Analysis of Factors Affecting the Grinding of Single Crystal Diamond Tools

1 Diamond tool grinding method
The manufacturing process of single crystal diamond tools generally includes material selection, orientation, sawing, blanking, card loading, rough grinding, fine grinding and inspection. After the selected diamond rough is oriented and cut along the largest plane, the blanks of the two tools can be obtained, which can improve the utilization of the diamond material and reduce the total grinding amount. The shape of the tool can be loaded (inlaid or brazed) by blanking. The blanking and roughing and finishing processes are all carried out by grinding.

The grinding of the diamond is carried out on a cast iron grinding disc. The grinding disc has a diameter of about 300 mm and is made of a high-phosphorus cast iron for grinding diamond which is optimized in shape, size and proportion of pores in the material structure. The surface of the grinding disc is inlaid with a diamond abrasive powder having a particle size ranging from less than 1 μm up to 40 μm. The coarse-grained diamond powder has a higher grinding rate, but the grinding quality is poor, so coarse powder is generally used for coarse 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 into an abrasive paste, and then coated on the surface of the grinding disc, and left for a period of time to allow the abrasive paste to fully penetrate into the cast iron pores of the grinding disc, and then use a larger diamond. The surface of the grinding disc is pre-polished back and forth to further enhance the inlaying effect of the diamond powder in the pores of the cast iron. When grinding, the ground diamond is generally embedded in a tin bucket to expose only the surface to be ground. The grinding disc rotation speed during grinding is about 2500 r/min, and the grinding pressure is 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, and the factors affecting the quality of grinding are also various. Some of the process issues of diamond tool grinding are discussed below.

2 Factors affecting the amount of grinding
It is found through experiments that the relationship between grinding amount and grinding condition 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 grain size of the abrasive, the inlay condition of the abrasive grains in the pores of the cast iron, etc., also change the grinding rate, thereby affecting the amount of grinding.

3 Factors affecting the quality of the grinding
Diamond has high hardness and brittleness. Although the surface roughness of the tool is easy to ensure during grinding, the blade is prone to chipping and the blade serration is not easy to reduce. Ultra-precision machining requires the cutting edge of the tool to be viewed without a breach under a 500x microscope, so the grinding process needs to be optimized from all aspects to achieve a straight and perfect edge.

Effect of abrasive particle size and surface condition of grinding disc on grinding quality
It can be seen that due to the large impact of the coarse powder on the blade, the saw blade has a large serration after grinding, and it is basically difficult to grind the blade without the chipping. When the fine powder is used, the blade changes after a few minutes of grinding. Straight, the sawtooth tends to zero.

Due to the limitation of processing precision, the newly manufactured grinding disc has a large unevenness on the disc surface and has a certain influence on the stability of the grinding. In addition, the abrasive grains just applied to the disk surface itself are also inferior in height. After a period of grinding, the high point on the disc surface is flattened, and the larger particles in the abrasive grains are also broken or shoveled off the disc surface, so that the homogeneity of the abrasive grains is improved, and the blade serration is stably reduced. Therefore, the key processes such as cutting or fine grinding of diamond tools must be carried out on a stable grinding disc surface.

Since the abrasive particles will be continuously broken or lost during the grinding process, if it is not replenished in time, the diamond will be directly contacted with the cast iron due to insufficient density of the abrasive grains on the disk surface, which will not only affect the quality of the tool grinding, but also the squeezing of the diamond. The action destroys or blocks the pores on the disc surface, thereby reducing the service life of the grinding disc. Therefore, it is necessary to frequently add a new abrasive paste to the disk surface during the grinding process.

In addition, it is also important to pretreat the surface of the grinding disc before powder coating. Generally, it is necessary to use the oil stone or coarse SiC abrasive powder to grind the disc surface to remove the turning groove and improve the flatness of the disc surface. Through the comparison test of grinding the disk surface with various oilstone and SiC powder, it was found that the TL280ZY1 whetstone was used to grind the disk surface for 1 hour and then coated with W1 fine diamond powder to obtain the most ideal grinding disk surface. At this time, the disk surface reached the stable time for the shortest time. The latter edge has the least amount of sawtooth. The use of free SiC abrasive powder tends to block the pores on the disc surface, making it difficult for the diamond powder to be embedded in the grinding disc in a large amount and firmly.

