1 Diamond tool grinding method
The manufacturing process of single crystal diamond tools generally includes material selection, orientation, sawing, blanking, carding, rough grinding, fine grinding and inspection. After the selected diamond raw stone is oriented and sawed along the largest plane, two blanks of the cutter can be obtained, so that the utilization rate of the diamond material can be improved and the total grinding amount can also be reduced. Through the blanking, the shape of the tool can be set to meet the requirement of mounting (brazing or brazing). Both blanking and coarse and fine grinding processes are ground.
Diamond grinding was performed on a cast iron grinding disc. The diameter of the grinding disc is about 300mm, which is made of special high-phosphorus cast iron for grinding diamonds, which is optimized for the shape, size and proportion of pores in the material structure. The surface of the grinding disk is inlaid with a diamond grinding powder whose particle size can be from less than 1 μm up to 40 μm. The coarse-grained diamond powder has a high polishing rate, but the grinding quality is poor. Therefore, coarse powder is generally used for rough grinding, and fine powder having a size of less than 1 μm is used for fine grinding. Before grinding, the diamond powder is first mixed with olive oil or other similar substances to form an abrasive paste, which is then applied to the surface of the grinding disk for a period of time to allow the abrasive slurry to fully infiltrate into the pores of the cast iron of the grinding disk, followed by a larger diamond. The surface of the grinding disk is pre-milled back and forth to further strengthen the diamond powder in the pores of the cast iron. During grinding, the diamond to be polished is generally embedded in a tin bucket and only the surface to be polished is exposed. The grinding disc has a rotational speed of about 2500 r/min and a grinding pressure of about 1 kg/mm2.
The grinding of diamond is very different from the processing of other tool materials. The grinding mechanism has not yet been convincingly explained. The factors affecting the grinding quality are also numerous. The following section discusses some of the technical issues of diamond tool grinding.
2 Factors affecting the amount of grinding
Through experiments, the relationship between the grinding amount and the grinding conditions is V=kvp
Where V - grinding volume k - grinding rate v - grinding speed p - grinding pressure
In addition, the grinding direction of the diamond, the particle size of the abrasive, and the inlaying condition of the abrasive grains in the cast iron pores also change the size of the grinding rate, thereby affecting the amount of grinding.
3 Factors affecting the grinding quality
The diamond has high hardness and high brittleness. Although the surface roughness of the cutter can be easily ensured during grinding, the edge of the cutter is liable to collapse, and the jaggedness of the cutting edge is not easily reduced. Ultra-precision machining requires that the cutting edge of the tool be observed under a microscope with a magnification of 500x. Therefore, it is necessary to optimize the grinding process in all aspects to obtain a straight and perfect cutting edge.
Effect of Grinding Particle Size and Grinding Disc Surface State on Grinding Quality
It can be seen that because of the large impact of the coarse powder on the blade, the jaggedness of the blade after grinding is also large, and it is basically difficult to grind a chipping-free blade; when fine powder is used, the blade is changed after a few minutes of grinding. Straightened, the zigzag tends to zero.
Newly-manufactured grinding discs have a large surface irregularity due to the limitation of machining accuracy, which has a certain influence on the stability of the grinding. In addition, the abrasive grains that have just been applied on the surface of the disk are also inferior in contour. After grinding for a period of time, the high spots on the plate surface were leveled and the larger particles in the abrasive grains were also broken or scooped away from the plate surface, so that the contour characteristics of the abrasive grains were improved, and the jaggedness of the blades was stably reduced. Therefore, the key operations such as sharpening or fine grinding of diamond tools must be performed on the surface of a stable grinding disk.
As the abrasive particles will be continuously broken or lost during grinding, if they are not added in time, it will lead to direct contact between the diamond and the cast iron due to the insufficient density of the abrasive grains on the surface, which will not only affect the grinding quality of the cutter, but also cause the scraping of the diamond. The function destroys or plugs the pores on the disk surface, thereby reducing the service life of the grinding disk. Therefore, it is necessary to add new polishing paste to the disk surface frequently during the grinding process.
In addition, the pre-treatment of the grinding disc surface before powder coating is also very important, generally need to use whetstone or coarse SiC grinding powder to mill the disk surface to remove the turning groove pattern and improve the flatness of the disk surface. Through comparative tests using a variety of whetstone and SiC powders to grind the disk surface, it was found that polishing the disk surface with the TL280ZY1 oil stone for 1 hour and then applying the W1 fine diamond powder can obtain the best grinding disk surface. At this time, the disk surface has reached the shortest stable time. The rear blade has the least jaggedness. However, the use of free SiC abrasive powder can easily block the pores on the disk surface, making it difficult for the diamond powder to be embedded in the grinding disk in large quantities and firmly.
