Rhombus is a crystal of a single carbon atom, and its crystal structure is the equiaxed face-centered cubic system with the highest atomic density. The bond between carbon atoms in the diamond crystal is sp3 hybrid covalent bond, which has strong binding force, stability and directionality. Diamond's unique crystal structure gives it the highest hardness, rigidity, refractive index and thermal conductivity in nature, as well as extremely high abrasion, corrosion and chemical stability.
The excellent properties of single crystal diamond can meet most of the requirements of precision and ultra-precision cutting tool materials, and it is an ideal material for precision cutting tools. The uniform crystal structure of diamond without internal grain boundary makes the cutting edge of the tool theoretically reach the atomic level flatness and sharpness. The cutting ability is strong, the precision is high, the cutting force is small; its high hardness and good wear resistance Strain, corrosion resistance and chemical stability ensure long tool life for long-term continuous cutting and reduce the impact of tool wear on part accuracy; its high thermal conductivity reduces cutting temperatures and parts Thermal deformation.
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, missiles and rocket navigation gyroscopes, computer hard disk substrates, and accelerator electron guns. Single crystal diamond can also be used in the manufacture of medical tools such as ophthalmology, brain surgery scalpels, and ultra-thin bio-dissection knives. In addition, the application of single crystal diamond tools in the processing of civilian products has become increasingly widespread, from the processing of watch parts, aluminum pistons, jewelry, etc. to the processing of pens, high-grip signs and non-ferrous metal mirror decorative parts, the application has entered the mechanical processing A variety of fields.
This article focuses on the design principles and application techniques of single crystal diamond tools.
1. Mirror processing mechanism of single crystal diamond cutter
The residual area height h=f/(ctgkr+ctgkr') after cutting is the theoretical roughness value of the machined surface, which decreases with the feed amount f, the tool lead angle kr and the secondary declination kr'. Reduced. Since the decrease of the lead angle kr causes the Fy force to increase rapidly to cause tool vibration, and the decrease of the feed amount f affects the cutting efficiency, the surface roughness value is generally reduced by reducing the secondary declination angle kr'.
The surface of the conventional cutting tool is rough and the edge of the cutting edge is poor. If the secondary declination is too small, the unevenness of the minor cutting edge will be reproduced on the machined surface. On the other hand, the secondary flank face will be intensified. The friction of the machined surface "pulls" the machined surface. Therefore, in conventional cutting, when the secondary declination is about 2°, the surface roughness value is the smallest, and further reducing the secondary declination causes the surface quality to deteriorate.
The surface roughness value of the single crystal diamond tool can be less than Ra0.01μm, the blade edge quality can be at least 100% under the microscope to observe no defects, and the friction coefficient is extremely small, so the auxiliary declination limit can be reduced to 0~2' Therefore, the theoretical value of the machined surface roughness can be made close to or equal to zero, and the actual surface roughness value can reach the requirements of a mirror surface or an ultra-smooth surface.
It can be seen that, in terms of the tool itself, the mirror surface machining mechanism of the single crystal diamond tool is that the theoretical value of the machined surface roughness is close to zero by the action of the ultra-clear surface of the tool and the minor cutting edge (shadowing edge). To get the mirror finish.
2 design of single crystal diamond cutter
The main factors to consider when designing a single crystal diamond tool are: 1 precision requirements of the machined part; 2 actual processing conditions; 3 characteristics of the diamond material.
When designing single crystal diamond tools, the following principles should be followed: 1 Because the hardness of single crystal diamond is high and the processing is difficult, the shape of the tool should be as simple as possible; 2 according to the characteristics of large single crystal diamond with high brittleness and poor impact resistance, it should be combined with actual conditions. Machining conditions, through the optimization of the tool geometry, improve the impact resistance of the cutter head; 3 design the length of the wiper edge according to the precision requirements of the machined parts, and consider the cutting ability of the cutter.
Let's take a few common tools as an example to discuss the design of single crystal diamond tools.
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Figure 1 Hard disk substrate turning tool head shape
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Figure 2 jewelry batch knife head shape
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Figure 3 contact lens cutter head shape
Computer hard disk substrate turning tool
The computer hard disk substrate is used for information storage, and the material thereof is aluminum or aluminum alloy. The lower the surface roughness value of the substrate, the larger the storage density, so it is critical to reduce the roughness of the processed surface. At the same time, since the thickness of the substrate is less than 0.9 mm, in order to prevent deformation thereof, the cutting force and extrusion during processing should be minimized.
The hard disk substrate is cut using an ultra-high precision disk lathe with good rigidity and stability. The cutting depth is about 0.01 to 0.02 mm, and the feed amount is about 0.5 mm.
