Titanium alloy bevel gears are widely used in high-end transmission systems such as aerospace and marine propulsion due to their high strength, low density, and excellent corrosion resistance. However, their turning process presents multiple challenges. Bevel gears feature a conical spiral structure, a complex tooth profile, and high precision requirements (pitch cumulative error ≤ 0.02mm ). Titanium alloy inherently has a low thermal conductivity (approximately 6.7 W/(m · K) , only one-sixth that of 45 steel ). Heat is concentrated on the tooth surface during cutting, leading to increased tool wear and burns on the workpiece surface (a blue oxide layer appears at temperatures exceeding 600 °C). Furthermore, titanium alloy is highly chemically active at high temperatures, easily bonding with the tool material, forming built-up edge and degrading tooth surface roughness ( Ra values can increase from 1.6μm to 6.3μm ). For example, when machining TC4 titanium alloy bevel gears, the life of conventional carbide tools is only 1/8-1/10 of that of steel gears. Therefore, tailored turning processes are required to balance machining accuracy and efficiency.
Specialized design of tool materials and geometry is central to turning titanium alloy bevel gears. The roughing stage requires significant stock removal (up to 2-3mm per tooth), requiring the use of ultrafine-grained carbide (such as a WC-TiC-TaC-Co alloy with a grain size of 0.6-0.8μm). Its flexural strength of 1900MPa or higher allows it to withstand high cutting forces. The addition of TaC reduces its affinity with titanium, minimizing adhesive wear. For finish turning, ensuring tooth profile accuracy is crucial, requiring the use of PCBN (cubic boron nitride) tools. These tools boast a hardness of up to HV3200 and wear resistance 5-8 times that of carbide, enabling tooth surface roughness to be controlled within Ra0.8μm. Tool geometry requires a large rake angle (10°-15°) to reduce cutting forces, and a clearance angle (8°-12°) greater than that of conventional turning tools to prevent friction between the flank and tooth surfaces. The lead angle is adjusted according to the taper angle (typically 45°-60°) to ensure that the cutting force is aligned with the tooth surface normal. An aircraft engine manufacturer used customized PCBN tools to improve the tooth profile accuracy of titanium alloy bevel gears from IT8 to IT6, extending tool life by three times.
Optimizing cutting parameters requires a balanced consideration of heat dissipation and tooth profile accuracy. During rough turning, the cutting speed is controlled at 80-100 m/min, the feed rate at 0.2-0.3 mm/r, and the back-cut depth at 1-1.5 mm. This results in short, fragmented chips, which facilitates heat dissipation. During finish turning, the speed is increased to 120-150 m/min (utilizing high-temperature softening of the material), the feed rate is reduced to 0.1-0.15 mm/r, and the back-cut depth is 0.1-0.3 mm to ensure tooth surface quality. Because bevel gear tooth surfaces are involute curves, turning requires CNC control of the tool’s generating motion, and the feed rate per revolution must match the helix angle (e.g., for a 30° helix angle, the axial feed rate must be converted to the normal feed rate). The cutting zone temperature must be strictly controlled below 500°C and can be monitored in real time using an infrared thermometer. The feed rate will automatically be reduced if the threshold is exceeded. Experimental data shows that when the finishing speed increases from 100m/min to 140m/min, the cutting resistance of titanium alloy decreases by 20%, but the tool wear rate increases by 15%, so the optimal balance needs to be found.
Enhanced cooling and lubrication system design is crucial for turning titanium alloy bevel gears. Conventional cooling methods struggle to penetrate the contact zone between the tooth surface and the tool, necessitating a high-pressure spray cooling system: pressure ≥15 MPa, flow rate ≥60 L/min. Extreme-pressure cutting oil (containing chlorine and phosphorus additives) is precisely sprayed onto the cutting zone through three nozzles spaced 120° apart, forming an oil film that isolates the workpiece from the tool. The cutting oil’s viscosity must be moderate (20-30 cSt at 40°C) to ensure lubrication while quickly dissipating heat. For deep cavities like tooth roots, internally cooled tools are required. Cutting oil flows directly through the tool’s center hole to the cutting edge, improving cooling efficiency by 60% compared to externally cooled tools. After implementing this cooling system at a shipbuilding machinery plant, the surface burn rate of titanium alloy bevel gears dropped from 25% to 3%, extending tool life by 2.5 times.
The clamping method and process must be adapted to the structural characteristics of bevel gears. Titanium alloy bevel gear blanks are mostly die-forged. During clamping, the large end face and inner bore are used as reference points. Positioning is achieved using a “one-side, two-pin” method: the end face is held in place by a vacuum suction cup (suction force 0.08 MPa), while a positioning pin is inserted into the inner bore (clearance ≤ 0.01 mm) to prevent radial movement during machining. The process follows a “rough turning – aging – semi-finish turning – finish turning” process. After rough turning, a beta heat treatment (TC4 alloy at 920°C for 1 hour, followed by air cooling) is performed to eliminate machining stresses. Semi-finish turning is performed to a 0.5mm allowance, ensuring a perpendicularity of the tooth tip circle to the reference plane of ≤ 0.01mm. Before finish turning, the gear blank dimensions are measured using a three-dimensional coordinate measuring machine (CMM) to adjust machining parameters. After finish turning, the tooth surfaces are polished (using a fine-grained grinding wheel) to reduce the surface roughness to Ra 0.4μm. After machining, pitch error, tooth profile error, and contact spot (contact area ≥ 70%) are inspected to ensure compliance with the GB/T 11365-2008 standard. Using this process, an aviation company has increased the transmission efficiency of titanium alloy bevel gears to 98%, far exceeding the 95% of steel gears.
