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, and they undergo dry cutting during the machining process. Hard turning of the slewing ring presents significant challenges due to the high machining allowance and strict requirements for precision on the raceway and bearing surfaces. The balance of toughness and wear resistance in CBN (cubic boron nitride) cutting tools is therefore critical in this application.
CBN inserts are among the hardest tool materials used in ferrous metal machining and are often referred to as "hot-hard" tools. However, their low impact toughness limits their use to small cutting amounts after quenching. In the context of large-scale hard-turning operations in the slewing bearing and wind turbine industry, CBN tools have the potential to significantly improve efficiency and performance.
For the hard turning of slewing bearings and wind power slewing rings, the machining allowance is determined based on prior heat treatment and quenching deformation. Typically, the remaining material after hardening ranges from 2–6 mm. During this process, the cutting tool must exhibit excellent impact toughness, especially when machining the inner and outer rings. At the same time, it needs to maintain high wear resistance and thermal stability to ensure dimensional accuracy and surface finish.
One of the main issues in this process is that traditional CBN inserts may either be too soft, leading to poor surface finish, or too brittle, causing chipping or breakage—especially during interrupted cuts. To address these challenges, new CBN grades with improved balance between wear resistance and toughness have been developed. Non-metallic (ceramic-based) bonding agents are particularly suitable for this purpose. They enhance the toughness of the CBN insert while avoiding premature softening that can occur with metal bonds during high-temperature cutting.
These ceramic-bonded CBN inserts have a Vickers hardness of up to HV3400 and a post-processing hardness range of HRC 45–79, making them ideal for long-duration, high-volume hard turning or grinding. Compared to traditional metal and ceramic bonded CBN tools, ceramic-based ones offer better performance in high-speed semi-finishing and finishing of large, hardened workpieces.
In practical applications, non-metallic CBN inserts have demonstrated remarkable advantages. For example, in processing a wind turbine pitch bearing with an outer diameter of φ1900 mm and thickness of 130 mm, the car grinding process reduced machining time from over 18 hours (using grinding) to just 4.5 hours. The cutting speed was maintained at 90–150 m/min, achieving a surface roughness of Ra 0.3 μm. Additionally, the tool life was extended by more than 30 cycles, and the overall cost was only one-fifth of the grinding method.
Another case involved a 50Mn slewing outer ring with a hardness of HRC 55 after quenching. Traditional cermet-based CBN inserts often experienced chipping due to irregularities in the bore. Using ceramic-bonded CBN inserts allowed a single pass to remove up to 2.5 mm of material, increasing efficiency by three times and reducing tool failure. The thermal conductivity of the ceramic bond also contributed to a 50% longer blade life compared to metal-bonded versions.
Compared to grinding, hard turning offers several benefits, including the avoidance of grinding burns, the possibility of dry cutting, and easier handling of metal chips, which reduces environmental impact. It’s also more energy-efficient and cost-effective.
When implementing the hard turning process for slewing bearings and wind power slewing rings, machine rigidity and tool geometry play a crucial role in minimizing tool breakage and ensuring dimensional accuracy. Blade edge treatments such as negative chamfers (e.g., 0.2 × 20°) and specific tool holder angles (6–7°) help reduce risks during operation. For smaller cuts, a 0.1 mm negative chamfer is usually sufficient, while the radius of the cutting tip should match the fixture to prevent vibration and tool damage.
In summary, the use of advanced ceramic-bonded CBN tools in the hard turning of slewing and wind power bearings not only improves efficiency and surface quality but also lowers production costs and environmental impact. This makes it a highly promising solution for the future of heavy-duty bearing manufacturing.
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