Turning of superhard materials

With the continuous development of the manufacturing industry, the world's machine tool manufacturers have been seeking to use new technologies to reduce the 

production costs of machined parts and finished products, making superhard cutting a new processing technology that manufacturers are currently paying attention to. 

It is expected that in the near future, superhard cutting technology will become more mature and widely used.

Superhard turning technology

Superhard turning is defined as the process of single-point cutting of high-hard material workpieces above HRC 45. Usually the hardness of the workpiece material can reach 

the range of HRC 58 to 68, and the cutting tool material is basically CBN (cubic boron nitride).

Superhard turning technology provides a new option for those processing and manufacturing that do not require extremely high precision grinding. Of course, for some 

workpieces with extremely high precision requirements, workpieces that are easily deformed, and workpieces with special requirements, grinding technology is still a more 

suitable choice. Although superhard turning cannot completely replace extremely high precision grinding at present, it can replace a considerable part of precision grinding, 

thereby reducing the expensive production costs in the grinding manufacturing process. At present, the surface roughness of superhard cutting workpieces can generally 

reach Ra0.2~Ra0.4, the roundness can reach 0.0005mm, and the dimensional accuracy can be controlled within 0.003mm. After cutting comparison, it was found that the 

production efficiency of superhard turning process is 4~6 times higher than that of general grinding process.

It can be clearly seen from Figure 1 that cutting a hard part of HRC 62 without using cutting fluid cooling will generate a lot of heat. In general, the temperature in the cutting 

point area can be as high as 926℃ during superhard cutting. In fact, local high-temperature heating can help complete the cutting process. The high heat of the tool cutting 

point pre-anneals and softens the cutting layer of the workpiece, making the workpiece easier to cut. In this cutting process, most of the heat is generated by chip peeling. In 

order to obtain exquisite surface processing quality, the cutting depth should be reduced as much as possible during the last cutting, generally controlled within 0.25 mm.

Currently, superhard turning has been widely used in the manufacture of automotive parts. Shanghai Automotive Gear Factory has successfully applied this technology to 

mass production. They combined gear carburizing and quenching of gears in gears 3, 4, and 5 with turning instead of grinding to perform final precision machining of 

finished products to meet the tolerance and surface roughness requirements of part design. The surface roughness of the workpiece can reach Ra0.2~Ra0.4, the roundness 

can reach 0.0005 mm, the tolerance band of 0.003 mm, and the Cpk value can reach 1.67.

Stability of continuous machining

An important sign of turning superhard materials is to ensure the stability of continuous machining. This is related to the overall dynamic rigidity of the machine tool, the 

cutting tool, and the heat treatment state of the workpiece.

Using polymer (artificial marble) to fill the main parts of the bed, increase its damping coefficient (generally 8 times that of the cast iron bed), and combine it with the use of 

linear rolling guides, will have a huge impact on the turning process of superhard materials, greatly reduce the vibration caused by cutting, and increase the time for rapid 

return to static rigidity (see Figures 2 and 3). The test results show that the improvement of the machine tool effectively suppresses the tool chipping caused by machine tool 

vibration, extends the service life of the tool, greatly improves the accuracy of the workpiece being processed, reduces the discreteness of the tolerance zone, and improves 

the surface quality of the workpiece.

Another important influencing factor is the performance and precision of the integrated moving axes of the machine tool, including the accuracy, geometric accuracy, 

electronic control function, error compensation, general adjustment and thermal deformation effect of the machine tool. This is because the final processing result of the 

workpiece is completely determined by the performance and precision of the machine tool.

Usually, ceramic knives are used for rough turning, and the cutting depth is greater than 0.25mm; CBN knives are used for fine turning, and the cutting depth is less than 

0.25mm.

In order to achieve the ideal effect of turning superhard materials, it is also necessary to control the heat treatment state of the workpiece. Generally, the change in 

quenching hardness is required to be less than the HRC between two points. If the workpiece is a carburized part, the consistency of the carburized layer depth must also be 

ensured. Generally, the depth should be controlled within 0.8-1.2mm.

Advantages of superhard material turning

Compared with the grinding process, superhard material turning has the following advantages:

1. Both "soft cutting" and superhard cutting can be performed on one lathe. One machine tool is equivalent to two machine tools, which not only saves plant space but also 

reduces the capital investment in purchasing machine tools;

2. The cutting efficiency of superhard turning is 4 to 6 times that of grinding;

3. In the process of superhard turning, the characteristics of single-point cutting of turning tools can be used to process complex-shaped workpieces, while grinders can only 

use forming grinding wheels for grinding;

4. Multiple cutting processes can be completed in one setting, saving the time of moving and reinstalling workpieces and reducing workpiece damage;

5. Superhard turning can easily achieve a surface roughness of Ra0.2 to Ra0.4;

6. Superhard turning lathes can adapt to workpieces of different specifications. Especially in the mold industry, it can meet the processing of different batches and complex 

workpieces;

7. Superhard turning chips are easier to handle than grinding chips, which meets environmental protection requirements;

8. The tool inventory cost is low.

Conclusion

Superhard turning is a feasible technology that can provide good economic benefits and better workpiece quality, especially on machine tools with high dynamic rigidity. The 

superhard turning process is not much different from the general turning process. Most manufacturers can introduce this new technology and apply it in actual production.