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Polycrystalline diamond (PCD) is a polycrystalline crystal obtained by mixing diamond micronized powder having a particle size of micron order with a small amount of metal powder (such as CO) and sintering at a high temperature (1400 ° C) and a high pressure (6000 MPa). Compared with other tool materials, polycrystalline diamond has the following performance characteristics: 1 high hardness and wear resistance; 2 high thermal conductivity and low thermal expansion coefficient, fast heat dissipation during cutting, low cutting temperature, small thermal deformation; 3 friction coefficient Small to reduce the surface roughness of the machined surface. However, since polycrystalline diamond has a strong affinity with iron group elements, it is not suitable for processing ferrous metals and their alloys. The PCD composite sheet that has been commercially supplied is prepared by sintering a 0.5-0.7 mm thick PCD layer on a cemented carbide substrate, so that it has both PCD high hardness, high wear resistance and good hardness of cemented carbide. Strength and toughness.
PCD cutters have excellent cutting performance in high-speed cutting of non-ferrous metals and their alloys and non-metal materials, so they have been widely used in automotive, aerospace, aerospace, building materials and other industrial fields. However, the high hardness and high wear resistance of PCD make the sharpening of the tool quite difficult, mainly reflected in the small material removal rate, large grinding wheel loss, low sharpening efficiency and sawtooth edge. The sharpening process of PCD tools has become one of the obstacles to its promotion and application. In order to break through this process bottleneck, domestic and foreign scholars have carried out a lot of research and development work.
2 PCD tool sharpening technology
The main sharpening process of PCD cutters includes discharge sharpening, diamond grinding wheel mechanical sharpening, electrolytic sharpening, etc. Among them, the discharge sharpening and the diamond grinding wheel mechanical sharpening are technically mature, and the following two sharpening methods are used. A comprehensive analysis.
2.1 Electric discharge sharpening (EDG)
Electrical discharge machining (EDM) (especially for wire EDM and electric discharge grinding) has been widely used in tool manufacturing. EDM technology for sharpening PCD tools is called discharge sharpening (EDG).
(1) Sharpening mechanism
The principle of discharge sharpening is fundamentally different from the traditional abrasive grinding principle, and it is also different from the principle of electrolytic sharpening (both abrasive mechanical and electrochemical). The discharge sharpening is to melt and vaporize the tool material by generating a transient high temperature by discharging between the grinding wheel electrode separated from the dielectric and the tool electrode. When sharpening a PCD tool, since the diamond is not electrically conductive, the tool electrode is a conductive network composed of a metal phase in the PCD. Thus, the discharge sharpening is a thermal erosion process. Since the temperature of the spark discharge can be as high as 8000 to 12000 °C, the PCD tool may cause heat loss and graphitization during sharpening, especially at the interface between the PCD and the cemented carbide substrate, which can form a depth of about 0.05 on the surface. Micro-cracks of mm, which is a major drawback of the discharge sharpening process. Since the discharge sharpening is a non-contact sharpening process, the grinding force is negligibly small, so the sharpening efficiency is high. The grinding rate obtained by R. Wyss et al. under certain experimental conditions was 4 mm 3 /min, and the abrasion ratio was 0.2 mm 3 /mm 3 ; while V. Baar et al. obtained an abrasion ratio of 1.0 mm 3 /mm 3 in the experiment.
(2) Sharpening equipment
In the case of electric discharge sharpening, hydrocarbon (such as paraffin) is usually used as the dielectric between the grinding wheel electrode and the tool electrode. The working voltage is generally 80-200V DC, and the grinding wheel electrode is made of conductive materials such as copper, tungsten and graphite. According to the position of the tool during sharpening, the discharge sharpening can be divided into a circumferential discharge sharpening and an end discharge grinding. During the sharpening process, the grinding wheel rotates to make it wear evenly. In the end face discharge sharpening, the grinding wheel also needs to swing left and right. Pulse power supply is the key equipment that affects the sharpening efficiency and sharpening quality. Therefore, the design of pulse power supply has become a research hotspot of discharge sharpening.
