Research on Deep-Hole Thread Tapping Technology for Titanium Alloys

Deep-hole thread tapping on parts made of exotic materials is highly challenging. For instance, tapping deep holes in a titanium alloy component poses significant difficulties. If a nearly completed component is scrapped due to the galling or scraping action caused by a broken tap, it is highly uneconomical. Therefore, to prevent scraping and tool breakage, the correct selection of cutting tools and tapping techniques is strictly required.

First, it is essential to define what constitutes a deep hole and why it demands special consideration. In drilling, holes with a depth greater than three times the hole diameter ($>3D$) are classified as deep holes. In tapping, deep-hole tapping implies that the thread depth exceeds 1.5 times the tap diameter ($>1.5D$). For example, when using a 1/4″ diameter tap to machine threads at a depth of 3/8″, this scenario is typically defined as deep-hole tapping.

Machining a deep-hole thread means prolonged contact between the cutting tool and the workpiece. Concurrently, significantly more cutting heat and higher cutting forces are generated during the process. Consequently, tapping small, deep holes in exotic materials (such as titanium components) is highly susceptible to tool breakage and thread inconsistency.

To address this issue, two primary solutions can be implemented:

1. Increasing the diameter of the pre-tap hole (minor diameter).

2. Utilizing taps specifically designed for deep-hole tapping.

1. Increasing the Pre-Tap Hole Diameter

An appropriately sized minor diameter (pre-tap hole) is vital for thread processing. A slightly oversized pre-tap hole effectively reduces the cutting heat and torque generated during tapping. However, it also reduces the thread engagement percentage.

The National Institute of Standards and Technology (NIST) stipulates that for deep holes, it is permissible to machine only 50% of the full thread height on the hole wall. This allowance is particularly critical when tapping small holes in exotic and difficult-to-machine materials. Although the thread engagement percentage decreases due to the reduced thread height on the hole wall, the reliability of the threaded connection is maintained by the increased length of the thread engagement.

The diameter increment of the pre-tap hole primarily depends on the required thread engagement percentage and the number of threads per inch (TPI). Based on these two values, the correct pre-tap hole diameter can be calculated using empirical formulas.

2. Cutting Parameters and Tool Geometry

Due to the poor machinability of titanium metals, meticulous consideration must be given to the cutting parameters and tool geometry.

Cutting Speed

Because titanium alloys exhibit high elasticity and deformation rates, relatively low cutting speeds must be adopted. When machining small holes in titanium components, the recommended peripheral cutting speed is 10 to 14 in/min. Lower speeds are not recommended, as they can cause work hardening of the workpiece. Additionally, attention must be paid to the cutting heat generated by tool wear.

Flutes (Chip Grooves)

In deep-hole tapping, the number of tap flutes should be reduced to increase the chip clearance volume per flute. Consequently, when the tap retracts, it can evacuate more chips, reducing the risk of tool breakage caused by chip clogging. Conversely, enlarging the tap flutes reduces the core diameter, which compromises the structural strength of the tap and subsequently limits the cutting speed. Furthermore, spiral flute taps evacuate chips more efficiently than straight flute taps.

Rake Angle and Relief Angle

• Rake Angle: A small rake angle enhances the cutting edge strength, thereby increasing tool life; whereas a large rake angle facilitates the cutting of long, continuous metal chips. Therefore, when machining titanium alloys, these two conflicting factors must be balanced to select an optimal rake angle.

• Relief Angle: A large relief angle reduces friction between the tool and the chips. Thus, a tap relief angle of 40° is sometimes required. When machining titanium, grinding a large relief angle on the tap facilitates chip evacuation. Additionally, fully ground taps and relieved-back taps are highly beneficial for the tapping process.

Coolant

When machining exotic materials, it is mandatory to ensure that the cutting fluid reaches the cutting edge. To improve the coolant flow rate, it is recommended to grind cooling slots on the heel of the tap. If the tap diameter is sufficiently large, internal coolant taps (through-coolant taps) should be considered.

3. Application Example

A certain aircraft component manufacturer required deep-hole tapping on a specific part. The component material was Grade 7 titanium alloy. During machining, a peripheral cutting speed of 13 in/min was applied along with cutting fluid.

To ensure component precision, the operator had to replace the taps proactively before they became dull. As a tap wears down, the acoustic signature generated during the cutting process changes. By monitoring these sounds, the operator could precisely determine the number of threaded holes that could be processed before tap failure.

The facility equipped each tapping machine with two identical tapping stations configured with the same taps. When one tap showed signs of wear, it could be replaced conveniently and promptly without delaying production.