What Are The Thread Processing Accuracy Requirements for Drill Pipes?

Drill pipe is a steel pipe with thread at the tail, which is a part of OCTG pipe. The thread processing accuracy of drill pipes refers to the accuracy of the size, shape and position of the threads after processing, which directly affects the performance and life of the drill pipes.

Dimensional accuracy requirements

1. Outer diameter tolerance

The outer diameter tolerance of drill pipe threads is generally between ±0.05 - ±0.15mm. For example, in some common oil drill pipe thread processing, the outer diameter dimension accuracy requirements are strictly controlled within a very small range. This is because the outer diameter size directly affects the tightness of the thread fit. If the outer diameter is too large, it will cause interference during assembly and it will be difficult to connect with other drill pipes or joints; if the outer diameter is too small, the connection will be loose, and it will not be able to effectively transmit torque and withstand axial force.

2. Inner diameter tolerance

The inner diameter tolerance of the thread is usually ±0.05 - ±0.1mm. The accuracy of the inner diameter size is important to ensure the strength of the thread root. Too large an inner diameter may reduce the cross-sectional area at the root of the thread, thereby reducing its load-bearing capacity; too small an inner diameter may affect the circulation of drilling fluid and other media inside the drill pipe.

3. Pitch accuracy

The pitch tolerance is generally controlled at ±0.03 - ±0.05mm. Accurate pitch is the key to ensuring continuous and uniform thread engagement. Pitch error will cause uneven force on the thread teeth. When transmitting torque during drilling, it is easy to cause some thread teeth to be overloaded and damaged, affecting the reliability of the drill pipe connection.

4. Thread length tolerance

The effective thread length tolerance is approximately ±3 - ±5mm. Appropriate thread length is an important factor in ensuring connection strength. If the thread length is insufficient, it may be stripped when subjected to large tension or torque; if it is too long, it may increase unnecessary material costs and processing difficulties.

Tooth profile accuracy requirements

1. Tooth profile angle tolerance

The drill pipe thread tooth profile angle tolerance is usually controlled within ±0.5°- ±1°. Taking the common API standard thread as an example, its tooth angle is 60°, and the tooth angle error needs to be strictly controlled during processing. If the tooth angle is inaccurate, the contact state of the thread teeth will change, resulting in a reduction in contact area, aggravated stress concentration, and reduced bearing capacity and sealing of the connection.

2. Tooth profile accuracy

The tooth profile should comply with the corresponding standard specifications, such as API standards. The top and bottom of the tooth profile should be smooth and flat, avoiding sharp edges or irregular shapes. Because these parts are prone to stress concentration during thread connection and force bearing, resulting in premature failure of the thread. For the precision control of the tooth profile, it is generally achieved through high-precision processing equipment and precise tool paths.

Surface roughness requirements

1. External thread surface roughness

The surface roughness (Ra value) of the drill pipe external thread is generally required to be between 3.2-6.3μm. Lower surface roughness can reduce the friction during thread connection, making assembly smoother, and at the same time, it is conducive to the sealing material (such as thread sealing grease) to better fill the thread gap and improve the sealing of the connection.

2. Internal thread surface roughness

The internal thread surface roughness (Ra value) is usually also in the range of 3.2 - 6.3μm. This helps to ensure a good fit between the internal and external threads and reduce the possibility of wear and leakage. In actual processing, the surface roughness is controlled by appropriate cutting parameters, tool materials and processing technology.

Summary

The drilling pipe thread processing accuracy requirements are extremely strict and cover many aspects. In terms of dimensional accuracy, the outer diameter tolerance is generally ±0.05 - ±0.15mm, the inner diameter tolerance is ±0.05 - ±0.1mm, the pitch accuracy is controlled at ±0.03 - ±0.05mm, and the thread length tolerance is about ±3 - ±5mm. In terms of tooth profile accuracy, the tooth profile angle tolerance is usually controlled within ±0.5° - ±1°, and the tooth profile must comply with the corresponding standard specifications to ensure that the top and bottom are smooth and flat. The surface roughness requirements for both internal and external threads are in the range of 3.2 - 6.3μm. These precision requirements ensure the sealing and reliability of the drill pipe threaded connection and meet the performance requirements in various drilling operations.