. In the grinding process of single crystal diamond tools, it is necessary to wear them to grind the tools that meet the requirements, but if there is unnecessary wear, it may damage the sharpened front and rear tool surfaces. The edge cracking (i.e. edge cracking) occurs when the stress on the edge exceeds the local bearing capacity of the diamond tool. It is generally caused by the micro cleavage damage of the diamond crystal along the (111) crystal plane. In ultra precision machining, the blunt radius of the cutting edge of diamond tools is relatively small, and it itself belongs to hard and brittle materials. At the same time, due to its anisotropy and the cleavage of (111) surface, with the vibration and the impact of grinding wheel sand particles on the cutting edge of tools, edge collapse often occurs

4 grinding test

the test is carried out on EWAG RS swing angle -12 knife grinder. In the test, due to the lack of effective crystal orientation means, only through the structural analysis of the scrapped tool, can the crystal direction of the tool be roughly determined, and then through the contact force, contact sound and other information between the tool and the grinding wheel surface during the grinding process, taking into account the grinding wheel speed, spindle reciprocating speed, swing and other parameters, carefully look for the appropriate grinding angle of the tool. When the sound of grinding is dull and laborious, and the hand feels that the machine tool has large vibration, the tool should be withdrawn immediately to prevent the tool body from damaging the grinding wheel, and the angle should be readjusted. After proper adjustment, the grinding sound is light and soft. The hand feels that the vibration of the machine tool is small, and the continuous cutting is 0.05mm, and the machine tool will not have vibration fluctuations

through the comparison of the grinding conditions of each time, it is determined that the more reasonable grinding direction of the main cutting edge and the auxiliary cutting edge is that the rotation direction of the grinding wheel should point to the pressure direction of the cutting edge, and form a 15 ~ 30 angle with it. According to the machine tool data and considering the material removal rate and grinding ratio, the recommended grinding wheel speed is 8 ~ 65m/s. Through the test, it is found that the grinding effect is the best when the grinding wheel speed is 22 ~ 28m/s; When the speed is 15 m/s, the RT value of the cutting edge is the smallest. Therefore, in the actual grinding process, place the cutter head on the grinding disc φ In the area around 140, select the wheel speed of 2100rev/min for rough grinding and 1000rev/min for fine grinding to ensure that the wheel speed is about 23m/s for rough grinding and about 15 m/s for fine grinding. The reciprocating swing amplitude of the main shaft should not be too large, which is generally slightly wider than the width of the sharpening edge, and the swing frequency should not be too fast

in order to obtain economical grinding effect, the grinding contact pressure needs to increase with the increase of blade length. In rough grinding, with the increase of contact pressure, there will be a positive mutation of material removal rate. In ultra precision grinding, the material removal rate first increases gradually with the increase of contact pressure, and then decreases gradually when the contact pressure increases to 180n. When the contact pressure between the tool and the grinding disc is 12 ~ 14N during fine polishing, it is most conducive to ensure the surface finish of the grinding surface. Therefore, there should be appropriate contact force between the tool and the grinding wheel surface during grinding. During rough grinding, try to use the pressure control of the machine tool. After setting the tool, load the tool as soon as possible, and press and hold the displacement control lever of the machine tool (the lever is used to control the workbench to switch between the working position and the measuring position), so as to ensure the required large contact force and avoid causing the vibration of the machine tool and leading to edge collapse

5 conclusion

in view of the anisotropic characteristics of diamond crystals, accurate crystal orientation is required before grinding. At the same time, during the grinding process, the temperature, machine vibration, grinding wheel particle size, rotating speed and reciprocating speed should be strictly controlled, and the grinding equipment with high rotation accuracy and the grinding disc with high plane accuracy should be selected to avoid unnecessary grinding due to the hard brittleness and poor thermal stability of diamond crystals