In the modern manufacturing industry, the gear is one of the most critical components in automotive transmissions, machine tools, industrial drives, and aerospace equipment. The performance of gears directly affects transmission efficiency, noise level, and service life of mechanical systems. With the continuous development of high-performance materials and the demand for high precision and efficiency, hard turning technology has become an important alternative to traditional grinding for finishing hardened gears.

Cubic Boron Nitride (CBN) inserts—as the cutting tools used in hard turning—have gained widespread recognition due to their outstanding hardness, thermal stability, and wear resistance. Compared to conventional carbide tools, CBN inserts are capable of machining hardened steel (above 55 HRC) with high dimensional accuracy and surface finish, effectively reducing process steps and production costs.

Moresuperhard PCBN inserts are specially designed for hard turning gears and have remarkable effects in the finishing of end faces and inner holes of automotive gears after carburizing and quenching.

Hard Turning Technology for Gears

What Is Hard Turning?

Hard turning refers to the cutting of materials with hardness typically above 45 HRC, usually performed after heat treatment (such as carburizing or induction hardening). The process is carried out on high-precision CNC lathes or turning centers, using superhard cutting tools such as CBN or PCBN inserts.

For gears, hard turning is commonly used to finish gear bore, end face, shaft shoulder, or gear teeth (tooth flanks) after hardening. This allows manufacturers to replace one or more grinding processes, achieving shorter production cycles and higher flexibility.

In short, hard turning offers high efficiency, flexibility, and environmental benefits, especially in small- and medium-batch production of hardened gears. It eliminates multiple steps such as grinding wheel dressing and coolant disposal, and it is easier to automate and integrate into modern production lines.

Why Use CBN Inserts for Gear Machining

What is CBN?

Cubic Boron Nitride (CBN) is a synthetic superhard material, second only to diamond in hardness. However, unlike diamond, CBN is chemically stable in contact with ferrous materials, making it ideal for cutting hardened steel.

Key properties of CBN:

• Hardness: 4000–5000 HV (comparable to diamond)

• Thermal stability: up to 1200°C

• Chemical stability: inert with iron-based alloys

• High wear resistance and low friction coefficient

 Advantages of CBN Inserts in Gear Hard Turning

1. Excellent Wear Resistance
   CBN maintains sharp cutting edges even at high temperatures, ensuring consistent quality over longer tool life compared to carbide or ceramic inserts.

2. Superior Surface Finish
   Hard turning with CBN can achieve a mirror-like finish (Ra < 0.4 μm), sufficient to replace grinding in many gear finishing operations.

3. Dimensional Stability
   Minimal tool wear leads to stable dimensional accuracy across multiple parts, meeting tight tolerance requirements of gear applications.

4. High Cutting Efficiency
   CBN inserts allow high cutting speeds (up to 200–300 m/min), reducing cycle times and improving productivity.

5. Dry Machining Capability
   Most hard turning operations with CBN can be done dry, eliminating coolant costs and environmental hazards.

6. Reduced Process Chain
   By replacing grinding, manufacturers can simplify the machining process, reduce handling, and shorten lead time.

CBN Insert Geometry and Grade Selection

 CBN Grade Selection

CBN inserts are divided into two main types according to their composition:

For gear machining, both continuous and interrupted cutting may occur, especially when turning near keyways or tooth roots. Therefore, a balanced CBN grade (around 60–70% CBN content) with fine grain structure is often ideal.

Applications in Gear Manufacturing

Turning Gear Bore and End Face

After hardening, the inner bore and end face of gears often require precise finishing to ensure concentricity and parallelism. CBN inserts can achieve:

• Dimensional accuracy: ≤5 μm

• Surface roughness: Ra 0.2–0.4 μm

Recommended insert:
Type: CCGW/CCGW09T308
Grade: MCBN600
Cutting: Continuous, dry, fine finishing

Turning Gear Tooth Flanks

For high-precision gears (e.g., automotive transmission gears), hard turning can replace grinding for the tooth flank finishing step, particularly in small batches or prototype production.

Challenges:

• Interrupted cutting at gear tooth gaps

• High hardness and vibration sensitivity

Solution:
Use a tougher CBN grade with a chamfered edge and optimized cutting path.

Recommended insert:
Type: TNGN160408 / CNGA120408
Grade: MCBN300
Edge prep: 0.1×20° chamfer
Coolant: Dry with air blast

Result:

• Ra: 0.4–0.8 μm

• Gear profile error ≤6 μm

• Tool life: 20–50 pcs/edge depending on material and hardness

Shaft Shoulder and Chamfering

CBN inserts are also suitable for finishing gear shaft shoulders or chamfers, ensuring smooth transitions and eliminating grinding burrs.

Recommended insert:
Type: DCGW11T304
Grade: MCBN600
Operation: Finishing / dry

Compared with traditional ceramic inserts, it effectively improves the finish turning efficiency while enhancing the surface quality. Its high durability feature is also widely recognized — during use, normal wear is the main form, and abnormal wear rarely occurs, which ensures the stability of the machining process and the reliability of product quality.

Gear Machining Case 1

Gear Machining Case 2