Despite the rapid development of PCD and PCBN tools, the grinding difficulties caused by the high hardness of tools have been puzzling most users, and the regrinding of blades is mainly completed by the original tool manufacturers. Not only the cutter price is high, the delivery time is long, but also it takes up the working capital of the enterprise. Therefore, it is necessary to seriously study the grinding characteristics of PCD and the grinding technology of PCD tools. The production process of PCD tools generally includes polishing, cutting, fixing, grinding and quality inspection. The diameter of PCD superhard material blank is usually 1/2, 1, 2, 3 and 4 inches, and the surface is generally rough (Ra2- 10μ m), so it can't be directly used to make tools. It needs grinding and polishing to mirror its surface (Ra≤0.0 1μm), and then it needs laser cutting or electric spark.
One of the keys of PCD tool manufacturing technology is the grinding quality of cutting edge. The lack of ideal grinding technology and technology for high-quality tool bit materials will waste resources. Adopting good grinding technology will improve the product quality of tools and reduce the use cost of tools. PCD is made of specially treated diamond and a small amount of binder by high temperature and ultra-high pressure sintering. The disordered diamond particles make PCD have uniform and extremely high hardness and wear resistance. PCD can be used for cutting tools, grinding wheel dressing, geological drilling, measuring tools, wire drawing tools, sand blasting tools and so on. However, the high hardness and wear resistance of PCD also bring great difficulties to its processing.
Scholars at home and abroad have done a lot of research and experiments on machining problems caused by high hardness and high wear resistance of PCD materials, including EDM, ultrasonic machining, electrochemical machining and laser machining, and achieved certain results. However, it is found that most of these processing technologies are suitable for the rough machining of PCD materials. In order to obtain good quality PCD cutting edge, the most ideal machining method is still grinding or grinding with diamond grinding wheel.
The grinding of PCD is mainly the result of the combined action of machinery and thermochemistry. The mechanical action is the micro-crushing, abrasion, shedding or cleavage of diamond formed by the continuous impact of diamond grinding wheel abrasive particles on PCD materials; Thermochemical action is that diamond is oxidized or graphitized by the high temperature formed by grinding PCD with diamond grinding wheel. The result of the mixing of the two leads to the removal of PCD material. Its grinding characteristics are mainly as follows:
(1) high grinding force.
Diamond has the highest hardness among the known minerals, and the wear amount when paired with various metallic and nonmetallic 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 cemented carbide. When grinding PCD with diamond grinding wheel, the initial cutting strength is very high, which is about 10 times that of cemented carbide (0.4MPa). Specific Mokeda1.2×104-1.4×105j/mm3; So the grinding force is much higher than grinding cemented carbide.
⑵ The grinding ratio is very small.
Because PCD has high hardness and good wear resistance (the relative wear resistance is 16- 199 times that of cemented carbide), the grinding ratio of PCD is only 0.005-0.033, which is about11000-1/of cemented carbide. The grinding efficiency is only 0.4-4.8 mm3/min. Therefore, in order to ensure the quality of cutting edge and the removal of tool, grinding time is very long and 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.
⑶ The particle size has a great influence.
According to the particle size, PCD materials used in cutting tools are mainly divided into three categories: coarse particle size (20-50μ m), medium particle size (about 10μm) and fine particle size (-5μ m), and their grinding force and grinding ratio differ by several times to dozens of times. Coarse-grained PCD has the highest grinding ratio and the most difficult grinding. After grinding, the serrated edge is the most serious and the quality is the worst, but the wear resistance is the strongest. The grinding ratio of fine-grained PCD is the lowest, which makes it easier to grind, and the cutting edge quality after grinding is the best. Based on the above grinding characteristics of PCD, the requirements for grinding equipment when grinding PCD with diamond grinding wheel are much higher than those of general tool grinding machines. Mainly includes:
(1) Machine tool has good rigidity of process system.
Because of the high hardness of PCD material, grinder must have high deformation resistance, especially spindle system and tool clamping system. The grinding force of PCD tool is generally 100-500 N, so it is required that the shaft diameter of the machine tool is large and the axial stiffness and strength of the bearing are high.
⑵ The machine tool has a short-range slewing mechanism with adjustable stroke and speed.
The grinding ratio of PCD is extremely low, and the grinding mechanism of PCD is mainly the result of micro-pulverization, wear, shedding and cleavage caused by the continuous impact of diamond grinding wheel on PCD materials, as well as the thermochemical effects of oxidation and graphitization. Therefore, the adoption of short-range swing mechanism is beneficial to improve grinding efficiency and cutting edge quality. Generally, the swing distance is 0-50 mm, and the swing speed is 20-60 times/minute.
