Types and use of cubic boron nitride tools

In the past, cubic boron nitride tools have been mainly used for finishing. In the past ten years, by improving the production process, controlling the purity and grain size of raw materials, using composite materials and hot pressing processes, the brittleness has been greatly improved, the toughness is improved, and the use is reliable. Greatly improved, it can be used as a conventional tool in production, which plays an important role in improving work efficiency and ensuring product quality.
Performance characteristics of cubic boron nitride tool material Cubic boron nitride tool refers to a tool that uses cubic boron nitride (CBN) to make a cutting part. Cubic boron nitride (CBN) is one of the isomers of boron nitride BN. Its structure is similar to that of diamond. It is a superhard material synthesized by ultra-high temperature and ultra-high pressure technology. Its hardness is second only to diamond (microhardness up to 8000 ~ 9000HV), and high thermal stability (up to 1250 ~ 1350 ° C), chemical inertness to iron elements, strong anti-adhesive ability, can be ground with diamond grinding wheel It is suitable for processing hard materials above 35HRC, such as various hardened steels (carbon tool steel, alloy tool steel, high speed steel, bearing steel, die steel, etc.), chilled cast iron, cobalt base and nickel base. High-hardness and wear-resistant materials such as high-temperature alloys, hard alloys, and surface-sprayed (welded) materials can also be used in the processing of titanium alloys, pure nickel, pure tungsten and other materials. It is one of the best tools for grinding instead of grinding. The surface roughness of the hardened steel processed by the cubic boron nitride tool is Ra1.60.4μm, and the precision is up to IT7IT6.
Shanghai General Motors milled the engine block plane (casting) with a cubic boron nitride CBN300 insert face milling cutter produced by Seco Tools (Shanghai) Co., Ltd. on a flexible engine production line with a cutting speed of up to 1600 m/min and a feed rate of 5000 mm. /min.
Cubic boron nitride tools also have limitations in use, such as greater brittleness, poor strength and toughness, and are not suitable for intermittent surface processing under impact loads. In addition, when it is used to process soft iron-based element materials, it is easy to generate long chips, scratching the rake face of the tool and forming a built-up edge, which may cause fluctuations in cutting force and damage the tool.
In order to improve the impact resistance of boron nitride tools, a new type of WBN material (WBN is a genus synthesized by graphite-based hexagonal boron nitride (g-BN or h-BN)). BN of lead-zinc ore structure. It has high toughness and sintering activity, can be easily converted into CBN, and is easily sintered together with CBN to form a polycrystalline crystal with strength and hardness close to CBN level.
The two-phase sintered body of WBN and CBN uses the toughness of WBN to compensate the brittleness of CBN. Its impact strength is 60% higher than that of pure CBN sintered body, the fracture toughness is increased by 2.53 times, and the flexural strength is increased by about 1 time. The compressive strength is comparable to that of cemented carbide, and the thermal stability is 1100~1200 °C. A workpiece that can be used to machine a discontinuous surface, such as the tooth face of a gear, a surfacing surface, or a grooved workpiece.
At present, WBN and CBN two-phase sintered body tools have been widely used in the automotive industry in the United States, Japan and Russia, such as for processing connecting rods, tappets, cylinder liners, brake brake wheels, crankshafts, flywheels, In the process of car, boring and milling of various pump casing parts.
1. Types of cubic boron nitride cutters There are three types of cubic boron nitride cutters commonly used in production: integrated polycrystalline cubic boron nitride (PCBN) cutters, polycrystalline cubic boron nitride and cemented carbide composite inserts (PCBN/ CC) and electroplated cubic boron nitride tools. Among them, the first two types of tools are used the most.
PCBN is a polycrystalline material obtained by polycrystallizing many CBN single crystal powders under high temperature and high pressure, and is a sintered body of CBN single crystal powder. PCBN/CC is a composite blade made of a 0.5~1mm thick PCBN on a cemented carbide substrate with good strength and toughness. It solves the low bending strength of the cubic boron nitride insert. Problems such as difficulty in welding. PCBN/CC can be made by welding or machine clamping. For example, the BZN blade produced by GE (General Electric) Co., Ltd., the Toma-10 blade of Russia, the BN blade of Sumitomo Electric Co., Ltd., and the LDP-J-CF, DLS-F, LF and LBN-Y series produced in China. The blades are all PCBN composite blades.
The electroplated cubic boron nitride tool is a method of burying sand by electroplating. The metal nickel, copper and cobalt are used as a bonding agent, and the fine particles of CBN are encapsulated on the surface of the tool base of a certain size and geometry, and then properly ground. production. The electroplated cubic boron nitride reamer produced by the Zhengzhou Abrasives Grinding Research Institute is such a tool.
