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.
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.
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