Effect of sharpening angle on grinding quality
The sharpening angle q is the angle between the direction of the line speed of the grinding and the cutting edge. When q>0°, the grinding direction is directed from the cutter body to the cutting edge, which is called smooth grinding; when q<0°, the grinding direction is directed from the cutting edge to the cutter body, which is called back grinding. Figure 2 shows the relationship between the blade serration and the sharpening angle. Since the tensile strength of diamond is extremely high, the cutting edge is subjected to tensile stress during the grinding, so the sawing degree is small after grinding; when the grinding is reversed, the cutting edge is subjected to compressive stress, so the sawing degree after grinding is large. It can be seen from Fig. 2 that when q is larger than and close to 0°, the blade can obtain the minimum sawtooth degree, and the stress direction at the cutting edge is substantially parallel to the cutting edge, and the blade has the highest tensile stress strength in this direction. . Another advantage of parallel to the edge grinding is that the wear marks on the flank are also parallel to the cutting edge and do not reproduce to the machined surface during machining, helping to improve the quality of the cutting.

Influence 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 can cause the impact of the disc facing the cutting edge during grinding, thereby destroying the straightness of the cutting edge, especially the effect of the disc surface end jump is more direct, because the impact direction caused by the end jump Vertical to the disk surface. Figure 3 and Figure 4 show the relationship between blade serration and disk end jump and machine vibration. It can be seen from the figure that there is a critical value for the influence of the disk end jump and the machine vibration on the blade serration. When it is less than the critical value, the blade serration tends to zero; and when it is greater than the critical value, the blade sawtooth sharply rises.

In order to reduce the disk end jump, when grinding the disk surface with oil stone, the end jump of the disk surface should be detected at the same time, and the end jump should be eliminated by grinding as much as possible. Then, the online dynamic balance of the grinding disc is also required to reduce the vibration caused by the imbalance of the mechanism during operation.

The end jump and vibration are also related to the accuracy and damping performance of the grinder. Conventional wooden top grinders require fiber mats between the tip and the shaft, and due to the rigidity of the wood, the disc will produce a dynamic jump of 0.05 to 0.1 mm 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, so that a gap is formed between the rotating shaft and the tip, so it is necessary to frequently adjust the gap, replace the pad or the wooden tip. Due to the unstable factors in the use of such equipment, the basic qualified natural diamond tools can only be ground in some short period of time when the equipment is in the best condition, even the skilled operators can only reach 30. % to 50% of the processing pass rate.

The accuracy of the static air bearing is higher than 0.5μm, the rotation is stable, and the high-pressure air supporting the main shaft has strong vibration absorption capacity. Therefore, the grinding machine using the static air bearing can obtain a perfect cutting edge even if the grinding tool has a diamond cutter with a wedge angle of only 45°. For general civilian diamond tools, it can basically achieve 100% processing pass rate.

Effect of deflection angle on grinding quality
The deflection angle w is the angle between the actual grinding direction and the best grinding direction on the surface to be polished of the diamond. For the (1 1 0) plane, when grinding in the best grinding direction (w=0°), the diamond surface is very smooth and the surface has large undulations, because the unevenness of the grinding disc surface is in the best grinding direction. The surface of the diamond is fully reappeared; when w=45°, the surface of the diamond is still quite smooth, but the degree of undulation is small, and fine groove marks appear; when w=60°, the surface of the diamond produces dense deep grooves, grinding The rate becomes very low; in the most difficult direction, the diamond surface is filled with pits and the polishing rate 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) plane, the well-ground area is w<15°.

In the well-ground area, the sawtooth tends to zero, and when w>45°, a large collapse occurs rapidly on the blade. Similar results were obtained for the (1 0 0) face.

The influence of the deflection angle on the surface quality can also be used to determine the best grinding direction of the diamond, because the diamond surface is bright and has large undulations when ground in the best grinding direction.

In summary, the grinding quality of diamond tools is quite sensitive to a variety of processing conditions, especially when grinding diamond tools with small blade wedge angles (such as ophthalmic scalpels, fiber knives, and bio-dissection knives). Therefore, it is necessary to carefully treat the surface of the grinding disc during grinding, use the fine diamond grinding powder to find the best grinding direction, and use a grinding machine with high precision, smooth operation and low vibration (such as an aerostatic bearing grinding machine). When the various processing conditions are in a satisfactory state, high-quality diamond tools with no chipping and small sawtooth edge can be ground.