(3) The tool rest of the machine tool has high-precision rotation function and online detection device.
Because PCD material is hard, brittle and wear-resistant, its tip is usually designed as an arc to reduce the relative vibration amplitude between the tool and the workpiece. In order to realize the machining of the tool tip arc, the tool rest of the machine tool should have a high-precision rotary function and an online detection device for the radius and quality of the tool tip arc. This can avoid the positioning error caused by multiple clamping and double the machining efficiency. 5. 1 Selection of grinding process
One of the purposes of tool sharpening is to obtain the edge quality with high performance-to-price ratio, and the key to quality lies in the choice of grinding wheel granularity. The finer the grinding wheel granularity, the smaller the edge collapse and the lower the grinding efficiency. Therefore, according to the precision, use (see table 1) or failure degree (see table 2) of the cutting edge, the grinding process of PCD tools can be divided into three processing stages: rough, fine and fine. Making a reasonable grinding process according to the specific situation can greatly improve the machining efficiency.
Rough machining does not require high cutting edge, so you can choose electric machining or grinding. Electro-machining efficiency is high, and it is suitable for machining complex tools, such as drill bit of printed circuit board, forming milling cutter for cutting laminated wood floor, etc. Coarse-grained grinding wheel can be used for grinding, with large contact area and high grinding force (300-400 N), which can quickly remove excess machining allowance. Fine grinding wheel is selected for finishing, which has the advantages of small contact area, low grinding force (100-200 N) and low grinding heat, but the material removal rate is low. At this stage, the quality of the cutting edge is further improved by grinding and polishing. Finishing is somewhere in between.
⒌2 Key points of grinding process
(1) The spindle accuracy is good. Generally, the end runout of the grinding wheel should be ≤ 0.02 mm, and the end runout of the grinding wheel is too large. When grinding, the grinding wheel impacts the cutting edge intermittently, which is easy to cause the cutting edge to collapse, and it is difficult to obtain a high-precision cutting edge.
2 grinding wheel should have good dynamic balance. The unbalance of grinding wheel will lead to the vibration of machine tool, and then affect the cutting edge quality and machining accuracy of the machined tool.
⑶ Ceramic bonded diamond grinding wheel should be the first choice for edging grinding wheel. Because ceramic bonding agent is easy to produce microcracks during grinding, abrasive particles are updated and self-sharpening, which makes the grinding process stable and is conducive to improving the accuracy and efficiency of machining surfaces; Secondly, a resin bonded diamond grinding wheel with high heat resistance can be selected.
(4) Pay attention to timely grinding the grinding wheel, and the granularity of oilstone should be appropriate. When machining PCD tools with diamond grinding wheel, the grinding wheel will be blocked, passivated and worn at high temperature and speed, which will lead to the decrease of machining speed, vibration, noise and burns. Usually, soft silicon carbide whetstone with a grain size smaller than that of the used grinding wheel 1-2 is selected as the whetstone.
5] Because diamond is easy to chemically diffuse with iron-based alloy and accelerate the wear of grinding wheel, it is necessary to avoid grinding metal and PCD at the same time.
[6] The turning of the grinding wheel must be from the rake face to the flank face of the tool. From the stress on the cutting edge of PCD tool in the grinding process, we can know that when the grinding wheel rotates from the rake face to the back of the tool, its grinding force (the sum of tangential force and normal force) acts inward on the cutting edge, that is, the tool is in a state of compressive stress and is not easy to collapse. On the contrary, it is tensile stress, and the cutting edge is easy to collapse. If reverse grinding is needed due to the tool structure, resin bonded grinding wheel is superior to metal and ceramic bonded grinding wheel.
1. In order to ensure the quality of the cutting edge and improve the grinding efficiency, the rake angle of the tool can be divided into large rake angle and small rake angle. The coarse-grained grinding wheel grinds the rear corner first, which has high grinding efficiency due to the large contact surface and large grinding force; Then use a fine grinding wheel to grind the small clearance angle, and control the sideband width of the small clearance angle to be about 0. 1-0.3 mm, so that the grinding quality of the small contact surface is good.
Being as far as possible in the cutting edge of clamping tool processing.
Levies water-based grinding fluid should be preferred as cooling fluid for ⑼PCD grinding. Due to the high hardness and poor heat resistance of PCD material, the cooling effect of water-based grinding fluid is better than that of oil-based grinding fluid, which can improve machining efficiency and cutting edge quality. In addition, in the grinding process, the cooling should be sufficient to avoid the large consumption (oxidation and graphitization) of diamonds (grinding wheels and tools) and the damage of tool edges caused by the lack or intermittent supply of grinding fluid.