CBN-FB2

2-flute CBN ball end mill for precision finishing

Tamotsu Nagai | OSG Corporation Product Development Engineer

OSG is accelerating its development in the micro‑precision machining sector. This field demands not only the ability to machine high‑hardness workpieces, but also superior dimensional accuracy. Diamond—the hardest material known—is generally considered the ideal tool material. However, diamond reacts with ferrous materials and wears rapidly, making it unsuitable for steel machining. For ferrous workpieces, the optimal material is cBN (cubic boron nitride), which provides hardness second only to diamond while enabling stable cutting performance. OSG’s latest solution for precision finishing, the CBN‑FB2, adopts this advanced tool material.

The CBN-FB2 is engineered primarily for mold machining. While mold types vary widely by application, demand for high value‑added, high‑precision molds has grown steadily. To support these evolving production requirements, OSG developed the CBN‑FB2, a 2‑flute cBN ball end mill designed specifically for superior finishing accuracy.

Improved Machined Surface Quality

Products manufactured using molds inherit the surface characteristics of the mold itself. As a result, polishing is often required after finish machining. This step is critical because it influences mold quality and lifespan, and in some applications, a mirror‑like surface is required. However, manual polishing significantly increases manufacturing costs and lead times.

Reducing polishing requirements depends on improving surface quality—both roughness and accuracy—during cutting. A common approach is to decrease the depth of cut (Pf), but this also extends machining time. The CBN‑FB2 integrates several design features that improve surface roughness without increasing cycle time.

Microrelief

Figure 1 shows the tool appearance and the CBN‑FB2’s microrelief structure. The microrelief incorporates a small relief angle and narrow relief width to generate a burnishing effect that reduces surface roughness. While a smaller relief angle enhances this effect, it also raises cutting resistance, which can cause tool deflection and reduce dimensional accuracy. The microrelief also helps suppress shape deformation caused by wear. In the CBN‑FB2, both the relief angle and relief width are optimized to strike the ideal balance between improved surface quality and stable machining accuracy.

Figure 1. Tool appearance and the CBN‑FB2’s microrelief structure.

Sharp Cutting Edges

Figure 2 compares the primary relief surface finish of the CBN‑FB2 with that of a conventional tool. The cutting edge ridge—formed by the primary relief and gash—is a crucial factor in determining edge sharpness. To enhance this characteristic, the CBN‑FB2 uses an improved grinding method that produces a sharper ridge (Figure 3), which in turn delivers superior surface finish.

During cutting, cBN particles may detach, accelerating wear. By improving the cutting edge ridge quality, the CBN‑FB2 suppresses wear caused by particle detachment, resulting in greater durability and longer tool life.

Figure 2. Comparison of primary relief.

Figure 3. Enlarged view of the cutting edge.

Glossy Finished Surfaces

Figure 4 compares finished surfaces produced by the CBN‑FB2 and a conventional tool. The conventional tool leaves a white, cloudy finish, while the CBN‑FB2 produces a highly glossy surface that reflects the camera lens used in the photograph. Surface roughness is also significantly improved. These results stem from the combined benefits of the microrelief and sharp cutting edges.

Figure 4. Comparison of machined surfaces produced by the CBN‑FB2 and a conventional tool.

Maintaining High Surface Quality

Figure 5 shows a comparison of surface roughness relative to cutting length between the CBN-FB2 and a competitor’s tool. Even after 800 meters of machining—approximately nine hours—using the same section of the cutting edge, the CBN‑FB2 maintained surface roughness comparable to that at the start of machining, despite visible edge wear.

Figure 5. Surface quality retention of the CBN-FB2.

Engineered for the increasing complexity and precision required in modern mold manufacturing, the CBN‑FB2 incorporates a range of advanced design enhancements. OSG remains committed to driving innovation in the precision tooling field, with new developments actively underway.

For more information on OSG’s CBN-FB2 2-flute CBN ball end mill