(3) Research results
Foreign scholars have carried out a large number of experimental research on the discharge sharpening technology of PCD tools. The research results of TBThoe et al. from the University of Birmingham in the United Kingdom are more representative.
Their experiments were conducted on an EDG machine developed by the School of Mechanical Engineering at the University of Birmingham. The PCD sample grades used for sharpening are Syndite CTB002, 010, 025 and Compax 1500, 1600. Through the test, the following conclusions can be drawn: 1 For fine-grained PCD samples, the end face discharge sharpening can obtain better edge quality; for the coarse-grained PCD sample, the circumferential discharge sharpening can obtain better edge quality. 2 Increasing the current, voltage or pulse width can increase the grinding rate and improve the sharpening efficiency, but at the same time lead to deeper and wider cracks on the surface of the PCD tool. 3 Fine-grained PCD samples are prone to discharge, the wear of the grinding wheel electrode is small, and the average size of the grains falling off during discharge is equal to the size of the grain itself, so that a good sharpening quality can be obtained. 4 The coarse grain PCD sample and the cemented carbide interface have a greater degree of erosion.
The pulse power supply and the sharpening process steps have a great influence on the quality of the PCD discharge sharpening. German scholar E. Beck et al. have done a lot of experimental research on this. They used a common and improved type of pulse power supply on the Vollmer QR20P special spark discharge tool grinding machine to compare the quality of PCD discharge sharpening. The test uses silicon-based synthetic oil as the dielectric and graphite as the grinding wheel electrode (negative electrode). The spindle speed is 500r/min and the sample material removal is 0.5mm. In addition, on the Microspark200 universal spark discharge grinder, the impact of the sharpening process on the quality of PCD sharpening was studied. The test used a pulsed PCD dedicated pulse power supply and pulsed according to the amount of grinding and the edge roughness after sharpening. The power supply is set to level 5; the test sample grades are SynditeCTC002, CTB002, CTB010, CTB025, and each sample of PCD is taken from 4 samples. The test uses different combinations of pulse power settings (ie different process steps) for sharpening, and then the edge is measured. And the roughness of the blade. Through the test, the following conclusions can be drawn: 1 The design and controllability of the pulse power supply can play a decisive role in the quality of the sharpening. The comparison test results show that the cutting edge and the knife of the PCD sample sharpened by the tool grinding machine equipped with the improved pulse power supply The surface roughness is close to the quality of the mechanical grinding of the diamond wheel. 2 By adjusting the setting of the pulse power supply for multi-stage sharpening, and reasonably assigning the proportion of each grinding amount and the grinding time, a higher sharpening quality can be obtained.
2.2 Diamond grinding wheel mechanical grinding
Diamond grinding wheel mechanical grinding is currently the most widely used PCD tool sharpening method. Compared with the discharge sharpening, its sharpening efficiency is lower (the grinding rate is about 1.5mm3/min), and the processing cost is higher (the wear ratio is about It is 0.02min3/min3), but good tool edge quality and complete smooth front and back flank are obtained.
(1) Sharpening mechanism
The mechanism of grinding the PCD cutter by the diamond grinding wheel is complicated. The domestic and foreign scholars have done a lot of research on it. At present, there are mainly the following viewpoints: 1 German scholar M.Kenter believes that the diamond grinding wheel has been engraved in the process of grinding the PCD tool. The action of sliding and sliding, the material is mainly removed by bonding, scoring, tribochemical reaction and surface fracture. Scanning electron microscopy (SEM) was used to observe the surface topography of three sharpened workpieces PCD-1 (particle size 2 μm), PCD-2 (particle size 10 μm) and PCD-3 (particle size 25 μm). The furrow was observed and no furrow was observed on the other two PCD workpieces. Therefore, Kenter believes that in most cases, the removal of PCD materials is dominated by tribochemical reactions and surface fractures. As the grinding progresses, the diamond abrasive particles are gradually passivated, and the furrow is not easily observed even on the PCD-1. Due to the high brittleness of the PCD material, cracks are easily induced under the extrusion of diamond abrasive grains. The cracks expand under mechanical and thermal stresses, eventually leading to the peeling of small pieces of PCD material, and the frictional heat causes graphitization and other tribochemical reactions of PCD. KGE Dunn et al. of 2GE Company observed the surface morphology of the sharpened PCD composite sheet by scanning electron microscopy. It is believed that the damage mechanism of PCD material is mainly micro-brittle fracture and fatigue damage.
