(1) Selection of PCBN tools
PCBN tools often do not show good cutting performance when processing medium-hard materials. For example, when processing medium and low-hardness workpieces, PCBN tool life is not as long as that of cemented carbide. The tool only starts when processing workpieces with hardness above 50HRC. Exerting its super-hard performance, such as processing workpieces with a hardness of about 60HRC, its life is more than 10 times longer than that of cemented carbide tools. Therefore, only when PCBN tools are used to process high-hardness materials can they exert their superior performance.
The effect of CBN grain size on the cutting performance of PCBN tool materials, and the wear resistance of PCBN tools is related to the size of CBN grains in the sintered body. The size of the CBN grains affects the strength of the tool. The fine grains can increase the grain boundary area of the grains, improve the sintering strength and the ability to resist crack growth, thereby increasing the wear resistance of PCBN tools. When the particle size is increased by 1 time, the tool life will be reduced by 30% to 50%. Figure 2-28 shows the tool wear curves of three grain sizes when cutting hardened steel. It can be seen from Figure 2-39 that the smaller the grain, the stronger the wear resistance. This is due to the smaller the grain. The higher the strength of the composite sheet, the higher the tool life.
1. Determination of PCBN geometric parameters Whether the geometric parameters of the cutting part are selected reasonably and properly will have a great impact on the life of PCBN tools, sometimes the difference can be as much as several times to more than ten times. Since the toughness of PCBN composite sheet is higher than that of ceramic but lower than that of cemented carbide, the strength of cutting edge and tip should be considered when designing and selecting PCBN tool geometric parameters.
(1) Rake angle: The generally recommended range is 0°～-10°, usually 0° rake angle is mostly used, and there are also negative or positive rake angles. As for the size of the negative rake angle, it depends on the specific conditions of the processed parts.
(2) Relief angle: generally choose a smaller one, choose within the range of 6°～10° to ensure that the wedge angle of the cutting part of the tool is large enough.
(3) Blade inclination: If there is no special requirement, it is usually 0°, and sometimes it can be a negative value to ensure that the cutting edge has a better stress state when cutting.
(4) Tool tip angle: Except for special requirements, generally it should not be less than 90°, and the tool tip should be ground into an arc of about rε=0.4～1.2mm to ensure that the tool tip has sufficient strength.
(5)Cutting edge: Generally, negative chamfering is used to increase the cutting edge strength. The width of the negative chamfer is related to the cutting amount and the properties of the processed material, and is generally taken as 0.2mm×(-15°～-30°).
(6) Main deflection angle and secondary deflection angle: The selection of these two parameters is mainly determined according to the specific processing conditions.
2. Use of cutting fluid Although the PCBN composite sheet can withstand a cutting temperature of 1200 to 1300°C, the welding part between the PCBN composite sheet and the cutter body can only withstand a temperature of about 800°C (in the case of brazing). At one point, it is best to use cutting fluid for cooling. However, at about 1000°C, CBN and water will hydrolyze, causing severe wear of PCBN tools. Therefore, it is not advisable to use water or other oxidizing cutting fluids to avoid aggravating oxidative wear. The most suitable cutting fluid is cutting oil with extreme pressure.
3.The particularity of the impact of workpiece hardness on PCBN tool life When cutting hardened steels with different hardnesses, as the hardness of the processed material increases, PCBN tool life does not decrease monotonously, but the tool life is the lowest at 40HRC. When the workpiece material has a high hardness After this hardness, PCBN tool life increases instead. When the hardness is lower than this hardness, PCBN tool life is lower than that of cemented carbide tools. This is mainly due to the small hardness loss of PCBN material at high temperature, while the hardness loss of the processed material is relatively large. The characteristic of PCBN is very suitable for processing high-hardness materials. The most representative one is the technology of turning instead of grinding in hard cutting to obtain the surface quality that can only be obtained by grinding in the past. At present, this cutting technology called “Red Crescent” (Red Crescent) is being researched and promoted.
4. PCBN tool life is high at high speed. When the cutting speed exceeds a certain limit, the higher the cutting speed, the higher the cutting temperature, the lower the hardness of the processed material, and the wear speed of PCBN tools will decrease, that is, the life of PCBN tools at high speeds is higher than that at low speeds. . Therefore, PCBN tools are most suitable for high-speed cutting of high-hardness materials. At present, high-speed and ultra-high-speed cutting technology has shown its superiority. As an advanced manufacturing technology, it has been widely valued. PCBN is the most promising tool material for high-speed cutting technology.
