Analyze how to perform deep hole thread processing on special material parts

It is more difficult to perform deep hole thread processing on special material parts. For example, deep hole tapping on a titanium alloy part is very challenging. If the part is scrapped due to the scraping effect caused by the broken tap on a part that is close to completion, it is very uneconomical. Therefore, in order to avoid scraping, the correct tool and tapping technique are required.

First, we need to define what a deep hole is and why it needs special consideration. In drilling, those holes with a 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. For example, when a tap with a diameter of 1/4' is used to machine a thread with a depth of 3/8

Machining a deep hole thread means a long time 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 special materials (such as titanium parts) is prone to tool breakage and thread inconsistency.

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

1. Increase the diameter of the hole before tapping

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

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 of special materials and difficult-to-process materials. Because although the thread contact rate decreases due to the reduction of the thread height on the hole wall, the thread length increases, so the reliable connection of the thread can still be maintained.

The diameter increment of the threaded bottom hole mainly depends on the required thread contact rate and the number of thread heads per inch. Based on the above two values, the correct threaded bottom hole diameter can be calculated using empirical formulas.

2. Cutting parameters

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

Cutting speed

Because titanium alloy has a large elasticity and deformation rate, it needs to adopt a relatively small cutting speed. When machining small holes in titanium alloy parts, the recommended circumferential cutting speed is 10-14 inches/min. We do not recommend using a lower speed, because that will lead to cold work hardening of the workpiece. In addition, pay attention to the cutting heat caused by tool breakage.

Chip flutes

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 is retracted, more iron filings can be taken away, reducing the chance of tool damage due to the clogging of iron filings. But on the other hand, the enlargement of the chip flute of the tap reduces the core diameter, so the strength of the tap is affected. So this will also affect the cutting speed. In addition, spiral flute taps are easier to remove chips than straight flute taps.

Front and rear corners

Small rake angle can improve cutting edge strength, thereby increasing tool life; while large rake angle is conducive to cutting metal with long chips. Therefore, when processing titanium alloys, these two factors need to be considered comprehensively, and a suitable rake angle should be selected.

The large clearance angle can reduce the friction between the tool and the chips. Therefore, sometimes the tapping angle is required to be 40°. When processing titanium metal, a large clearance angle is ground on the tap, which is conducive to chip removal. In addition, the fully ground taps and the taps with the blade back grinding are also beneficial for tapping.

Coolant

When processing special materials, it must be ensured that the cutting fluid reaches the cutting edge. In order to improve the flow of 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

A certain aircraft parts manufacturer needs to perform deep hole tapping on a part. The part material is grade 7 titanium alloy. During processing, the circumferential cutting speed is 13 inches/min, and coolant is used at the same time.

In order to ensure the accuracy of the parts, the operator should replace the taps in time before they become blunt. When the tap is worn, the sound produced during the cutting process will change. By listening to these sounds, before processing, the operator can determine the number of threaded holes that can be processed before the tap wears.

The factory has 2 tapping stations on each tapping equipment, equipped with the same taps. When one of the taps is worn out, it can be easily replaced in time.