Application of PCBN cutting tool in machining brake disc

Automobile brake disc requires high smoothness and machining efficiency. Generally, the surface roughness is required to be Ra1.6, the thickness difference between the two brake surfaces is generally required to be controlled within 0.009mm, the parallelism is required to be within 0.04mm, and the run-out is required to be within 0.025mm of error scope, and these shape and position tolerances are guaranteed by finishing. The traditional grinding process has been difficult to meet the market demand for shape and position tolerance turning. PCBN cutting tool has extremely high hardness and wear resistance, which shows great superiority when machining brake disc. It can not only obtain satisfactory surface machining quality (surface roughness is Ra1.6), improve production efficiency, but also reduce the cost of single piece processing, and has remarkable economic benefits.

1.Materials and properties of brake disc

The material of automobile brake disc is generally gray cast iron, commonly used material is HT250, its minimum tensile strength of mechanical properties is 250MPa, hardness is 209HB, strength, wear resistance and heat resistance are good, shock absorption is good, but the ductility of brake disc is poor, which belongs to short chip material; and the cutting force is usually relatively low. In addition, due to the uneven distribution of graphite, the hardness of the brake disc changes, which will lead to fluctuations in the machining of PCBN cutting tools, even tool chipping and other phenomena, reducing the machining efficiency of the brake disc. In addition, the non-plastic material in gray cast iron will accelerate the cutting tool wear, so PCBN cutting tools are required to have high wear resistance. The typical chemical constituents of HT250 are shown in Table 1.

Table 1: Chemical constituents of HT250

2.Matters needing attention in the process of using PCBN cutting tools

① First of all, we should pay attention to the rigidity of the whole process system of machine tools, fixtures and cutting tools. The fixture itself should have sufficient strength and rigidity, and the protruding length of the tool holder and head should be as short as possible to avoid the vibration of tool holder in the machining process.

② When installing the cutting tool, the cutting tip should be aligned with the center of the workpiece to avoid vibration when the center is too high or too low.

③ When installing the cutting tool, the cutting tip should be aligned with the center of the workpiece to avoid vibration when the center is too high or too low.

3.Case study of brake disc processing

① For rough car end face and upper brake surface, take brake disc diameter D=278mm as an example, processing parameters are V=650m/min, ap=2.5mm, f=0.5mm/r.

②For rough inner face and lower brake surface, take brake disc diameter D=278mm as an example; machining parameters are V=650m/min, ap=2.5mm, f=0.5mm/r

③For fine turning, take brake disc diameter D=278mm as an example; machining parameters are V=500m/min, ap=0.1-0.2mm, f=0.25mm/r