5. Points for reference selection of PCBN
(1) Select the appropriate cutting amount
1) Optimization of cutting speed. PCBN tool material has high hardness at room temperature and high temperature, and good thermal stability. The mechanism of cutting hard materials is to soften the metal in the small area of the cutting area, so that the hardness difference between the tool and the workpiece is increased, and the cutting is more efficient. Easy to proceed. The cutting speed is too low to generate enough cutting heat to soften the metal in the cutting area, so that the cutting performance of the PCBN tool cannot be exerted. Therefore, the selected value of the cutting speed is much higher than that of cemented carbide tools. In addition, PCBN shows special laws when used for cutting at higher speeds. Many experiments with PCBN tools for cutting hardened steel, high-speed steel, bearing steel and other high-hard materials and cast iron have shown that the cutting speed is higher than a certain level. After the value, the relationship between cutting speed and tool life no longer conforms to Taylor’s formula, that is, the tool life will increase when the cutting speed is higher than a certain value. Therefore, PCBN tools are very suitable for high-speed and ultra-high-speed cutting.
Generally speaking, the cutting speed should be selected according to the material to be processed. For example, when cutting various hardened steels (50～64HRC), the turning speed can be selected in the range of 70～200m/min, while cutting Ni-based spraying (welding) parts ( 50～62HRC), the cutting speed can be selected in the range of 50～80m/min. The tool can be used in the optimal cutting speed range to obtain a higher service life.
2) The optimal feed amount, because PCBN cutters generally have negative chamfering, therefore, when possible, the choice of feed amount should be greater than the chamfering width. In addition, PCBN tools are mainly used for finishing, and the amount of back-grabbing is generally small. In order to produce the metal softening effect, the feed amount should be relatively large. To make the PCBN tool produce a metal softening effect when cutting, it is necessary to make the cutting area large enough. However, the feed rate of PCBN tool should generally not be greater than 0.2mm/r.
3) Optimizing the amount of back-grabbing. PCBN tools are mainly used for finishing and semi-finishing, so the amount of back-grabbing is mostly below 1mm. When PCBN tools are used to cut hard materials with a hardness greater than 50HRC, a small amount of back-grabbing will easily cause rapid tool wear. When a larger amount of back-grabbing is used, there is enough heat to soften the metal in the cutting area (hardness is reduced) ), on the contrary, the wear rate of the PCBN tool is reduced. Therefore, the selection of the amount of back-grabbing knife should consider the softening effect of metal, and it is not suitable to choose too small. The amount of the back knife is generally chosen not to be less than 0.3mm.
The choice of cutting amount should pay attention to the comprehensive influence between the three. Although the PCBN composite sheet is resistant to high temperature, the welding strength of the composite sheet and other parts often restricts the improvement of cutting efficiency. For example, the welding of the composite sheet and the cutter body is often due to cutting Excessive temperature will cause desoldering, causing falling off or hitting a knife, and damaging the quality of the processed surface.
(2) Requirements for processing equipment. Because PCBN tools are more brittle than carbide tools, there must be certain requirements on the process system of the machine tool when using it. First, the machine tool spindle must be small in deflection, the tool holder and the entire processing system must be rigid, and the vibration of the machine tool must be small to prevent it from hitting. Knife. Since PCBN tools are mostly used for cutting hard-to-machine materials such as hardened steel and wear-resistant cast iron, the tools have negative chamfers, so the radial force is large, which requires the rigidity and precision of the machine tool to be good, and the system vibration should be small.
When installing PCBN tools, the tool tip should be aligned accurately, the cantilever should be small, and the extension length of the tool head should be short, generally less than 25-30mm. Careful operation should be performed when setting the tool to avoid collisions; it is not suitable for processing on waste surfaces. The workpiece flash is best to be chamfered; the workpiece should be clamped well, and the factors that cause vibration during cutting should be eliminated as much as possible, such as adding a center frame when processing a slender shaft.
The recommended PCBN grades are CB7105, CB7025, CB7035, CB7525, as shown in Figure 2-41.
CB7015 is used for continuous to light interrupted cutting;
CB7025 is used for light to medium interrupted cutting;
CB7035 is used for medium to heavy intermittent cutting;
CB7525 is used for heavy-duty interrupted cutting.
(2) Selection of PCD tools
The thermal stability of diamond is relatively poor, and its hardness will be lost when the cutting temperature reaches 800°C. Diamond tools are not suitable for processing steel materials, because diamond has a strong chemical affinity with iron. At high temperatures, iron atoms easily interact with carbon atoms to convert them into graphite structures, and the tools are extremely easy to damage. PCD tools are limited to processing non-ferrous materials, such as high silicon aluminum, metal matrix composite materials (MMC) and carbon fiber reinforced plastics (CFRP). PCD using a large amount of cutting fluid can also be used for super finishing of titanium materials. Using single crystal diamond tools, mirror processing can be achieved on ultra-precision lathes. Single crystal diamond tools are currently the most important tool in the field of ultra-precision machining. The cutting edge can be sharpened. The radius of the cutting edge can reach 20~30nm. The surface roughness of the processed workpiece is small and the tool life is very long. , Sharpening can be used for several hours at a time. At present, single crystal diamond tools are widely used for processing computer disk substrates, video recorder drums, laser mirrors, various astronomical telescopes, microscopes, and optical instruments.