Diamond is the crystal of single carbon atom, and its crystal structure belongs to the cubic crystal system with equiaxed plane center (one kind of crystal system with highest atom density).Since the bonding bond between carbon atoms in diamond is sp3 hybrid covalent bond, it has strong binding stability and directivity.The unique crystal structure makes diamond have the highest hardness and rigidity, refractive index, thermal conductivity, excellent abrasion resistance, corrosion resistance and chemical stability, etc.The excellent properties of natural diamond can meet most requirements of precision and ultra-precision cutting tool materials, and it is an ideal precision cutting tool material.The uniform crystal structure of natural diamond without internal grain boundary makes the cutting edge theoretically reach the straightness and sharpness of atomic level.The hardness, abrasion resistance, corrosion resistance and chemical stability of natural diamond ensure the long service life of the cutting tool, ensure the continuous long normal cutting, and reduce the impact of tool wear on parts precision;Its higher thermal conductivity can reduce cutting temperature and thermal deformation of parts.Natural diamond as superhard cutting tool material, therefore, has an important position in the field of mechanical processing and has been widely applied, especially in the field of super precision machining, such as processing used in nuclear reactors, and other high technology in the field of all kinds of mirror, used for missile or rocket in the navigation gyroscope, computer hard disk substrate, electron gun accelerator and ultra-precision components), using natural diamond tool no matter on price or on the precision than the traditional machining method has obvious advantages.Table 1 compares the accuracy and price of various mirrors processed by natural diamond tools and traditional grinding and polishing methods.

In addition to the high-tech fields, natural diamond cutting tools in the application of ordinary industrial and civil products machining also increased year by year, has been developed from traditional watch parts processing to the aluminum piston, jewelry, pen, glossy plates, non-ferrous metal decoration parts processing, can say natural diamond cutting tools has been further mechanical processing fields, plays a more and more important role.

On the other hand, due to the unique properties of natural diamond, it is very difficult to process it.Due to the high hardness of natural diamond, it must be ground by special methods, which also requires high technical requirements for operators.Due to the good chemical stability of natural diamond, it was unable to be welded for a long time in the past, and only the mechanical method was used to clamp large-grain diamond, which resulted in the waste of diamond materials and the high price of cutting tools, and also affected the precision of cutting tools and parts.

In the past 100 years, scientists and technicians have made long-term research and development on the important role, application prospects and implementation methods of diamond tools in mechanical processing, and achieved fruitful results.The development of natural diamond cutting tool technology is reviewed in the following stages.

Traditional processing technology of natural diamond tools

Natural diamond tools were developed after world war ii to meet the processing needs of precision parts, light ornaments and jewelry carving. The manufacturing process of natural diamond tools originated from the grinding and polishing technology of diamond jewelry. The development of natural diamond tools has brought about significant changes in the manufacturing technology of watches and related products.

Lapping of diamond jewelry process method is: on the cast iron grinding plate surface coated diamond powder grinding paste mixed with olive oil and into, and made diamond powder inlaid in the tiny pores on the surface of the cast iron, fixed in clamp skateboard with diamonds on it since the weight of grinding on the grinding plate rotating at high speed.

1. Blade processing and testing: the quality of the blade will directly affect the quality of the processed parts.2. Orientation: the most violent wear surface of the tool is placed on the hardest crystal surface of diamond, so as to ensure the longest service life of the tool.The traditional orientation method generally USES the naked eye orientation.3. Mounting: in the process of cutting, natural diamond tools should bear cutting forces from all directions. In order to ensure continuous and stable cutting, diamond tools must be firmly installed on the tool bar.

Diamond brazing technology had not yet been invented, so mechanical clamping was the only way.Because of the simple process and low cost of traditional diamond cutting tools, it is still used in rough machining of diamond cutting tools today.In order to further improve the diamond tool processing technology, in half a century of science and technology personnel on the physical and chemical properties of diamond crystal and diamond tool grinding mechanism, formation mechanism of blade, cutting theory, brazing technology, a lot of work on precision grinding equipment, and other subjects, as the natural diamond tool in ultra-precision machining technology development has laid a solid foundation, many research topic today is continuing.

Ultra-precision machining technology for natural diamond tools

In the late 1970s, in the research of laser nuclear fusion technology, a large number of high-precision soft metal reflectors need to be processed, and the surface roughness and shape precision of soft metal are required to reach the level of ultra-precision.Such as the use of traditional grinding, polishing processing methods, not only long processing time, high cost, difficult operation, and is not easy to achieve the required accuracy.Therefore, it is urgent to develop new processing methods.In real demand, driven by natural diamond ultra-precision mirror cutting technology is developing rapidly, on the basis of the existing diamond turning technology, by improving the accuracy of machine tool and rigid, strictly control the process of vibration and temperature drift, development of ultra-precision natural diamond cutting tools, etc., to form the lens cutting process, and developed into a specialized technology.As one of the key technologies for ultra-precision mirror cutting, natural diamond cutting tool technology has achieved significant innovation and development in both theory and practice, mainly reflected in the following aspects:

1. Development of ultra-precision diamond tools

(1) a polished edge is introduced between the main cutting edge and the auxiliary cutting edge, so that the theoretical roughness of the machined surface is close to zero and the polished edge is free from defects when detected under a microscope of 500 or higher.(2) the machining accuracy of the key Angle of the diamond cutter reaches 2 ".(3) the arc precision of the arc cutter used for turning internal surface reaches the micron level.

