Grinding characteristics of PCD and PCD tool sharpening technology

1 Introduction With the rapid development of modern science and technology, the variety of tools made of superhard materials such as polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN) is becoming more and more abundant, and its performance has been continuously developed and improved. . The abrasive grains of the blade are from tens of micrometers to several micrometers to nanometers; the content of diamond and cubic boron nitride is divided into low content, medium content and high content; the bonding agent is composed of metal, non-metal or mixed material; the thickness of PCD layer is from From millimeter to micrometer; the combination of PCD layer and cemented carbide substrate has flat and corrugated surface; PCD layer has different characteristics such as high wear resistance, high toughness and high heat resistance. At present, the application range of PCD and PCBN tools has been extended to the automotive, aerospace, precision machinery, home appliances, wood, electronic and electrical industries for the production of turning tools, boring tools, milling cutters and drill bits, reamers, boring tools and saw blades. Sickles, razors, etc.
Despite the rapid development of PCD and PCBN tools, the difficulty of tool sharpening caused by its high hardness has been plaguing most users. The re-grinding of the blades is mainly done by the original tool manufacturers. Not only the high price of the tool, the long delivery time, but also the liquidity of the company. Therefore, it is necessary to seriously study the grinding characteristics of PCD and the sharpening technology of PCD tools.
2 Manufacturing Process of PCD Tool The production process of PCD cutting tool generally includes polishing, cutting, fixing, sharpening, quality inspection and so on. PCD superhard material blanks are usually 1/2, 1, 2, 3, 4 inches in diameter, and their surface is generally rough (Ra2 ~ 10μm). It can not be directly used to make tools. It needs to be polished to make the surface mirror surface (Ra ≤0.01μm), then processed into blades of certain geometry and size by laser cutting or wire EDM, and further mechanical and chemical treatment of the blade and the substrate to be fixed, then silver-based brazing It is fixed to the substrate and finally sharpened by a diamond grinding wheel.
One of the keys to PCD cutting tool manufacturing technology is the sharpening quality of the cutting edge. The lack of ideal sharpening technology and technology for high-quality cutter head materials will result in waste of resources. The use of a good sharpening process will improve the quality of the tool and reduce the cost of tool use.
3 Grinding processing characteristics of PCD material PCD is made by special treatment of diamond and a small amount of binder sintered under high temperature and high pressure. The disorderly arrangement of diamond grains gives the PCD a uniform, extremely high hardness and wear resistance. PCD can be used for cutting tools, grinding wheel dressing, geological drilling, gage measuring heads, wire drawing tools, sand blasting tools and so on. However, the high hardness and high wear resistance of PCD also bring great difficulties to its processing.
Domestic and foreign scholars have carried out a large number of research and experiments on the processing problems caused by the high hardness and high wear resistance of PCD materials, including EDM, ultrasonic machining, electrochemical machining, laser processing, etc., and have achieved certain effects. . However, comprehensive analysis found that these processing techniques are currently more suitable for roughing of PCD materials. To get a good PCD cutting edge quality, the ideal machining method is still grinding or grinding with a diamond wheel.
Grinding of PCD is mainly the result of a combination of mechanical and thermochemical effects. The mechanical action is the micro-crushing, abrasion, shedding or cleavage of the diamond formed by the continuous impact of the diamond grinding wheel on the PCD material; the thermochemical action is the high temperature formed by the grinding of the PCD by the diamond grinding wheel to oxidize or graphitize the diamond. The result of the mixing of the two causes the PCD material to be removed. Its grinding processing features are mainly:
(1) Large grinding force Diamond is the hardest material among minerals. The wear amount of friction with various metal and non-metal materials is only 1/50~1/800 of cemented carbide; the hardness of PCD ( HV) is 80-120KN/mm2, second only to single crystal diamond, much higher than hard alloy. When grinding PCD with diamond grinding wheel, the initial cutting strength is very high, about 10 times higher than that of cemented carbide (0.4MPa); the specific grinding energy can reach 1.2×104～1.4×105J/mm3; therefore, the grinding force is much higher. For grinding cemented carbide.
