In the manufacturing of  titanium alloy parts, deep-hole thread machining is a crucial and highly difficult key process. Definition: A deep hole is defined as a hole with a depth greater than 3 times its diameter, and deep-hole tapping refers to tapping with a depth greater than 1.5 times the tap diameter.

1.Concentrated cutting heat and rapid tool wear:

The thermal conductivity of titanium alloys is only 20-30 W/(m·K), and the temperature in the cutting zone can reach 800-1000℃. This not only causes tool softening, but also forms an adhesive layer through high-temperature chemical reactions, accelerating tool failure.

3.Difficult chip evacuation and high risk of tool breakage:

Titanium alloy cutting is prone to producing ribbon chips, which can easily block the narrow chip evacuation channel in deep holes, leading to a sudden increase in cutting force and tool breakage. The risk is even higher in blind hole machining. In addition, titanium alloys have poor electrical conductivity, making it difficult to remove broken tools by electrical discharge machining (EDM), which often results in part scrapping.

3.Significant work hardening:

The hardness of the hardened layer produced by cutting can reach 2-3 times that of the matrix. Low-speed cutting will further aggravate hardening, increasing machining difficulty and accelerating tool wear.

4.Large elastic deformation and difficult precision control:

The elastic modulus of titanium alloys is only half that of steel. Tool deflection is prone to occur during cutting, resulting in undersized thread dimensions, and springback after machining will affect the thread fitting accuracy and connection strength.

1. Conventional Tapping

Tools: High-speed steel with cobalt or ultrafine-grained cemented carbide, with TiCN/TiAlN coatings

Parameters: Cutting speed 9-18 m/min, adopting pecking cycle, thread engagement rate 65%-75%

Structure: Large rake and relief angles, fewer flutes, 48° spiral flutes for blind holes, and back taper on the calibration section

2. Thread Milling

Relying on three-axis CNC linkage machining, it is increasingly widely used.

Low risk of tool breakage, excellent chip evacuation, high precision, strong flexibility and good surface quality

Slightly lower efficiency, requires a three-axis machining center, and has high equipment cost

3. Cold Form Tapping

It is a chip-free machining method that fundamentally solves the chip evacuation problem, and produces threads with high strength and good wear resistance. However, titanium alloys have high deformation resistance, and the traditional process has a large torque which is prone to tool breakage. After the introduction of longitudinal-torsional ultrasonic vibration-assisted technology, the tapping torque is reduced by 20%-37%, and the machining effect is significantly improved.

4. Comprehensive Comparison of the Three Methods

• Cutting speed: Excessively high speed will accelerate tool wear due to temperature rise, while excessively low speed will aggravate work hardening. Conventional tapping: 9-18 m/min; Thread milling: 24-37 m/min.
• Feed rate: Excessively large feed rate is easy to cause tool breakage, while excessively small feed rate reduces efficiency and accelerates wear. Thread milling: feed per tooth 0.025-0.076 mm.
• Tap drill diameter: A recommended thread engagement rate of 65%-75% can reduce torque by 15%-20% without affecting strength. For example, for M6×1.0 threads, a 5.1 mm tap drill can be used instead of the standard 5.0 mm for titanium alloy machining.
• Cooling and lubrication: Sulfur/chlorine-containing extreme pressure cutting oils must be used for conventional tapping; 5-20 MPa high-pressure cooling or internal tool cooling is adopted for thread milling.

Ultrasonic Vibration-Assisted Machining

Principle: High-frequency ultrasonic vibration of 20-40 kHz is applied to the tool or workpiece, causing periodic separation and contact between the two, which changes the mechanical characteristics of the cutting process.

Core Advantages:

Significantly reduce cutting force and torque: Longitudinal-torsional ultrasonic-assisted cold form tapping can reduce the maximum tapping torque by 20%-37%

• Improve chip evacuation: Make chips easier to break and discharge, reducing the risk of blockage
• Enhance surface quality: Effectively inhibit built-up edge and reduce the surface roughness of machined parts
• Extend tool life: Reduce cutting force and temperature, and decrease friction between the tool and workpiece

In addition, advanced technologies such as laser-assisted machining and cryogenic cooling machining have also been researched and applied in the field of titanium alloy machining.

Ruihang Group mainly produces titanium products with the complete industry chain,including smelting,forging, straightening,rolling,surface treating,testing process. For any purchasing needs, feel free to contact us at email:Sam.Rui@bjrh-titanium.com