Sumitomo BN Series CBN: The Complete Guide to Hard Turning Insert Selection
Hard turning has become a mainstream machining process, replacing grinding operations in many production environments. Sumitomo Electric Industries, one of the world’s leading CBN (Cubic Boron Nitride) insert manufacturers, offers the BN Series — a comprehensive family of CBN grades engineered for hard turning applications ranging from 45 HRC to 70 HRC. This guide walks through the key grades, their applications, and how to select the right one for your operation.
Why CBN for Hard Turning?
CBN is the second hardest material after diamond, with a Knoop hardness of approximately 4,500 HV. Unlike diamond, CBN does not react chemically with iron-based materials at cutting temperatures, making it the ideal superabrasive for machining hardened steels, cast irons, and superalloys. The thermal stability of CBN up to 1,400°C allows it to maintain edge integrity at the high temperatures generated during hard turning.
Sumitomo BN Series Grade Overview
Sumitomo’s BN Series covers a broad range of CBN content levels, binder systems, and grain sizes. The CBN content directly affects thermal conductivity, wear resistance, and toughness. Higher CBN content generally means better thermal conductivity and wear resistance, while lower CBN content offers improved chemical stability and surface finish capability.
| Grade | CBN Content (%) | Grain Size | Hardness (HRC Range) | Primary Application | Binder Type |
|---|---|---|---|---|---|
| BN100 | 50 | Fine (1-3 µm) | 55-68 | Hardened steel finishing | TiCN ceramic |
| BN200 | 65 | Medium (3-5 µm) | 50-65 | Hardened steel general | TiCN ceramic |
| BN300 | 70 | Medium (3-8 µm) | 45-62 | Interrupted cuts | Co ceramic |
| BN500 | 85 | Coarse (8-15 µm) | 45-58 | Gray cast iron high-speed | Co/Al ceramic |
| BN600 | 90 | Coarse (10-20 µm) | 45-55 | Gray cast iron roughing | Metallic |
| BNX25 | 60 | Fine-Medium (2-5 µm) | 50-66 | Hardened steel continuous | TiCN ceramic |
| BNX60 | 80 | Medium-Coarse (5-12 µm) | 45-60 | Sintered iron, hard cast iron | Co ceramic |
Grade Selection by Workpiece Material
Hardened Steels (55-68 HRC)
For continuous turning of hardened bearing steels (SUJ2/AISI 52100), gear steels (SCM420/SAE 8620), and die steels (SKD11/D2), BN100 delivers the best surface finish due to its fine grain structure and high chemical stability. Typical cutting parameters: Vc = 120-200 m/min, f = 0.08-0.15 mm/rev, ap = 0.1-0.3 mm.
When moderate interruptions are present — such as keyways or oil holes — BN200 offers a balance between wear resistance and chipping resistance. Its medium grain size provides fracture toughness while maintaining acceptable tool life on interrupted cuts.
Hardened Steels with Heavy Interruption (45-62 HRC)
BN300 is engineered for severe interrupted cutting. The cobalt-based binder provides metallic-phase toughness that absorbs impact loads. Use BN300 on gear tooth profiles, splined shafts, and components with cross-holes. Cutting speeds should be reduced to 80-140 m/min to manage thermal shock at interruption points.
Gray Cast Iron (FC250, FC300 / GG25, GG30)
High-CBN grades BN500 and BN600 dominate cast iron machining. The metallic or cobalt-aluminum binder provides excellent thermal conductivity, preventing built-up edge formation common with iron workpieces. BN500 can achieve cutting speeds of 400-800 m/min on gray cast iron, with tool life often exceeding 500 pieces per edge in automotive brake disc and cylinder bore applications.
BNX Series: Next-Generation Grades
The BNX25 and BNX60 grades represent Sumitomo’s latest development in nano-structured CBN. BNX25 uses a proprietary fine-grain sintering process that achieves higher density at the CBN-CBN grain boundaries, improving crater wear resistance by approximately 30% over BN200 in continuous hardened steel turning. BNX60 extends this technology to higher-CBN formulations for sintered powder metal components and chilled cast iron.
Insert Geometry Selection
| Geometry Code | Edge Prep | Rake Angle | Best For |
|---|---|---|---|
| T-type (Standard) | Honed 0.02mm | 0° | General hard turning, continuous |
| GH-type | Chamfer + hone 20°×0.1mm | -5° | Interrupted cuts, heavy duty |
| GS-type | Chamfer + hone 25°×0.13mm | -5° | Heavy interruption, roughing |
| Wiper (WH) | Wiper edge + chamfer | 0° | High-feed finishing, Ra < 0.4µm |
Cutting Parameter Guidelines
| Application | Grade | Vc (m/min) | f (mm/rev) | ap (mm) |
|---|---|---|---|---|
| Hardened steel finishing | BN100 | 150-200 | 0.08-0.15 | 0.1-0.3 |
| Hardened steel general | BN200 | 120-180 | 0.10-0.20 | 0.2-0.5 |
| Hardened steel interrupted | BN300 | 80-140 | 0.10-0.25 | 0.2-1.0 |
| Cast iron high-speed | BN500 | 400-800 | 0.15-0.40 | 0.5-2.0 |
| Cast iron roughing | BN600 | 300-600 | 0.20-0.50 | 1.0-3.0 |
Competitor Comparison
| Sumitomo | Kennametal | Sandvik | Seco | Application |
|---|---|---|---|---|
| BN100 | KD050 | CB7015 | CBN010K | Hardened steel finishing |
| BN200 | KD100 | CB7025 | CBN020K | Hardened steel general |
| BN300 | KD200 | CB7050 | CBN060K | Interrupted hard turning |
| BN500 | KD140 | CB7115 | CBN050K | Cast iron high-speed |
| BNX25 | KD080 | CB7020 | CBN015K | Continuous hardened steel |
Key Takeaways
- BN100 is the go-to grade for precision finishing of hardened steels above 55 HRC
- BN300 is essential for interrupted cuts — never use high-CBN grades on heavy interruption
- BN500/BN600 unlock extreme cutting speeds on gray cast iron, often replacing ceramic inserts
- Edge preparation is critical: wiper geometries enable Ra below 0.4 µm, replacing finish grinding
- The BNX series represents the latest generation with 20-30% longer tool life than standard BN grades
Selecting the correct Sumitomo BN grade requires matching CBN content, grain size, and binder system to your specific workpiece material, hardness, and cut interruption level. When in doubt, start with the more interruption-resistant grade and increase CBN content only when edge chipping is not a concern.
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