Deep-hole tapping of parts made from special materials is a difficult process. For example, deep-hole tapping on a titanium alloy part is particularly challenging. It is highly uneconomical if a nearly finished part is scrapped due to gouging caused by a broken tap. Therefore, to prevent gouging, it is essential to use the correct cutting tools and tapping techniques.
First, we need to define what constitutes a deep hole and why it requires special consideration. In drilling, holes with a depth greater than three times their diameter are referred to as deep holes. Deep-hole tapping, on the other hand, refers to tapping depths that exceed 1.5 times the tap diameter. For example, when using a 1/4″-diameter tap to cut a 3/8″-deep thread, this is typically referred to as deep-hole tapping.
Machining a deep-hole thread involves prolonged contact between the tool and the workpiece. At the same time, the machining process generates more cutting heat and greater cutting forces. Consequently, tapping small deep holes in special materials (such as titanium parts) can easily lead to tool breakage and thread inconsistencies.
To address this issue, two solutions can be adopted:
(1) Increase the diameter of the pilot hole before tapping;
(2) Use taps specifically designed for deep-hole tapping.
1. Increasing the Diameter of the Pilot Hole Before Tapping
A properly sized pilot hole is crucial for threading operations. A pilot hole that is slightly larger than required can effectively reduce the cutting heat and cutting forces generated during tapping. However, it also reduces the thread engagement ratio.
National standards and technical committees stipulate that, in deep holes, it is permissible to tap only 50% of the full thread height on the hole wall. This is particularly important when tapping small holes in special or difficult-to-machine materials. Although the reduction in thread height on the hole wall decreases the thread engagement ratio, the increased thread length ensures a reliable threaded connection.
The required increase in the pilot hole diameter depends primarily on the required thread engagement ratio and the number of threads per inch. Based on these two values, the correct pilot hole diameter can be calculated using empirical formulas.
2. Cutting Parameters
Because titanium parts are difficult to machine, careful consideration must be given to cutting parameters and tool geometry.
Cutting Speed
Because titanium alloys exhibit high elasticity and deformation rates, relatively low cutting speeds should be employed. When machining small holes in titanium alloy parts, a recommended peripheral cutting speed of 10–14 inches per minute is advised. We do not recommend using lower speeds, as this can lead to workpiece work hardening. Additionally, care must be taken to prevent cutting heat caused by tool breakage.
3. Coolant
When machining special materials, it is essential to ensure that the coolant reaches the cutting edge. To improve coolant flow, it is recommended to mill cooling grooves on the rake face of the tap. If the diameter is large enough, consider using an internally cooled tap.
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