Because the traditional grinding equipment can not meet the machining requirements of ultra-precision diamond tools, the arc and edge grinding machine with air bearing has been developed, and the grinding precision can reach 0.1 m.3. Precision orientation technology and equipment are used to orientate diamond tools for the purpose of not only providing the longest tool life, but also minimizing the friction between the tool surface and the machined surface and the stress on the cleavage surface near the blade.To do this, a more sophisticated X-ray diffractometer is needed.

Vacuum brazing is one of the most important breakthroughs in diamond tool manufacturing technology.On the one hand, the traditional mechanical clamping method may lead to the tiny displacement and vibration of diamond cutter in the cutting process, which will affect the machining quality.On the other hand, due to the extremely high chemical stability of diamond itself, it is difficult to react with other metals to achieve welding under general conditions, that is, diamond is non-weldable.In order to solve this problem, the special condition of brazing diamond (high vacuum environment) and brazing alloy (silver-based alloy with titanium as active element) were found after long-term research and exploration.

The establishment of tool wear mechanism through the study found that in the cutting process, the diamond tool wear is mainly chemical bond wear, and there is a small amount of mechanical wear and other forms of wear.The establishment of tool wear mechanism determines the principle of tool orientation: the crystal surface with the best chemical stability is placed on the back surface of the tool

6. Systematic research on mechanical grinding method

Through the in-depth study of diamond theory, it is found that the macroscopic plastic deformation of diamond (in the past, it was believed that diamond only had a small amount of elastic deformation but no plastic deformation);The trace impurities in diamond crystal were studied. According to the different impurities, the diamond was divided into four kinds, so different kinds of diamond could be selected according to different USES.In addition, through the systematic theoretical research on the fracture characteristics, cleavage characteristics and surface formation mechanism of diamond, a large amount of data has been obtained, various theories have been formed, and a more scientific and profound understanding of diamond has been gained.The development characteristics of diamond cutting tool technology in this stage are as follows: as diamond cutting tool is applied in national defense, high-tech and other fields, a large amount of funds are invested, and cutting-edge equipment, instruments and the latest scientific methods are used in the research, making a leap in development.

The latest processing technology of natural diamond tools

In the late 1980s, micro - machinery as a new research field developed rapidly.The use of mechanical processing for the manufacture of micro-robot micro-components (such as?For 0.1mm micro precision gear, 0.3mm micro motor, etc.), the tool tip arc radius of 3 ~ 5 m is required, and the arc precision can be controlled, and can reach a considerable length of tool life.

From the analysis of cutting tool materials, only single crystal natural diamond can meet the above requirements.At the same time, after nearly ten years of rapid development, the theory and technology of diamond cutting tools have accumulated a wealth of experience, and they are basically equipped with the ability to develop the high-precision cutting tools mentioned above.However, there is still a gap between the requirements of diamond cutting tools processed by the aforementioned ultra-precision machining technology of diamond cutting tools and those of micro-machining tools, and more advanced processing methods need to be further developed.In recent years, a variety of methods have been developed for grinding diamond tools with various chemical mechanisms.

1. The thermochemical method for the mechanism of thermal chemical methods: when the temperature is 800 ℃, if the diamond surface contact with iron, diamond crystals in to be able to get rid of its lattice of carbon atoms, spread to the iron in the crystal lattice.Using thermal chemical method of grinding process is: the iron grinding disc in hydrogen atmosphere heated to 800 ℃, the diamond in contact with the grinding disc and the relative sliding, diamond lattice of carbon atoms will spread to the iron in the crystal lattice, achieve the purpose of grinding diamond;Carbon atoms entering the iron lattice react with hydrogen to form methane, which is released into the air by air currents.Using thermochemical method, the grinding speed is 40 ~ 2000 atomic layers per second.

2. The vacuum plasma chemical polishing, first of all, using vacuum plasma physical vapor deposition method on the grinding disc surface plating a layer of fine grained silicon oxide, and then activate the surface of diamond in high vacuum, at the same time into contact with the rotation of the grinding disc, diamond surface in the activation state of carbon atoms will react with the silica on the grinding disc, grinding of diamond formation;The resulting carbon monoxide or carbon dioxide gas is pumped out of the reaction chamber.The reaction rate of this method is 1 ~ 3000 m3/s (about 0.25 ~ 750 atomic layers per second).

3. Non-damaging mechanical chemical polishing method: a certain amount of fine diamond powder and even finer (nanometer level) silicon powder were added into NaOH solution, and the silicon powder with strong negative electrostatic force would be adsorbed on a single diamond particle much larger than it to form diamond grinding powder with silicon adsorption layer, and then it was coated on the porous cast iron grinding disc.During the grinding of diamond, the silicon powder adsorbed on the diamond grinding powder on the one hand prevents the direct impact of the diamond powder on the surface of the processed diamond and protects the surface of the processed diamond from deep damage; on the other hand, it reacts with the surface of the processed diamond and removes the reaction layer by weak grinding action.The grinding speed of this method is very low, only 1 atomic layer per minute.