The surface hardness of slewing bearings and wind power slewing bearing rings after quenching typically reaches HRC 55–62, with a hardened layer depth of approximately 3–5 mm. These components are commonly made from materials like 50Mn or 42CrMo. After hardening, the bearings undergo dry cutting using turning processes. The hard turning of these rings presents significant challenges due to the large machining allowance and strict dimensional requirements for the raceway and bearing surfaces. As a result, the performance balance of CBN (cubic boron nitride) inserts has become a critical factor in this process.
CBN is the hardest tool material used for ferrous metal machining and is often referred to as a "hot-hard" cutting tool. However, its poor toughness limits its use to small cuts after quenching. In the context of large-scale hard-turning in the slewing bearing and wind power industry, CBN tools are expected to make significant progress. The challenge lies in balancing wear resistance and impact toughness, especially when machining complex and irregular surfaces.
For the hard turning of slewing rings and wind power bearing rings, the machining allowance typically ranges between 2–6 mm after heat treatment. During this process, the blade must possess excellent impact toughness to handle the high loads and irregularities caused by quenching deformation. At the same time, the tool must maintain high wear resistance and thermal stability to ensure precise finishing and dimensional accuracy.
Traditional CBN tools often face issues such as low hardness, poor wear resistance, or excessive brittleness, leading to chipping or tool failure—especially during discontinuous cutting. To address these challenges, new CBN insert grades with non-metallic (ceramic-based) bonding agents have been developed. These materials offer better toughness and thermal stability compared to traditional metal or ceramic bonds, making them ideal for high-speed semi-finishing and finishing of high-hardness workpieces.
CBN polycrystalline inserts can achieve a hardness of up to HV 3400, with a post-processing hardness range of HRC 45–79. They are well-suited for long-term heavy-duty hard turning or grinding operations. Compared to conventional CBN tools, ceramic-bonded inserts demonstrate superior performance in terms of tool life, efficiency, and cost-effectiveness.
Case studies show that using non-metallic CBN inserts for wind power slewing bearings significantly improves processing efficiency. For example, in a 1.5MW wind turbine pitch bearing (outer diameter φ1900mm, thickness 130mm), hard turning with CBN inserts reduces machining time from over 18 hours (with grinding) to just a few hours. The cutting speed is around 90–150 m/min, achieving a surface roughness of Ra 0.3 μm. Additionally, the tool life increases by more than 30 sets, while energy and material consumption are reduced to about 1/5 of the grinding process.
Another case involved a 50Mn outer ring with a hardness of HRC 55 after quenching and a machining allowance of 2.5 mm. Traditional cermet-based CBN inserts struggled with chipping due to irregular deformation, requiring multiple passes and increasing costs. With ceramic-bonded CBN inserts, a single pass was sufficient, boosting efficiency by 3 times and reducing tool failure risks. Thermal conductivity of the non-metallic bond also extended tool life by 50% compared to metal-bonded inserts.
In addition to efficiency and cost benefits, hard turning with CBN tools offers environmental advantages. It avoids grinding burns and allows for dry cutting, which is more economical and eco-friendly. Machining chips are easier to recycle, reducing waste and pollution.
When implementing hard turning in slewing bearing and wind power applications, it's crucial to consider machine tool rigidity and clearance, as they directly affect tool performance and workpiece accuracy. Blade geometry, including edge preparation (such as negative chamfers and tip radii), plays a key role in minimizing tool failure risks. For large-diameter bearings, non-metallic CBN tools are recommended, while smaller parts may still use standard cermet-based inserts.
In conclusion, the use of advanced CBN tools with optimized bonding agents and blade geometries has transformed the hard turning process in the slewing bearing and wind power industries. This technology not only meets demanding precision and durability requirements but also delivers substantial improvements in productivity and sustainability.
CEPAI Group Co., Ltd. , https://www.jscepai.com