(2) The grinding ratio is very small. Because the hardness and wear resistance of PCD are very high (relative wear resistance is 16 to 199 times that of cemented carbide), the grinding ratio is only 0.005 to 0.033 when grinding PCD. 1/1000~1/100000 of cemented carbide; grinding efficiency is only 0.4~4.8mm3/min. Therefore, in order to ensure the cutting edge quality and removal amount of the cutting tool, the grinding time is long and the processing efficiency is low. In addition, when the hardness, content, and particle size of PCD are different, the grinding time is also very different.
(3) The influence of particle size is very large. The PCD materials are mainly divided into three types according to the particle size: coarse grain size (20～50μm), medium grain size (about 10μm) and fine grain size (~5μm), grinding force and grinding. It is several times to several tens of times different. The coarse-grained PCD has the highest grinding ratio and the most difficult grinding. After grinding, the cutting edge is the most severe and the worst quality, but the wear resistance is the strongest. The fine-grained PCD grinding ratio is relatively low, and the grinding is easy. The quality of the cutting edge is best after grinding.
4 PCD cutting tool sharpening requirements for tooling Based on the above-mentioned grinding characteristics of PCD, the requirement for sharpening equipment when grinding PCD with diamond grinding wheel is much higher than that of general tool grinding machine. There are:
(1) Machine tool has good process system rigidity Due to the high hardness of PCD material, the grinding machine must have high resistance to deformation, especially the spindle system and tool clamping system. When the PCD cutting tool is sharpened, the grinding force is generally 100-500N. Therefore, the machine tool has a large shaft diameter and a high axial rigidity and strength of the bearing.
(2) The machine tool has a short stroke swinging mechanism with adjustable stroke and adjustable speed. The PCD grinding ratio is extremely low. The grinding mechanism of PCD is mainly caused by the continuous impact of the diamond grinding wheel on the PCD material, resulting in micro-crushing, wear, shedding and solution. The result of mixing mechanical action with oxidation and graphitization thermochemistry. Therefore, the use of a short-range swing mechanism is beneficial to improve the grinding efficiency and improve the cutting edge quality. Generally, the swing distance is 0 to 50 mm, and the swing speed is 20 to 60 beats/min.
(3) The tool holder of the machine tool has high-precision rotation function and on-line detection device. Because the PCD material is hard and brittle and wear-resistant, the tool tip is usually designed in an arc shape to reduce the amplitude of the relative vibration of the tool and the workpiece. In order to realize the machining of the cutting edge arc, the tool holder of the machine tool should have a high-precision turning function and an on-line detecting device for the size and quality of the tool nose arc radius. This avoids the positioning error caused by multiple clamping and can double the processing efficiency.
5 Grinding process of PCD and PCBN tools 5.1 Selection of sharpening process One of the purposes of cutting tool sharpening is to obtain cost-effective cutting edge quality, and the key to quality is the choice of sharpening wheel size. The finer the grain size of the grinding wheel, the smaller the cutting edge of the cutting edge, and the lower the grinding efficiency. For this reason, according to the accuracy and use of the cutting edge of the tool (see Table 1) or its failure degree (see Table 2), the PCD cutting tool sharpening process is divided into three processing stages: coarse, fine and fine. According to the specific situation, the reasonable sharpening process can greatly improve the processing efficiency.
