Design and application of the hottest single cryst

2022-08-17
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Design and application of single crystal diamond tools

diamond is the crystal of a single carbon atom, and its crystal structure belongs to the equiaxed face centered cubic system with the highest atomic density. The bond between carbon atoms in diamond crystal is SP3 hybrid covalent bond, which has strong binding force, stability and directivity. The unique crystal structure of diamond makes it have the highest hardness, rigidity, refractive index and thermal conductivity in nature, as well as extremely high wear resistance, corrosion resistance and chemical stability

The excellent characteristics of single crystal diamond can meet most of the requirements of precision and ultra precision cutting for tool materials, and it is an ideal tool material for precision cutting. The uniform crystal structure of diamond without internal grain boundary makes the cutting edge theoretically reach the flatness and sharpness of atomic level, and the cutting ability is strong, the precision is high, and the cutting force is small; Its high hardness, good wear resistance, corrosion resistance and chemical stability can ensure that the tool has a long service life, so that it can carry out continuous cutting for a long time, and reduce the impact of tool wear on the accuracy of parts; Its high thermal conductivity can reduce the cutting temperature and the thermal deformation of parts

Single crystal diamond tools play an important role in the field of machining, and are widely used in the processing of ultra precision mirror parts such as mirrors, navigation gyroscopes of missiles and rockets, computer hard disk substrates, accelerator electron guns and so on. Single crystal diamond can also be used to make medical knives such as ophthalmology, brain surgery scalpels, ultra-thin biological slicers and so on. In addition, the application of single crystal diamond tools in the processing of civil products is also becoming more and more extensive. From the processing of watch parts, aluminum pistons, jewelry, etc. to the processing of pens, high gloss signs and non-ferrous metal mirror decorative parts, its application has entered a variety of fields of machining

this paper focuses on the design principle and application technology of single crystal diamond tools

1. Mirror machining mechanism of single crystal diamond tools

the residual area height after cutting h=f/(ctgkr+ctgkr') is the theoretical roughness value of the machined surface, which decreases with the decrease of feed rate F, tool main deflection angle Kr and auxiliary deflection angle kr'. Since the reduction of the main deflection angle Kr will rapidly increase the FY force and cause tool vibration, and reducing the feed rate f will affect the cutting efficiency, the surface roughness value is generally reduced by reducing the secondary deflection angle kr'

the surface of the conventional cutting tool is rough and the flatness of the blade is poor. If the selected secondary deflection angle is too small, on the one hand, the unevenness of the secondary cutting edge will be reflected on the machined surface; On the other hand, it will also aggravate the friction between the auxiliary flank and the machined surface, which will "roughen" the machined surface. Therefore, in conventional cutting, when the secondary deflection angle is about 2 °, the machined surface roughness value is the smallest, and further reducing the secondary deflection angle will deteriorate the machined surface quality

the surface roughness value of single crystal diamond tools can be less than ra0.01 m, the blade quality can at least reach no defects under 100 times microscope, and the friction coefficient is very small, so the limit value of the secondary deflection angle can be reduced to 0 ~ 2 ', so that the theoretical value of the machined surface roughness can be close to or equal to zero, and the actual surface roughness value can meet the requirements of mirror or super smooth surface

it can be seen that as far as the tool itself is concerned, the mirror machining mechanism of single crystal diamond tools is to obtain the mirror machining effect by making the theoretical value of the machined surface roughness close to zero through the action of the super smooth surface of the tool and the flawless auxiliary cutting edge (polishing edge)

2 design of single crystal diamond tools

the main factors to be considered in the design of single crystal diamond tools are: ① accuracy requirements of machined parts; ② Actual processing conditions; ③ Characteristics of diamond materials

when designing single crystal diamond tools, the following principles should be followed: ① due to the high hardness and difficult processing of single crystal diamond, the shape of the tools should be as simple as possible; ② According to the characteristics of high brittleness and poor impact resistance of single crystal diamond, the impact resistance of the tool head should be improved by optimizing the geometry of the tool in combination with the actual processing conditions; ③ The length of the polishing blade shall be designed according to the accuracy requirements of the machined parts, and the thinning ability of the tool shall be considered at the same time

next, taking several common tools as examples, the design scheme of single crystal diamond tools is discussed

turning tool for computer hard disk substrate

computer hard disk substrate is used for information storage. Its material is aluminum or aluminum alloy. The lower the surface roughness value of the substrate, the higher the storage density. Therefore, it is very critical to reduce its machined surface roughness value. At the same time, because the thickness of the substrate is less than 0.9mm, in order to prevent the deformation of the Malaysian Civil Aviation Administration Director Azhar Harding said yesterday that the cutting force and extrusion during processing should be minimized

ultra high precision disk lathe with good rigidity and stability is used for cutting hard disk substrates. The cutting depth is about 0.01 ~ 0.02mm, and the feed rate is about 0.5mm

the shape of the tool head of the single crystal diamond turning tool on the hard disk substrate is shown in Figure 1. The cutter head has two main cutting edges, which can feed left and right during machining. The longer polishing blade can ensure that the wear of the back surface of the polishing blade will not interfere with each other when machining in the left and right directions. The two main rake faces are inclined downward by 5 ° in the feed direction, so as to obtain a negative rake angle of about 2.5 °, so that the chips flow to the surface to be machined, so as to avoid the chips scratching the machined surface. The diamond turning tool is very sharp, and the arc radius of the blade is less than 100nm. The 5 ° back angle can fully reduce the extrusion and friction between the back surface and the machined surface

