Grinding characteristics of the hottest PCD and PC

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PCD grinding characteristics and PCD tool grinding technology

1 introduction

with the rapid development of modern science and technology, there are more and more varieties of tools made of super hard materials such as polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN), and their performance has also been continuously developed and improved. The abrasive particle size of the blade ranges from tens of microns, a few microns to nanometer; The content of diamond and cubic boron nitride can be divided into low content, medium content and high content; Binders include metal, non-metal and mixed materials; The thickness of PCD layer ranges from millimeter level to micron level; The bonding modes of PCD layer and cemented carbide substrate include plane and corrugated surface; PCD layer has different characteristics such as high wear resistance, high toughness and high heat resistance. At present, the application scope of PCD and PCBN tools is expanded to automobile, aerospace, precision machinery, home appliances, wood, electronics and electrical industries, and they are used to make turning tools, boring cutters, milling cutters, drill bits, reamers, spot facers, saws, router cutters, razors, etc

despite the rapid development of PCD and PCBN tools, the difficulty of tool grinding caused by their high hardness has plagued most users, and the regrinding of blades is mainly completed by the original tool manufacturers. Not only the price of cutting tools is high and the delivery time is long, but also the working capital of the enterprise is occupied. Therefore, it is necessary to study the grinding characteristics of PCD and the grinding technology of PCD tools

2 manufacturing process of PCD cutting tools

the production process of PCD cutting tools generally includes polishing, cutting, fixing, grinding, quality inspection, etc. The diameter of PCD superhard material blank is usually 1/2, 1, 2, 3 and 4 inches, and its surface is generally rough (ra2 ~ 10 μ m) It can not be directly used to make tools. It needs to be polished to make its surface reach the mirror surface (RA ≤ 0.01 μ m) Then, the blade with certain geometric shape and size requirements is machined by laser cutting or WEDM, and then the surface to be fixed between the blade and the substrate is further mechanically and chemically treated, and then the blade is fixed to the substrate by silver based brazing, and finally grinded by diamond grinding wheel

One of the keys of PCD cutting tool manufacturing technology is the grinding quality of cutting edge. The lack of ideal grinding process and technology for high-quality tool head materials will cause a waste of resources. The adoption of good grinding process will improve the product quality of the tool and reduce the use cost of the tool

3 grinding characteristics of PCD materials

pcd is made of specially treated diamond and a small amount of binder sintered under high temperature and ultra-high pressure. The disordered diamond grains make PCD have uniform, extremely high hardness and wear resistance. PCD can be used for cutting tools, grinding wheel dressing, geological drilling, measuring tool probe, wire drawing tool, sand blasting tool, etc. However, the high hardness and wear resistance of PCD also bring great difficulties to its processing

domestic and foreign scholars have conducted a lot of research and tests on the machining problems caused by the high hardness and wear resistance of PCD materials, including EDM, ultrasonic machining, electrochemical machining, laser machining, etc., and achieved certain results. However, through comprehensive analysis, it is found that these machining technologies are mostly suitable for rough machining of PCD materials. In order to obtain good PCD cutting edge quality, the most ideal machining method is still grinding or grinding with diamond wheel

pcd grinding is mainly the result of the combination of mechanical and thermochemical effects. The mechanical action is the micro breaking, wear, falling off or cleavage of diamond formed by the continuous impact of the abrasive particles of diamond wheel on PCD materials; Thermochemical action is that the high temperature formed by grinding PCD with diamond wheel makes diamond oxidized or graphitized. The mixed effect of the two results in the removal of PCD materials. Its grinding characteristics are as follows:

(1) great grinding force

diamond is a known mineral. 4. it has the highest hardness in the function of power failure, and the wear amount of paired friction with various metal and non-metal materials is only 1/50 ~ 1/800 of that of cemented carbide; The hardness (HV) of PCD is 80 ~ 120kn/mm2, which is second only to single crystal diamond and much higher than that of cemented carbide. When PCD is grinded with diamond wheel, the initial cutting strength is very high, about 10 times higher than that of cemented carbide (0.4MPa); Specific grinding energy reaches 1.2 × 104~1.4 × 105J/mm3; Therefore, the grinding force is much higher than that of cemented carbide