Table 1 According to the accuracy of the cutting edge, the classification of the use number - cutting edge precision - optional sharpening wheel particle size - use a coarse - 0.05mm - 230 / 270 # ~ 320 / 400 # - rough processing b fine - 0.02mm - M20 ~ M40—semi-finishing c--0.005mm—M5～M10—finishing table 2 Classification according to blade failure degree (re-grinding tool)
Serial number—degree of failure—optional sharpening wheel particle size—remarks a coarse—cutting edge damage 0.5mm—230/270#～320/400#—or electrical machining b fine—cutting edge collapse 0.3mm—M20～M40
c fine - cutting edge wear 0.1mm - M5 ~ M10
Roughing requires less edge and can be machined or ground. High electrical processing efficiency, suitable for processing complex tools, such as drill bits for printed circuit boards, forming cutters for cutting laminate flooring. Grinding processing can choose coarse-grained grinding wheel. When grinding, the contact area is large, the grinding force is high (300~400N), and the excess machining allowance can be quickly removed. The fine-grained grinding wheel is used for fine machining, and the contact area is small when sharpening. Low grinding force (100 ~ 200N), less heat generation, but low material removal rate. This stage is mainly to improve the cutting edge quality by grinding and polishing. Finishing is in the middle of both.
5.2 Sharpening process points (1) The accuracy of the spindle should be good. Generally, the end face of the grinding wheel should be ≤ 0.02mm. The end face of the grinding wheel is too large, and the grinding wheel intermittently impacts the cutting edge during grinding, which makes it easy for the cutting edge to collapse, and it is difficult to obtain a high-precision cutting edge.
(2) The grinding wheel should have a good dynamic balance. The imbalance of the grinding wheel will cause the vibration of the machine tool, which will affect the cutting edge quality and machining accuracy of the tool being machined.
(3) The ceramic grinding wheel should be preferred for sharpening grinding wheels. Because the ceramic bond is prone to micro-cracking during the grinding process, the abrasive grains are renewed and sharpened, which makes the grinding process smooth, which is beneficial to improve the precision and efficiency of the machined surface. Secondly, the resin binder with higher heat resistance is selected. Diamond grinding wheel.
(4) Pay attention to the grinding wheel at the appropriate time, and the grain size of the edged stone should be suitable. When machining PCD tools with diamond grinding wheels, the grinding wheel will become clogged, passivated, high temperature and rapid wear, resulting in reduced processing speed and vibration, noise and burns. Soft silicon carbide whetstones having a finer grain size of 1 to 2 than the used grinding wheel are usually selected as the edged whetstone.
(5) Since diamond is easily chemically diffused with iron-based alloys to accelerate wear of the grinding wheel, it is therefore necessary to avoid grinding metal and PCD at the same time.
(6) The direction of rotation of the grinding wheel must be swung from the tool nose to the flank face. From the force of the cutting edge of the PCD tool during grinding, when the grinding wheel rotates from the tool nose to the flank face, the grinding force (the sum of the tangential and normal forces) acts on the cutting edge inward, that is, the tool is affected. Compressive stress, not easy to collapse; vice versa, tensile stress, the cutting edge is easy to collapse. If it is necessary to reverse the sharpening due to the structure of the tool, the resin bond grinding wheel is preferred over the metal and ceramic bond grinding wheel.
(7) In order to ensure the cutting edge quality and improve the sharpening efficiency, the back angle of the tool can be divided into a large relief angle and a small relief angle. Grinding the large back angle with a coarse-grained grinding wheel, because the contact surface has large grinding force and high sharpening efficiency; then the small back angle is sharpened with a fine-grained grinding wheel, and the width of the small back angle is controlled to be about 0.1-0.3 mm. The contact surface has a good quality.
(8) The machining of the cutting edge of the tool is completed in one setup as much as possible.
(9) The PCD sharpening coolant should preferably be a water-based grinding fluid. Due to the high hardness and poor heat resistance of the PCD material, the water-based grinding fluid has better cooling effect than the oil-based grinding fluid, which can improve the processing efficiency and the quality of the cutting edge. In addition, during the grinding process, the cooling should be sufficient, and the flow should not be stopped, so as to avoid the large consumption (oxidation, graphitization) of the diamond (grinding wheel, cutter) and the cutting edge of the cutter due to the small amount of grinding fluid or intermittent supply.