Figure 1 shape of hard disk substrate turning tool head

the key quality requirement of this tool is that the blade and tip are free of defects under 500 times microscope. The design principle is also applicable to other ultra precision mirror machining tools

jewelry applicator

jewelry applicator is used to mill tree leaf patterns on gold and silver jewelry. It is actually a forming milling cutter, and its head shape is shown in Figure 2. The knife tip angle is between 110 ° ~ 150 ° to adapt to the processing of patterns of different sizes and depths. Due to the simple structure, poor rigidity and large vibration of the patterning machine tool, and the poor processing conditions of intermittent dry cutting, the patterning tool head needs to have strong impact resistance. A negative front angle of 5 ° and a back angle of 1 ° ~ 1.5 ° can effectively increase the strength of the blade. At the same time, a small back angle can also make the tool and the workpiece squeeze properly, so that the cutting will not be "virtual", and prevent the cutting vibration from forming "vibration lines" on the machined surface

Figure 2 shape of the head of the jewelry applicator

the design principle of the jewelry applicator is roughly the same as that of the tool used to process watch cases and some high gloss decorative parts. This kind of tool requires that the blade should be observed under a 200 ~ 500 times microscope without chipping

contact lens turning tool

the material used for contact lenses is very soft and has a certain tensile strength. General material tools summarize the successful experience in the past. Because the arc of the blade is too large, it is difficult to cut it. Since there is another grinding process after turning, the surface roughness of turning is only required to reach ra0.1 m. The main purpose of turning is to obtain a concave convex arc surface with certain shape accuracy, and the main requirement for the tool is the sharpness of the blade. The shape of the cutter head of the contact lens turning tool is shown in Figure 3. In order to meet the needs of cutting concave arc surface, the back angle of 15 ° is adopted, and the blade is required to observe without chipping under a 100 times microscope

Figure 3 contact lens turning tool head shape

3 use and maintenance of single crystal diamond tools

single crystal diamond tools are brittle, and the blade is very sharp, which is easy to collapse when impacted, so they should be used under stable and vibration free working conditions as far as possible; At the same time, the clamping rigidity of workpiece and cutter and the rigidity of the whole system should be improved as much as possible to increase its anti vibration ability. The cutting amount should not exceed 0.1mm

higher cutting speed can reduce the cutting force, while low-speed cutting will increase the cutting force, thus accelerating the failure of tool edge collapse. Therefore, the cutting speed should not be too low when using diamond tools

avoid contacting the diamond tool with the workpiece or other hard objects in the static state to prevent damaging the cutting edge of the tool. Hard objects such as sand particles are easy to be entrained in the fingernails and fingerprints of operators. Scraping the blade directly with fingernails or fingers may also damage the diamond blade. Non contact measurement methods such as optical instruments should be used for the detection and adjustment of diamond tools

the diamond cutter should be cleaned with absorbent cotton and an appropriate amount of alcohol or acetone. When the cutter is not in use, it should be covered with a rubber or plastic protective sleeve and placed in a separate knife box

when using a straight-line polishing edge diamond tool for mirror cutting, the secondary deflection angle is required to be less than a few minutes, and the installation error of the tool may be up to 1 ° during the installation process, so the tool needs to be aligned before machining, and the best HRD machining surface roughness can be obtained by adjusting the parallelism between the polishing edge of the tool and the feeding direction. The tool setting method is as follows: first, machine a small section of surface on the workpiece, and then adjust the tool under a 10 ~ 30 times microscope to make the polishing edge parallel to its mirror image in the new machined surface. Due to the high requirements for parallelism, patient and precise repeated adjustment must be carried out. It should be noted that when screwing the clamping screw of the tool bar, the tool may rotate slightly, resulting in tool setting failure

because the tool setting process of the straight-line polishing tool is time-consuming and laborious, for some methods that require a slight analysis of the iron content in the lubricating oil, the circular arc edge tool with a radius of 10 ~ 30mm can be used to replace the straight-line polishing tool for mirror cutting with low accuracy requirements. The shape of this circular arc cutting tool is similar to that of the contact lens turning tool, but the back angle is reduced and the circular arc radius of the tip is increased. Using this tool can not only simplify the tool setting process, but also when one arc is worn, another new arc can be used after slightly rotating the tool, thus increasing the tool life. High precision single crystal diamond arc cutter is also a necessary tool for machining concave mirrors. (end)

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