Hard turning is a machining process that involves single-point cutting of workpieces with a hardness exceeding HRC 45. It has long been a key focus in the development of cutting tools, especially as new high-hardness and complex materials have emerged. Initially, hard turning was mainly used for hardened steels such as gear steel, bearing steel, and die steel after heat treatment. However, it has now expanded to include materials like thermal spray coatings, chrome-nickel alloys, heat-resistant nickel base alloys, and stellite cobalt alloys.
Currently, PCBN (Polycrystalline Cubic Boron Nitride) tools are considered the most wear-resistant materials for machining ferrous metals. Their service life is typically several times longer than that of cemented carbide or ceramic inserts. PCBN tools have become a pioneer in localized tool production, offering improved performance and cost efficiency. For instance, Valin's BN-K10 PCBN grade can machine tungsten carbide with hardness above HRC 70—marking a first in China—and is widely used in mining machinery, wear-resistant castings, and rolling mill components, as well as in the hard turning of high-hardness pump parts like high-chromium alloy cast iron impellers.
Compared to grinding, hard turning offers significant advantages. It reduces capital investment in machine tools, supports flexible production, and improves equipment utilization. In terms of processing quality, approximately 80% of the heat generated during hard turning is carried away by the chips, significantly reducing the risk of grinding burns. In terms of efficiency, hard turning can be 4 to 6 times faster than grinding, and the chips produced are easier to recycle.
Hard turning is often used as the final machining step, replacing grinding and its associated risks. The precision, rigidity, and integration of machine tool components—including accuracy, geometric precision, and electronic control functions—play a critical role in determining the success of the hard turning process. These factors influence tool wear and the dimensional and shape tolerances of the machined part. Therefore, when using hard turning instead of precision grinding, it’s essential to consider the specific requirements of the workpiece. For ultra-precise or special applications where machine tool limitations exist, grinding may still be the preferred option.
Although hard turning cannot fully replace ultra-precision grinding, it has already replaced a large portion of precision grinding, significantly lowering production costs. A notable example is the use of Valin’s BN-H20 PCBN tool in the hard turning of hardened steel gears, achieving a surface roughness of Ra 0.4 (equivalent to Rz 3.2), which fully meets gear grinding standards.
BN-H20 and BN-H10 are typical PCBN grades used for finishing hardened components. They are available not only as standard CNC inserts but also as custom boring tools, grooving cutters, and other forming tools, as shown in Figure 2.
The term “grinding cars†has been around for a long time, but PCBN tools are now commonly used for finishing. When dealing with high-hardness workpieces and large machining allowances, traditional methods often involve annealing or heavy grinding. However, Valin’s BN-S20 PCBN grade is designed to handle up to 10mm of material removal efficiently. This tool uses a pure ceramic bonding agent, providing better impact resistance and avoiding the softening issues seen in metal-bonded PCBN tools. As a result, BN-S20 offers superior high-temperature wear resistance and is more than four times longer-lasting in heavy-duty hard turning applications.
At the 2007 launch of BN-S20, it broke previous limitations, proving that PCBN tools could perform strong intermittent roughing, challenging the belief that hard materials were difficult to machine. With ongoing research into ceramic-bonded CBN tools, the BN-S20 grade now maintains good impact fracture toughness while improving wear resistance. This allows for efficient machining of hard materials without the need for traditional annealing or secondary quenching processes, making it ideal for mold repair and large shaft and disc components.
BN-S20 is also effective for machining austenitic stainless steel (45HRC), high-temperature alloys, high-manganese steel, high-strength steel, high-nickel alloys, high-chromium steel, and surfacing materials with poor surface finish. Its versatility makes it a valuable tool in various industrial applications.
With advancements in tool materials, structural design, and manufacturing techniques, many Chinese-developed tools have achieved excellent performance. PCBN tools have led the way in localizing advanced cutting solutions, driving innovation in tool grades. As the manufacturing industry continues to prioritize cost reduction and efficiency, the reliance on imported tools has decreased. It is expected that the variety and number of locally produced tools will continue to grow, further strengthening China’s position in the global tool market.
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