Correct selection of the type and grade of the blade Different types of PCBN tools, due to their different composition, the cutting performance is very different, must pay attention to the selection. At present, PCBN tools are not classified internationally like cemented carbide. Each production plant has its own variety and grade. When using it, it must be selected according to the manufacturer's product sample.
The performance of PCBN is related to the particle size, content and type of binder of CBN. According to its structure, it can be roughly divided into two categories: one is directly sintered from CBN grains without binder, and the content of CBN is more than 70%. ), high hardness, suitable for cutting of heat-resistant alloys, cast iron and iron-based sintered metals; the other is based on CBN grains, through ceramic bonding agents (mainly TiN, TiC, TiCN, AlN, Al2O3, etc.) Sintered, this kind of PCBN has less CBN content (less than 70%) and low hardness, which is suitable for continuous cutting of hardened steel.
For the high-speed interrupted cutting of hardened steel, the newly developed new PCBN variety Sumidoron BNX25 can be used. It has high resistance to crater wear, and it can cause tool failure caused by fracture and breakage during high-speed interrupted cutting. High resistance. The recommended cutting conditions for machining hardened steel with BNX25 are cutting speeds of 120 to 200 m/min, feeds of 0.15 to 0.3 mm/r, and backing tools of 0.2 to 0.5 mm.
The grain size of CBN is roughly classified into three types: coarse grains (average grain size of 20 to 50 μm), medium grains (10 to 20 μm), and fine grains (2 to 10 μm). The larger the grain size, the better the wear resistance, the higher the tool life, but the poor quality of the cutting edge makes it difficult to make a high-precision tool. On the contrary, the fine-grain cutter has a good cutting edge quality and a good surface quality. Therefore, the crystal grains of the polycrystal are continuously refined, and there are fine crystals of 1 μm or even 0.5 μm or less. It is necessary to select crystal grains of different sizes according to different process requirements such as roughing and finishing.
2. Selecting reasonable tool geometry parameters Because the cubic boron nitride material has high brittleness and low strength, the tool rake angle is usually γ0=0°~-5°, and the back angle α0=6°~15°. In order to improve the blade strength, the chamfer must be ground with a negative chamfer. The width of the chamfer can be br1=0.1~0.3mm, and the chamfering angle γ0=-6°~-20°; the tip of the blade needs to be properly rounded, and the radius of refining rε = 0.2 to 0.8 mm. However, the larger the radius of the tool nose and the larger the negative chamfer, the greater the cutting force and the greater the chance of chattering. Therefore, when the system rigidity of the machine tool-clamp-tool-workpiece is insufficient, especially when machining an elongated workpiece, it is not appropriate to use an excessive tool nose radius and a negative chamfer.
When milling steel and castings above 45HRC with cubic boron nitride indexable face milling cutter, the combination of the front rake angle and the side rake angle often adopts a double negative rake angle structure, generally -5° to -7°, the main deflection The angle is 45° to 75°, the negative chamfering dimension is br1×γ01=0.2 mm×-20°, and the cutting edge blunt radius rn=0.05 to 0.13 mm.
3. Choosing the right amount of cutting Cubic boron nitride tools are mainly used for finishing and semi-finishing. Therefore, the amount of backing knife is generally small, usually less than 2mm; the feed is 0.05~0.4mm/r. However, since the tool has a negative chamfer, the feed amount must be selected to be larger than the width of the negative chamfer.
In addition, when cutting high hardness materials such as hardened steel and chilled cast iron with a cubic boron nitride tool, when the cutting speed is higher than a certain value, the relationship between the cutting speed and the tool life no longer conforms to the Taylor formula, that is, with the cutting speed. The improvement of the tool life will not decrease, but will increase.
This is because, as the cutting speed increases and the cutting temperature rises, the metal of the cut layer softens and the hardness decreases, while the cubic boron nitride tool does not soften due to the high thermal stability, resulting in a difference in hardness between the tool and the workpiece. Large, tool wear slows down, making cutting easier, this feature is called metal softening. Tool life is reduced only when the cutting temperature exceeds the tool's heat resistant temperature.
Therefore, when cutting different workpiece materials with different grades of cubic boron nitride tools, there is an optimum cutting speed range, which must be paid attention to when using. For example, the optimum cutting speed for cutting high speed steel (62~64HRC) with DLS-F tool is 60~70m/min.
Due to the metal softening effect, the cubic boron nitride tool is most suitable for advanced cutting processes such as high-speed cutting and dry cutting of hard materials, such as machining workpieces with hardness above 60HRC, and its life is 10 times higher than that of cemented carbide tools. the above.