3 Ai Xing and others in China used a slotted diamond wheel to grind PCD. At the same time, ultrasonic vibration and laser irradiation were used to simulate the mechanical impact and thermal shock during grinding. According to the test results, the removal methods of PCD materials were summarized as follows: At the moment when the grinding wheel is in contact with the PCD, the grinding force suddenly increases, and the severe mechanical impact causes cracks on the surface of the PCD, and even fragments are generated. During the stable grinding period, the abrasive grains of the grinding wheel are pressed and rubbed on the surface of the PCD. When the pressure reaches a certain level, cracks will form on the surface of the PCD. When the friction temperature reaches a certain level, the PCD will undergo graphitization and other chemical reactions. It has been found through experiments that when grinding with a grooved diamond grinding wheel, the grinding force is discontinuous, and the periodic cooling of the coolant is beneficial to the crack propagation, so that the grooved grinding wheel has higher grinding efficiency than the non-grooved grinding wheel. 1 to 2 times.
(2) Sharpening equipment
The characteristics of the PCD material determine that the requirements for the PCD tool sharpening machine are different from those of the ordinary tool grinding machine, namely: 1 requires the grinding wheel spindle and the machine tool as a whole to have high rigidity and stability to maintain the constant pressure of the grinding wheel on the PCD material during sharpening. . 2 The grinding wheel frame can be oscillated laterally to ensure uniform wear of the grinding wheel end face; the swinging frequency and the swinging amplitude of the grinding wheel frame are adjustable. 3 The optical projector and high-precision rotary table should be equipped on the machine. 4 special diamond grinding wheel should be used.
(3) Research results
German scholar M.Kenter studied the influence of the process parameters of the grinding wheel PCD on the grinding rate and the wear ratio. Since PCD tool sharpening is a constant pressure grinding, M.Kenter uses the grinding rate and the wear ratio as the test evaluation criteria. According to the test results, the following conclusions can be drawn: 1 In order to make the radial wear of the cup-shaped diamond grinding wheel uniform, the coincidence degree of the grinding wheel and the tool should be ≥1, which can be achieved by adjusting the swing amplitude and frequency of the grinding wheel of the sharpening machine tool. 2 Increase the grinding wheel rotation speed VC, constant pressure FA and PCD particle size respectively, and the grinding rate and the wear ratio increase. Since these three process parameters have the greatest influence on the wear rate and the wear ratio, the grinding efficiency can be improved and the sharpening cost can be reduced by changing the size. 3 Grinding wheel size, diamond concentration, binder type, coolant concentration, etc. have a certain impact on the grinding rate and wear ratio.
3 Research direction
The discharge sharpening of PCD tools and the mechanical grinding technology of diamond grinding wheels are still not perfect. The main research directions in the future are as follows: 1The quality of the cutting edge and the poor surface quality of PCD cutters after sharpening are the existence of the discharge sharpening process. The problem is highlighted; the main disadvantage of the diamond grinding machine mechanical sharpening process is that the PCD tool has low sharpening efficiency and high cost of sharpening. In order to solve these key problems, it is necessary to establish a mathematical model for the key parameters affecting the quality of sharpening, sharpening efficiency and sharpening cost, and deeply study the mechanism and relationship of the process elements, and improve the sharpening process by comprehensively optimizing the process parameters. System performance. 2 Computer-aided design for the sharpening process of major PCD products. 3 Theoretically further study the PCD sharpening mechanism to guide the development and improvement of the practical sharpening process. According to the characteristics of PCD materials,
Conduct a composite study on the existing sharpening process to explore a new and effective sharpening process.
With the continuous expansion of the application field of PCD tools, the research on the sharpening process of PCD tools is becoming more and more important, and the results of this research will also promote the development and popularization of PCD tools.
Polycrystalline diamond tool sharpening process: tool sharpening is more difficult
1 Introduction