Effect of Sharpening Angle on Grinding Quality
The sharpening angle q is the angle between the linear speed direction of the grinding and the cutting edge. When q>0°, the grinding direction is from the cutter body to the cutting edge, which is called smooth grinding; when q<0°, the grinding direction is from the cutting edge to the cutter body, which is called counter grinding. Figure 2 shows the relationship between blade jaggies and sharpening angle. Due to the extremely high tensile strength of diamonds, the cutting edge is subjected to tensile stress during grinding, and therefore the degree of sawing is small after grinding. When the wear is reversed, the cutting edge is subjected to compressive stress, so that the degree of jaggedness after grinding is large. It can be seen from Fig. 2 that when q is greater than and close to 0°, the blade can obtain the minimum jaggedness. At this time, the stress direction at the cutting edge is basically parallel to the cutting edge, and the cutting edge has the highest tensile stress strength in this direction. . Another advantage of grinding parallel to the cutting edge is that the wear scar on the rake face is also parallel to the cutting edge and will not be reconstructed on the machined surface during the cutting process, which will help improve the quality of the cutting process.
Effect of Disk End Jump and Machine Vibration on Grinding Quality
The end-jump of the grinding disc surface and the vibration of the machine tool will cause impact of the disc on the cutting edge during grinding, thus destroying the straightness of the cutting edge, and the influence of disc end jump is more direct, which is due to the impact direction caused by the end jump. Vertical to the disk surface. Fig. 3 and Fig. 4 are the relationship between the blade sawtooth degree and the disc end jump and machine vibration. As can be seen from the figure, there is a critical value for the influence of disk end hops and machine vibrations on the jaggedness of the blade. When it is less than the critical value, the jaggedness of the blade tends to zero; when it is greater than the critical value, the jaggedness of the blade sharply rises.
In order to reduce the disk end-jump, when grinding the disk surface with Whetstone, the end-jump condition of the disk surface should be detected at the same time, and the end-jumping should be eliminated by grinding as much as possible. Then, the on-line dynamic balance of the grinding disk needs to be performed to reduce the vibration caused by the mechanism unbalance during operation.
End jumps and vibrations are also related to the precision and vibration damping performance of the grinder. Traditional wooden top grinders require fiber matting between the tip and the shaft, and due to the limitation of the rigidity of the wood, the grinding disc will produce a 0.05 to 0.1 mm dynamic beating at high speeds; in addition, wood and fiber materials The heat resistance is poor, and it is easy to wear under high-speed sliding conditions to create a gap between the shaft and the tip. Therefore, it is necessary to frequently adjust the clearance, replace the gasket, or the wooden tip. Since there are many unstable factors when using such equipment, only basic qualified diamond tools can be ground within a certain period of time when the equipment is in the best condition. Even a skilled operator can only achieve 30. % to 50% processing pass rate.
The precision of the static pressure air bearing is higher than 0.5μm, the rotation is stable, and the high-pressure air supporting the main shaft has strong vibration absorption capability. Therefore, a grinding machine using a static air bearing can obtain a perfect cutting edge even if it grinds a diamond cutting tool with a wedge angle of only 45°. For general civil diamond tools, basically 100% processing yield can be achieved.
Effect of Angular Angle on Grinding Quality
The deflection angle w is the angle between the actual grinding direction and the best grinding direction on the polished surface of the diamond. For the (1 1 0) plane, the diamond surface is very smooth when grinding in the best grinding direction (w=0°), and the surface has large undulations because of the unevenness of the grinding disk surface in the best direction of the grinding. The diamond surface has been fully reconstructed; when w=45°, the surface of the diamond is still smooth, but the degree of undulation is small, and small groove marks appear; when w=60°, the diamond surface produces a dense deep groove, grinding The rate becomes very low; in the hardest-to-grind direction, the diamond surface is filled with individual pits and the rate of grinding is essentially zero. The area of w<45° can be considered as a good grinding direction and a smooth surface can be obtained. For the (100) face, the good grinding zone is w<15°.
In the good grinding area, the sawtooth tends to zero, and when w>45°, a large collapse occurs quickly on the blade. Similar results can be obtained for the (100) surface.
The law of the influence of the angling angle on the surface quality can also be used to determine the best direction of diamond wear, because the surface of the diamond is bright and has more undulations in the direction of the best grinding.
In summary, the grinding quality of diamond tools is quite sensitive to various processing conditions, especially when grinding diamond tools with small cutting edge wedge angles (such as ophthalmic scalpels, fiber cleavers, bioslicers, etc.). Therefore, when grinding, the surface of the grinding disc must be carefully treated, and the finest diamond grinding powder should be used to find the best grinding direction, and a grinding machine (such as an aerostatic bearing grinding machine) with high precision, stable operation, and low vibration should be used. To ensure a variety of processing conditions in a more ideal state, you can grind a high-quality diamond cutting tool with no chipping and small blade teeth.