The shape of the cutter head of the hard disk substrate single crystal diamond turning tool is shown in Fig. 1. The cutter head has two main cutting edges, which can be fed to the left and right during machining. The longer wiper ensures that the flank wear of the wiper does not interfere with each other when machining in the left and right directions. The two main rake faces are tilted 5° in the feed direction to obtain a negative rake angle of about 2.5°, allowing the chips to flow to the surface to be machined to avoid chipping the machined surface. The diamond turning tool is very sharp, the radius of the cutting edge is less than 100nm, and the back angle of 5° can sufficiently reduce the extrusion and friction of the flank and the machined surface.
The key quality requirement for this tool is that the blade and tip are observed without defects under a 500x microscope. The design principles are equally applicable to other ultra-precision mirror processing tools.
Jewelry batch knife
The jewellery batch knife is used to mill a leaf-shaped pattern on gold and silver jewellery. It is actually a forming cutter with the shape of the cutter head as shown in Fig. 2. The angle of the knife tip is between 110° and 150° to adapt to the pattern processing of different sizes and depths. Due to the simple structure, poor rigidity and large vibration of the batch machine, and the use of intermittent dry cutting, the processing conditions are very poor, so the batch cutter head needs to have strong impact resistance. A negative rake angle of 5° and a relief angle of 1° to 1.5° can effectively increase the strength of the blade, while the smaller relief angle can also properly squeeze the tool and the workpiece, so that the cutting does not “defect†and prevent cutting vibration. Forming "vibration" on the machined surface.
The design principle of the knives for the knives and the processed case and some high-gloss decorative parts is about the same. The knives require the blade to observe no rupture under the microscope of 200-500 times.
Contact lens cutter
The materials used in contact lenses are very soft and have a certain tensile strength. Generally, the tool of the material is difficult to cut due to the excessive arc of the blade. Since there is still a grinding process after turning, the surface roughness of turning machining is only required to reach Ra 0.1μm. The main purpose of turning machining is to obtain the concave and convex arc surface with certain shape accuracy. The main requirement for the cutting tool is the sharpness of the cutting edge. Sex. The shape of the head of the contact lens cutter is shown in Figure 3. In order to meet the need to cut the concave arc surface, a 15° relief angle is required, and the blade is required to observe no chipping under a 100-fold microscope.
3 use and maintenance of single crystal diamond tools
The single crystal diamond cutter is brittle and the blade is very sharp. It is easy to cause chipping when impacted. Therefore, it should be used under stable and vibration-free working conditions. At the same time, the rigidity of the workpiece and the tool should be increased as much as possible. The rigidity of the system increases its anti-vibration ability. The cutting amount is preferably not more than 0.1 mm.
Higher cutting speeds reduce the cutting force, while low-speed cutting increases the cutting force and accelerates the tool chipping failure. Therefore, the cutting speed should not be too low when using diamond tools.
Avoid touching the diamond tool with a workpiece or other hard object at rest to prevent damage to the tool edge. It is easy for the operator's nails and fingerprints to carry hard objects such as sand, and scratching the blade directly with nails or fingers may damage the diamond blade. The detection and adjustment of diamond tools shall be carried out by non-contact measurement methods such as optical instruments.
The diamond cutter should be cleaned with absorbent cotton and an appropriate amount of alcohol or acetone. When the tool is not in use, it should be covered with a rubber or plastic protective cover and placed in a separate knife box.
When using a straight line wiper diamond tool for mirror cutting, the secondary declination is required to be less than a few minutes, and the tool may have an installation error of 1° during the installation process. Therefore, the tool needs to be tooled before machining, and the tool is trimmed by adjusting the tool. The parallelism of the blade and the feed direction for optimum surface roughness. The tool setting method is as follows: First, a small surface is machined on the workpiece, and then the tool is adjusted under a microscope of 10 to 30 times so that the wiper is parallel to the mirror image in the newly machined surface. Due to the high requirements for parallelism, it is necessary to make patient and fine repeated adjustments. It should be noted that twisting the tool holder clamping screw may cause the tool to rotate slightly and cause the tool to fail.
Since the tool setting process of the straight line wiper blade is time-consuming and laborious, for some mirror surface cutting with lower precision requirements, a circular edge cutter with a radius of 10 to 30 mm can be used instead of the straight line wiper tool. The shape of the arc-blade cutter is similar to that of a contact lens cutter, except that the relief angle is reduced and the radius of the tool nose arc is increased. The use of this tool not only simplifies the tool setting process, but also increases the tool life when a circular arc is worn and a new section of the arc can be used after slightly turning the tool. High-precision single crystal diamond arc tools are also essential tools for machining concave mirrors.
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