Selection and Orientation of Single Crystal Diamond Tools

In the diamond tool manufacturing process, the first step is to choose the material. According to different processing conditions, selecting suitable diamond raw materials can avoid the increase of tool manufacturing costs due to the use of high-grade rough stones on the premise of guaranteeing the use performance of the tools. At the same time, for high-quality diamond tools used for ultra-precision machining, tool performance requirements can be fully guaranteed through the choice of materials.
The anisotropy of single crystal diamonds varies greatly in different crystal planes and in different directions. Correct orientation not only simplifies the machining process, reduces manufacturing costs, but also doubles (or even tens of times) the tool life.
Therefore, when manufacturing single-crystal diamond tools, scientific and rational material selection and orientation are of great significance for giving full play to the superior performance of diamond tools and improving processing quality and economic benefits.

First, the diamond cutting tool selection

1. Difference in impurity free diamond contained <br> The diamond crystals, diamond can be divided into four categories:
(1) The vast majority of Type Ia natural diamonds are of this type, with a nitrogen content of about 3000 ppm. Nitrogen is present in an aggregated form and produces one or more point defects in the diamond crystals.
(2) Type Ib natural diamond has very little Ib type, but almost all artificial diamonds are Ib type, and its nitrogen content is 40 to 500 ppm. Nitrogen exists in the form of a substitutional solid solution, and is evenly distributed in the diamond lattice, making diamonds Yellowish green.
(3) Type IIa This type is only present in a small amount in natural diamonds. Its nitrogen content is only 20ppm and it is a kind of high-purity diamond.
(4) Type IIb semiconductor diamond, which has a nitrogen content of 20 ppm but contains sufficient boron to form a P-type semiconductor. This type is rarely seen in natural diamonds, but it can be obtained using a special denitrogenation-plus-boron method when artificially synthesizing diamonds.

2. Selection principle I a type is most common in natural diamonds, and most single crystal diamond tools use this material. Since the natural conditions for the formation of natural diamonds are different, the dispersion of their quality is very large. Generally, natural diamonds are classified into different categories and grades based on the crystal particle size (weight), shape, integrity, transparency, number of cracks and inclusions, color and uniformity. According to the use of natural diamonds in China, it is divided into nine categories: gemstones, wire drawing dies, cutters, grinding wheel cutters, and abrasives. Some of these categories can be subdivided into several levels (JC220-79) according to their quality or application requirements.
The quality requirements for diamonds for cutters are as follows: crystals are complete, shape is dodecahedron, arc octahedron or transitional crystals, the minimum diameter of the crystal must not be less than 4mm, the color is colorless, light green, yellow, brown, etc., cracks are not allowed The crystal surface is allowed to have inclusions and etch pits of not more than 0.5 mm and weigh 0.7 to 3 carats.
With the ever-increasing use of single-crystal diamond tools and continuous improvement in manufacturing technology, the actual range of choice of raw materials is not limited to the above standards. If advanced single crystal diamond brazing technology is used, diamond knives weighing only 0.05 carats can be made, so that the requirements for crystal size are greatly reduced. For jewellery tools, piston tools, contact lens tools, and most consumer products that require less precision, only the blade part is free of cracks, impurities, and wraps, and its shape, color, and the quality of the head and tail are not strict requirements. Appropriate diamonds can also be selected from the three-stage wheel cutter materials. It has also been found that dark brown diamonds have a higher tool life.
For ultra-precision cutting tools, ophthalmic scalpels, etc., which require extremely high precision, the material must be selected from grade I or even gem grade rough diamonds, and polarization microscopy or more sophisticated instruments must be used to select diamonds with low internal stress. raw material.
In recent years, breakthroughs have been made in the manufacture of large-grained single-crystal diamonds using synthetic methods. Both De Beers and Sumitomo Corporation of Japan have manufactured large-size synthetic diamond products with a crystal length of 13 mm. The nitrogen atoms in the synthetic Ib-type diamond are uniformly distributed in the diamond lattice in the form of a single replacement of the carbon atoms in the diamond crystals. On the one hand, they reduce the possibility of nitrogen atoms accumulating on the cutting edge of the cutter to form tiny collapsed pores. Uniform distortion of the crystal lattice increases the hardness of the diamond. For synthetic diamond products that are specifically used for the manufacture of cutting tools, their internal stresses are also optimized to make the product more stable, reliable, and less discrete. For general-purpose synthetic diamond products, there is basically no need for material selection. Since the crystal orientation of the rough stone has been precisely specified at the time of shipment, crystal orientation is not required.
Edge Technologies of the United States used Sumitomo Corp.'s synthetic diamond (Sumicrystal) to manufacture cutting knives for optical components such as high-precision mirrors, and achieved good results. The service life of various tools made from Monodite of De Beers is 20% to 200% longer than that of natural diamond.
The disadvantage of Type Ib artificial single crystals is that the brittleness is relatively large and the processing is more difficult. A fine sharpening method is required to obtain a qualified cutting edge quality, and the price is higher than that of natural diamond.