(2) the grinding ratio is very small

due to the high hardness and wear resistance of PCD (the relative wear resistance is 16 ~ 199 times that of cemented carbide), the grinding ratio of PCD is only 0.005 ~ 0.033, about 1/1000 ~ 1/100000 of cemented carbide; The grinding efficiency is only 0.4 ~ 4.8mm3/min. Therefore, in order to ensure the cutting edge quality and removal of cutting tools, the grinding time is very long and the machining efficiency is very low. In addition, when the hardness, content and particle size of PCD are different, the grinding time is also very different

(3) particle size has a great influence

pcd materials used for cutting tools are mainly divided into three categories according to particle size: coarse particle size (20 ~ 50 μ m) Medium particle size (10 μ M) and fine grained (~ 5 μ m) The grinding force and grinding ratio are several times to tens of times different. Coarse grain PCD has the highest grinding ratio and the most difficult grinding, and the serration of the grinding edge is the most serious and the quality is the worst, but the wear resistance is the strongest; Fine grained PCD has the lowest grinding ratio, easier grinding and the best edge quality after grinding

4 requirements for tooling for PCD cutting tool grinding

based on the above grinding characteristics of PCD, the requirements for grinding equipment when grinding PCD with diamond grinding wheel are much higher than that of tool grinding machines for officials of China General Administration of quality supervision, inspection and quarantine who have met in Beijing to supervise import and export business. Mainly:

(1) the machine tool has good process system rigidity

due to the high hardness of PCD materials, the grinder must have high deformation resistance, especially the spindle system and tool clamping system. The grinding force of PCD cutting tool is generally 100 ~ 500N. Therefore, it is required that the shaft diameter of the machine tool should be large and the axial rigidity and strength of the bearing should be high

(2) the machine tool has a short-range swing mechanism with adjustable stroke and speed

pcd grinding ratio is very low. The grinding mechanism of PCD is mainly the result of the combination of mechanical effects such as micro crushing, wear, falling off and cleavage formed by the continuous impact of diamond wheel on PCD materials and the thermochemical effects of oxidation and graphitization. Therefore, the use of short-range swing mechanism is conducive to improve the grinding efficiency and improve the quality of tool edge. Generally, the swing distance is 0 ~ 50mm, and the swing speed is 20 ~ 60 times/min

(3) the tool holder of the machine tool has high-precision rotation function and detection device

because the PCD material is hard, brittle and wear-resistant, its tool tip is usually designed as a circular arc to reduce the amplitude of the relative vibration between the tool and the workpiece. In order to realize the machining of the tool tip arc, the tool holder of the machine tool should have a high-precision rotation function and a tool tip arc radius size and quality detection device. In this way, the positioning error caused by multiple clamping can be avoided, and the processing efficiency can be doubled

5 grinding process of PCD and PCBN tools

5.1 selection of grinding process

one of the purposes of cutting tool grinding is to obtain cutting edge quality with high cost performance, and the key to quality is the selection of grinding wheel particle size. The finer the grain size of the grinding wheel, the smaller the chipping of the cutting edge, and the lower the grinding efficiency. For this reason, the PCD cutting tool grinding process can be divided into three processing stages: rough, fine and fine according to the precision, purpose (see Table 1) or the failure degree (see Table 2) of the cutting edge of the tool. According to the specific situation, the reasonable grinding process can greatly improve the machining efficiency

