company news about Introduction of titanium alloy deep hole threading technology

Deep hole threading of special material parts can be difficult. For example, deep hole tapping on a titanium part is very challenging. It is very uneconomical to scrap a part that is near completion due to the scraping action created by a broken tap. Therefore, in order to avoid scraping, the correct tool and tapping technique is required.

The first thing that needs to be defined is what a deep hole is and why it requires special consideration. In drilling, those holes that are deeper than three times the hole diameter are called deep holes. And deep hole tapping means tapping a depth greater than 1.5 times the tap diameter or more. For example, when a tap with a diameter of 1/4″ is used to machine a thread with a depth of 3/8″, the situation is often referred to as deep hole tapping.

Machining a deep hole thread means that there is a long period of contact between the tool and the workpiece. At the same time, more cutting heat and higher cutting forces are generated during the machining process. Therefore, tapping in small deep holes in special materials (e.g. titanium-like parts) is prone to tool breakage and thread inconsistency.

1, increase the diameter of the hole before tapping

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

National standards and technical committees state that in deep holes, it is permissible to tap only 50% of the full height of the thread on the hole wall. This is especially important when tapping small holes in specialty and difficult-to-machine materials. This is because despite the decrease in thread contact rate due to the reduction of the thread height on the hole wall, a reliable threaded connection is maintained due to the increase in thread length. 

The diameter increment of the threaded bottom hole depends primarily on the required thread contact ratio and the number of thread heads per inch. Based on these two values, the correct diameter of the threaded bottom hole can be calculated using an empirical formula.

2、Cutting parameters

Since titanium parts are difficult to machine, cutting parameters and tool geometry need to be fully considered.

Cutting speed

Because titanium alloy has a large elasticity and deformation rate, it is necessary to adopt a relatively small cutting speed. When machining small holes in titanium parts, the recommended circumferential cutting speed is 10 to 14 inches/minute. Smaller speeds are not recommended because they can lead to cold work hardening of the workpiece. It is also important to be aware of cutting heat due to tool breakage.

Chip Accommodation
When tapping deep holes, the number of tap slots needs to be reduced so that there is more room for chips in each slot. In this way, when the tap retracts, more chips can be taken away, reducing the chance of tool breakage due to chip clogging. But on the other hand, the increase of the chip holding space of the tap makes the diameter of the core decrease, so the strength of the tap is affected. So this will also affect the cutting speed. In addition, spiral fluted taps are easier to remove chips than straight fluted taps.

Front and Back Angle

A small rake angle improves cutting edge strength and thus increases tool life, while a large rake angle is good for cutting metals with long chips. Therefore, when machining titanium alloys, it is necessary to comprehensively consider these two factors and choose the appropriate front angle.

Large back angle can reduce the friction between the tool and the chip. Therefore, sometimes the back angle of the tap is required to be 40°. When machining titanium, grinding a large back angle on the tap is conducive to chip removal. In addition, fully ground taps and taps with a spade ground back of the cutting edge are also good for tapping.

Coolant

When machining special materials, it is important to ensure that the cutting fluid reaches the cutting edge. In order to improve the flow of coolant, it is recommended to open a cooling channel on the back of the tap's cutting edge. If the diameter is large enough, an internally cooled tap can be considered.