Application of Cubic Boron Trioxide (CBN) in Machining

Cubic Boron Nitride (CBN) is a superhard material that has been developed as an alternative to synthetic diamond. It ranks second in hardness after diamond and is widely used in cutting and grinding operations, especially for hard materials. Its high thermal stability and wear resistance make it ideal for machining applications where traditional tools would fail. In the machining of hardened steel, CBN tools are particularly effective when the workpiece hardness exceeds 45 HRC. The optimal cutting speed ranges from 80 to 120 m/min, but this should be reduced for harder materials—such as 70 HRC workpieces, which require speeds between 60 and 80 m/min. Finishing cuts typically have a depth of 0.1–0.3 mm and feed rates of 0.05–0.025 mm/rev, resulting in surface roughness as low as Ra 0.3–0.6 μm. With CNC lathes, dimensional accuracy can reach 0.01 mm, matching the performance of grinding machines. No coolant is usually needed during finishing, as the heat generated is carried away by chips, minimizing thermal distortion. For discontinuous surfaces, PCBN composite blades are preferred, especially when using low cutting speeds and rigid machine tools. A finishing allowance of about 0.3 mm helps ensure uniform cutting and prolongs tool life. Fine turning inserts should be made with 80° diamond blades, with a nose radius of 0.8–1.2 mm. Before use, the tool edge must be chamfered to prevent chipping. Quenching hard workpieces is a relatively new process, requiring careful testing before full implementation. For example, FAW Group uses PCBN tools to machine 20CrMnTi gears with hardness between 58–63 HRC, achieving high efficiency with parameters like υc = 150 m/min, f = 0.1 mm/rev, and αp = 0.2–0.3 mm. When machining hard cast iron or gray cast iron, CBN tools offer significant advantages. For instance, automotive valve seats made of high-chromium alloy cast iron (around 44 HRC) are efficiently machined using PCBN tools, with a surface roughness of Ra 0.4 μm and minimal runout. Dongfeng Motor Company has successfully used CBN tools since 1988, improving local production capabilities. High-speed milling of gray cast iron also benefits from CBN tools, especially in finishing operations. Using symmetric PCBN inserts on end mills can increase cutting speed fourfold while reducing cutting force by 75%, leading to better tool life and surface quality. For high-speed boring of casting holes, paired PCBN inserts enhance rigidity and allow higher metal removal rates. In China, companies like Shanghai Volkswagen use PCBN tools to bore cylinder bores at 500 m/min, achieving high precision and long tool life. Reaming hardened steel or cast iron holes is another area where CBN reamers excel. These reamers are designed with precise dimensions and spiral grooves for chip removal and cooling. For example, a factory achieved a surface roughness of Ra 0.2 μm and cylindrical tolerance of 0.005 mm using electroplated CBN reamers. For difficult-to-machine materials like austenitic stainless steel or high-temperature alloys, CBN tools with positive rake angles and appropriate cutting parameters (υc = 100 m/min, f = 0.05–0.2 mm/rev) can significantly improve efficiency and surface quality. In grinding applications, CBN wheels are essential for high-efficiency and high-precision operations. They are used in camshaft and crankshaft grinding, internal hole grinding, and gear grinding. Compared to traditional abrasives, CBN wheels offer longer life, higher efficiency, and better surface finish. Promoting the use of CBN tools requires collaboration between manufacturers, users, and research institutions. Joint development projects help address issues related to tool selection, application, and performance optimization. By focusing on real-world applications and continuous improvement, the adoption of CBN technology can lead to significant gains in productivity and product quality.

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