4. Method for preventing hydrolysis Although the cubic boron nitride tool can withstand the cutting temperature of 1300 to 1400 ° C, it should be noted that CBN reacts with water vapor and oxygen in the air at high temperature (about 1000 ° C) to form ammonia. And boric acid (BN + 3H2O → H3BO3 + NH3), this chemical reaction is called hydrolysis, which will accelerate the wear of the tool. Therefore, in wet cutting, avoid using an aqueous solution as a cutting fluid, and use an aqueous solution with extreme pressure additives or extreme pressure cutting oil to attenuate the hydrolysis.
It should also be pointed out that due to the high thermal stability of the CBN tool, the cutting fluid has little effect on the tool life and the surface roughness of the machine. In addition, the metal has a softening effect when cutting the hard material at high speed, so high-speed dry cutting can be used when conditions permit. And dry cutting also contributes to environmental protection and operator health and reduces production costs.
5. Choosing the right machine tool The machine tool using cubic boron nitride tool must have high rigidity, high power and high speed to fully exert its performance and achieve good economic benefits. In addition, the accuracy of the machine tool is also good, and the fixture and clamping device for clamping the workpiece must be highly reliable, so as to avoid vibration during machining and damage the tool.
It must be pointed out that many of the current machine tools in production cannot meet the processing requirements of cubic boron nitride tools, so the potential of cubic boron nitride tools has not been fully utilized. In the future, with CNC machine tools (NC) and machining centers (MC) The increase in the use of high-efficiency equipment will further promote the use of cubic boron nitride tools.
6. Sharpening and inspection before use of the tool Before using the cubic boron nitride tool, the front and back flank surfaces should be ground to reduce the friction during the cutting process, and also improve the straightness of the cutting edge and prevent the tool from being straight. Abnormal wear. In addition, before use, check the cutting edge quality of the cutting edge with a magnifying glass of 50 times or more to control the serrated edge of the cutting edge, otherwise it will affect the tool life and processing quality.
7. Unconventional use of PCBN tools It is generally believed that PCBN tools are not suitable for cutting low carbon steel workpieces because they produce long chips, scratch the rake face of the tool and generate high heat, which accelerates the thermal chemical wear of the tool. . However, this situation only occurs at high temperatures. The hardness of the PCBN tool will be reduced to about 200 °C, and it will be equivalent to the hard alloy at room temperature. When the cutting temperature is not high, the PCBN tool will not produce thermal wear. .
Therefore, PCBN tools can also cut low carbon steel with a higher service life than coated carbide tools and cermet cutters with the right amount of cutting.
New Trends in Cubic Boron Nitride Tools The Cubic Boron Nitride Tool is an advanced cutting tool with a broad application prospect. First of all, in the case of finishing and semi-finishing, in some cases even in the roughing process, it is effective to process hardened steel, chilled cast iron, cobalt-based and nickel-based superalloys of 35HRC or higher and other difficult-to-machine materials. . For example, Russia's T-10 (PCBN) inserts have good impact resistance. They can be used for both finishing and semi-finishing, as well as for roughing up to 6mm. At present, the annual consumption of cubic boron nitride in the world is increasing by about 10% to 15%.
Artificial cubic boron nitride super-hard tool material, the current development of single crystal to coarse particles, high strength, multi-functional direction. The single crystal CBN produced by Japanese inorganic materials has a diameter of 3 mm. De Beers' PCBN products have a maximum diameter of 101.6mm and are then laser cut to any shape required to process 70HRC high hardness materials.
Cubic boron nitride tool materials are more expensive. In order to expand their application range, cubic boron nitride coated tools have been developed abroad at a relatively low price. It is a superhard material coated with a thin layer of cubic boron nitride on a general-purpose tungsten-cobalt (K) type cemented carbide substrate (in principle, it can be coated on any substrate) by chemical vapor deposition (CVD). The UK has also developed a CVD coating of a mixture of nickel, copper, titanium, chromium, niobium and cobalt on a PCBN insert, as well as a protective coating of TiN or TiC, which is more wear resistant than conventional PCBN inserts. 3 times. Lellond has developed a superhard composite of ceramic and CBN that combines the advantages of both materials and is ideal for high speed machining of high hardness wear resistant cast iron.
The wide use of cubic boron nitride tools depends on high-speed, high-rigidity, high-vibration metal cutting machine tools, as well as high-strength new varieties of blade grades, and should be selected according to the specific processing conditions, so that it is compatible with the material being processed. "match". At present, the development of cubic boron nitride tools, on the one hand, must try to improve the quality of the blade and reduce the cost of the blade; on the other hand, it is necessary to further develop a finer grain material to increase its density, reduce the edge of the sawtooth, and enable it to edge. Grinding sharper, and at the same time to solve the problem of improving the grinding efficiency and quality of cubic boron nitride tools.

 

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