Second, the diamond tool orientation

　　 The orientation of the diamond tool includes two aspects: (1) Tool orientation: Determine the front and back face of the tool on which crystal surface of the diamond, so as to obtain higher performance and longer tool life; (2 Crystal Orientation: For a diamond single crystal, how to find the orientation of its crystal axis, so that the knife surface is accurately placed on the designed crystal surface.

1. Determine tool orientation <br> tool orientation program and the wear mechanism in machining concerned.
Early theorized that diamond cutters wear in the form of chipping and mechanical wear. The orientation of tools is often (110-110). That is to say, the rake face and back face of the cutter are placed on two mutually perpendicular (110) faces at the same time. At this time, the flow direction of the rake face, the flank face and the rubbing direction of the machined surface are all in the hardest grinding direction of the (110) plane. Since the soft direction of the (110) plane is the softest direction in the entire crystal, this scheme is the best.
Further studies have shown that diamond tool wear patterns should also include chemical degradation such as thermal degradation and corrosion, and in some cases, chemical wear patterns dominate. A large number of experimental results also show that the (100) plane has higher stress, corrosion, and thermal degradation resistance than other crystal planes. Based on these facts, the (110-100) orientation scheme is widely adopted. In this scheme, the rake face of the tool is (110) face, and the flank face is (100) face, although the rake face and the chip friction direction are in the softest direction of the (110) face, but in the most severely worn back blade On the surface, the rubbing direction is in the direction of the most difficult to wear (100), while maintaining the ability to resist mechanical wear, while improving the ability of the flank to resist other wear. Another advantage of this orientation scheme is that diamond material can be saved because diamond roughs generally have a maximum length in the <100> direction of the (110) plane.
The wear of diamond tools is a very complicated physical and chemical process. When different workpieces are processed under different conditions, the proportion of various types of wear will change, so the orientation of diamond tools should be based on the main wear in the processing Form to choose a reasonable orientation program.
When machining ceramics, glass or other hard and brittle materials, or when the vibration is large due to machine accuracy, etc., the tool wear is mainly due to micro-cracking. Therefore, the orientation of the tool should be such that the blade has the highest strength and can be selected (100-100 ) Orientation.
When processing silicon-aluminum alloy workpieces such as pistons, because the material contains a lot of silicon compound hard spots, the mechanical wear of the tool is larger, so the (110-110) orientation can be used to achieve better processing results.
When processing some non-metallic materials with complex components, chemical wear may become the most important form of wear. Therefore, (110-100) or even (100-100) orientation schemes are mostly adopted.
The determination of the orientation plan also requires a comprehensive consideration of the tool manufacturing process. The easy-grinding direction of the (110) face is softer than that of the (100) face, so the (110) face has better processability. However, any direction of the (111) surface is not easy to grind and should generally be avoided.

2. The method of crystallographic orientation <br> crystallographic orientation and the orientation can be divided into the instrument using artificial visual orientation. The error of the orientation of the instrument can be controlled within 1° and the orientation accuracy is high, but it requires expensive equipment and the operation is complicated. In addition, whether the tool with the orientation of the instrument has more advantages than the tool with artificially determined orientation, how much the life can be improved, and how much the orientation error exceeds will significantly reduce the tool performance. The research literature on this aspect is rare. At present, general diamond tool manufacturers mostly use artificial eyes to measure.
The direction of the artificial eye is to determine the crystal axis position and direction of the crystal based on the number and relative position of the original crystal faces. For octahedral crystals, the three perpendicular lines connected by three pairs of symmetrical vertices are the X, Y, and Z axes of the crystal. Therefore, the octahedral crystal plane is (111) plane; the eight squares obtained by grinding its apex perpendicular to the axis are (100) planes; and the two planes intersecting their edges are equidistantly angled to remove the edges, ie, (110) face.
Dodecahedron crystals have six vertices that intersect at four edges. The line connecting the symmetrical vertices is the crystal axis. The twelve surfaces are (110) surfaces; the intersection of six four edges represents the (100) surface; the intersection of eight three edges represents the (111) surface.
Manually visually detecting crystal orientation is a fast, easy-to-use, time-saving, and time-saving method of orientation. When encountering incomplete or irregular crystals or machined tools, it can also be easily ground and hardened according to the crystal faces and faces of the diamond. The mutual positional relationship between the grinding directions, through some local, incoherent crystallographic clues to find the desired crystal face.