Deep hole threading of material parts is difficult. For example, deep hole tapping in a titanium alloy part is very challenging. If on a nearly finished part, the part is scrapped due to the scraping effect of the broken tap, which is very uneconomical. Therefore, in order to avoid scraping, it is required to use the correct tool and tapping technology. Kan

First you need to define what a deep hole is and why it needs to be considered. In drilling, those holes whose hole depth is greater than 3 times the hole diameter are called deep holes. Deep hole tapping means that the tapping depth is more than 1.5 times the diameter of the tap. For example, when a tap with a diameter of 1/4 "is used to process a thread with a depth of 3/8", this situation is usually called deep hole tapping.

Machining a deep hole thread means a long time of contact between the tool and the workpiece. At the same time, more cutting heat and greater cutting force will be generated during the machining process. Therefore, tapping in small deep holes of materials (such as titanium metal parts) is prone to tool breakage and thread inconsistency. Kan

In order to solve this problem, two solutions can be adopted:(1) increase the diameter of the hole before tapping;(2) use a tap designed for deep hole tapping. Kan

1. Increase the diameter of the hole before tapping

Proper thread bottom hole is very important for thread processing. A slightly larger size of the threaded bottom hole can reduce the cutting heat and cutting force generated during the tapping process. But it also reduces the thread contact rate. Kan

The National Standards and Technical Committee stipulates that in deep holes, only 50% of the full height of the thread is allowed to be tapped on the hole wall. This is especially important when tapping small holes in materials and difficult-to-machine materials. Because although the thread contact rate decreases due to the reduction of the thread height on the hole wall, the thread can still maintain a reliable connection due to the increase of the thread length. Kan

The diameter increment of the threaded bottom hole depends mainly on the required thread contact rate and the number of threads per inch. According to the above two values, the correct diameter of the thread bottom hole can be calculated by using the empirical formula. Kan

2. Cutting parameters

Because titanium metal parts are difficult to process, it is necessary to fully consider the cutting parameters and tool geometry.

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Cutting speed one

Due to the large elasticity and deformation rate of titanium alloy, it is necessary to have a relatively small cutting speed. When machining small holes in titanium alloy parts, the circumferential cutting speed used is 10 to 14 inches/minute. We do not use a smaller speed, because that would lead to cold work hardening of the workpiece. In addition, it is also necessary to pay attention to the cutting heat caused by tool breakage. Kan

Chip Groove

In the deep hole tapping, the need to reduce the tap slot number, so that each slot chip space increased. In this way, when the tap is retreated, more iron filings can be taken away, reducing the chance of tool breakage due to iron filings. On the other hand, however, the core diameter is reduced by enlarging the chip flute of the tap, and therefore, the strength of the tap is affected. So this will also affect the cutting speed. In addition, the spiral flute tap is easier than the straight flute tap chip. Kill

Front and back corners

A small rake angle can increase the strength of the cutting edge, thereby increasing tool life; while a large rake angle is conducive to cutting long-chip metal. Therefore, in the processing of titanium alloy, it is necessary to consider these two factors and select the appropriate rake angle. Kan

The large back angle can reduce the friction between the tool and the chip. Therefore, it is sometimes required that the tap rear angle is 40 °. When processing titanium metal, a large rear angle is ground on the tap, which is beneficial to chip removal. In addition, the full grinding tap and blade back shovel grinding tap is also conducive to tapping.

Coolant

When machining the material, ensure that the cutting fluid reaches the cutting edge. In order to improve the flow of coolant, a cooling groove is opened on the blade back of the tap. If the diameter is large enough, consider using an internal cooling tap.

3. Application Examples

An aircraft parts manufacturer needs to tap deep holes in one part. The part material is grade 7 titanium alloy. In the process, the peripheral cutting speed is 13 inches/minute, and the coolant is used. Kan

In order to ensure the accuracy of the parts, the operator should replace the tap in time before it is blunt. When the tap is worn, the sound generated during the cutting process will change. By listening to these sounds, before machining, the operator can determine the number of threaded holes that can be machined before the tap wears. Kan

The factory in each tapping equipment, there are 2 tapping station, equipped with the same tap. When one of the taps is worn, it can be easily replaced in time.