Deep hole threading of special material parts is difficult. For example, deep hole tapping on a titanium alloy part is very challenging. It is very uneconomical to scrap a part on a near-finished part due to the scraping action caused by tap breakage. Therefore, in order to avoid scraping, it is required to use the correct tool and tapping technology.

First you need to define what is a deep hole and why it requires special consideration. In drilling, those holes with a hole depth 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. This is often referred to as deep hole tapping when machining a 3/8" deep thread with a 1/4" diameter tap.

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

To solve this problem, two options are available: (1) increasing the diameter of the hole before tapping; and (2) using a tap designed for deep hole tapping.

1. Increase the diameter of the hole before tapping

A suitable threaded bottom hole is very important for threading. A slightly larger threaded bottom hole can effectively reduce the cutting heat and cutting forces generated during the tapping process. But it also reduces the thread contact rate.

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 wall of the hole. This is especially important when tapping small holes in special materials and difficult-to-machine materials. Because the thread contact rate is lowered due to the reduction in the thread height on the wall of the hole, the threaded connection is maintained because of the increased length of the thread.

The increase in diameter of the threaded bottom hole is primarily dependent on the required thread contact rate and the number of threads per inch. Based on the above two values, the correct thread bottom hole diameter can be calculated using the empirical formula.

2. Cutting parameters

Since titanium parts are difficult to machine, it is necessary to fully consider the cutting parameters and tool geometry.

Cutting speed

Due to the large elasticity and deformation rate of titanium alloys, relatively small cutting speeds are required. When machining small holes in titanium parts, the recommended circumferential cutting speed is 10 to 14 inches per minute. We do not recommend a smaller speed because it will cause the work piece to harden. In addition, it is also necessary to pay attention to the damage of the tool and cause the cutting heat.

Chip flute

In deep hole tapping, it is necessary to reduce the number of tap grooves to increase the chip space of each groove. In this way, when the tap retracts, more iron filings can be taken away, reducing the chance of damage to the tool due to clogging of the iron filings. On the other hand, the increase in the tap chip pocket reduces the core diameter and, therefore, the tap strength is affected. So this will also affect the cutting speed. In addition, the spiral groove tap is easier to chip than the straight groove tap.

Front and back angle

The small rake angle increases the cutting edge strength and thus increases tool life; while the large rake angle facilitates cutting long chip metal. Therefore, in the processing of titanium alloy, it is necessary to comprehensively consider these two factors, and select a suitable rake angle.

The large relief angle reduces the friction between the tool and the chip. Therefore, it is sometimes required that the taper angle be 40°. When processing titanium metal, a large relief angle is ground on the tap to facilitate chip evacuation. In addition, full-grinding taps and taps for back grinding are also beneficial for tapping.

Coolant

When machining special materials, it must be ensured that the cutting fluid reaches the cutting edge. In order to improve the flow rate of the coolant, it is recommended to open a cooling groove on the back of the tap. If the diameter is large enough, an internal cooling tap can be considered.

3. Application examples

An aircraft parts manufacturer needs deep hole tapping on one part. The part material is a grade 7 titanium alloy. During processing, the circumferential cutting speed was 13 inches per minute while using a coolant.

In order to ensure the accuracy of the parts, the operator should replace them in time before the taps are blunt. When the tap wears, the sound produced during the cutting changes. By listening to these sounds, the operator can determine the number of threaded holes that can be machined before the tap wears before machining.

The plant has two tapping stations on each tapping device, equipped with the same tap. When one of the taps is worn, it can be easily replaced in time.

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Tag: Threading Deep hole Tap Cutting speed Tapping

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