Table 1 according to the precision of cutting edge Application classification under the impact force exerted by the punch

serial number - cutting edge accuracy - grain size of optional grinding wheel - Application

a rough - 0.05mm - 230/270 320/400 a rough machining

b fine - 0.02mm - M20 ~ M40 - semi finish machining

c fine - 0.005mm - M5 ~ M10 - finish machining

Table 2 classification according to the degree of blade failure (regrinding tool)

serial number - failure degree - grain size of optional grinding wheel - remarks

a coarse - cutting edge damage 0.5mm - 230/27 and the heat treatment method adopted to meet these requirements 0 320/400 . High machining efficiency, suitable for machining complex cutting tools, such as drill bits for printed circuit boards, forming milling cutters for cutting laminate floors, etc. Coarse grain grinding wheel can be selected for grinding, with large contact area and high grinding force (300 ~ 400N) during grinding, which can quickly remove excess machining allowance; Fine grained grinding wheel is selected for fine machining. During grinding, the contact area is small, the grinding force is low (100 ~ 200N), the grinding heat is small, but the material removal rate is low. In this stage, the quality of cutting edge is further improved by grinding and polishing. Finishing is one of the two

5.2 key points of grinding process

(1) the spindle accuracy is good. Generally, the end face runout of the grinding wheel should be ≤ 0.02mm. If the end face runout of the grinding wheel is too large, the grinding wheel will impact the cutting edge intermittently during grinding, which is easy to cause the cutting edge to collapse, and it is difficult to obtain a high-precision cutting edge

(2) the grinding wheel shall have good dynamic balance. The unbalance of the grinding wheel will lead to the vibration of the machine tool, and then affect the cutting edge quality and machining accuracy of the machined tool

(3) ceramic bonded diamond wheel shall be preferred as the grinding wheel. Because the ceramic bond is easy to crack in the grinding process, so that the abrasive particles can be renewed and self sharpened, making the grinding process stable, which is conducive to improving the accuracy and efficiency of the machined surface; The second choice is resin bonded diamond wheel with high heat resistance

(4) timely pay attention to the cutting edge of the grinding wheel, and the particle size of the cutting edge oilstone shall be appropriate. When PCD tools are processed with diamond grinding wheel, the grinding wheel will be blocked, passivated, high temperature and rapid wear, resulting in the reduction of processing speed and the generation of vibration marks, noise and burns. Usually, the soft silicon carbide oilstone 1 ~ 2 smaller than the grinding wheel used is selected as the cutting edge oilstone

(5) since diamond is easy to chemically diffuse with iron-based alloy, which accelerates the wear of grinding wheel, it is necessary to avoid grinding metal and PCD at the same time as much as possible

(6) the rotation direction of the grinding wheel must rotate from the front cutter face to the rear cutter face. From the force on the cutting edge of PCD tool during grinding, when the grinding wheel rotates from the front edge of the tool to the back surface, its grinding force (the sum of tangential and normal forces) acts on the cutting edge inward, that is, the tool is under pressure stress and is not easy to collapse; Otherwise, it is tensile stress, and the cutting edge is easy to collapse. If it is necessary to reverse grinding because of the tool structure, the resin bonded grinding wheel is better than the metal and ceramic bonded grinding wheel

(7) in order to ensure the cutting edge quality and improve the grinding efficiency, the back angle of the tool can be divided into large back angle and small back angle. Using coarse-grained grinding wheel to grind large back angle first, because the contact surface is large, the grinding force is large, and the grinding efficiency is high; Then use the fine-grained grinding wheel to grind the small back angle, and control the width of the blade belt of the small back angle to about 0.1 ~ 0.3mm. The small contact surface has good grinding quality

(8) finish machining the cutting edge of the tool in one clamping as far as possible

(9) the PCD grinding coolant should preferably be water-based grinding fluid. Due to the high hardness and poor heat resistance of PCD materials, the cooling effect of water-based grinding fluid is better than that of oil-based grinding fluid, which can improve the machining efficiency and edge quality. In addition, the cooling shall be sufficient in the grinding process, and the flow shall not be cut off, so as to avoid large consumption (oxidation and graphitization) of diamond (grinding wheel and cutting tool) and damage of cutting edge of cutting tool due to small amount of grinding fluid or intermittent supply. (end)

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