How is sealing achieved for TBG threads in oil pipelines?
Thread Content
Add tape? Apply thread sealant? Should I just tighten it directly?
I’ve never seen it. What is a TBG thread?
Oil tubing threads include unthickened TBG and thickened UPTBG types; refer to \"GBT 9253.2-1999 Petroleum and Natural Gas Industry – Machining, Measurement, and Inspection of Threads for Casing, Tubing, and Pipeline Pipes\" or the Chinese version of \"API 5B\".
I just searched on Baidu as well, but I couldn’t find any relevant materials
I read an article stating that the sealing mechanism of this type of thread relies on a self-sealing principle based on interference fit; therefore, high precision is required in the machining of such threads. In the past, since oil field users allowed the use of polytetrafluoroethylene sealing tape around the threaded connections of pumping pumps, problems with the sealing performance of these threaded connections were rare. Since the 1990s, with the deepening of reform and opening up as well as changes in oil field operations, oil field users have placed increasingly strict requirements on the threaded connections used for pumping pumps. It is not allowed to use sealing tape or sealant; only a small amount of SF thread sealant may be applied. Moreover, the connections must be tightened using friction pliers by one person, and they must withstand a pressure test of 30 MPa without any leakage, while also remaining undamaged upon disassembly.
Full article: Control measures to ensure the sealing performance of API tubing thread connections. Author: Anonymous. Source: Unknown. Based on years of experience in the machining and quality control of tubing threads, this article outlines the control measures taken to ensure the sealing performance of API tubing thread connections, covering aspects such as the regrinding and installation of tubing thread cutters, as well as errors in taper, pitch, tooth profile, and quality inspection. It focuses on analyzing the advantages and disadvantages, as well as the profile errors, of machining oil tube threads directly using a comb cutter for rough and finish turning, or using a regular thread cutting tool for rough turning and a comb cutter for finish turning. It points out that the correct method for machining oil tube threads is to first use a regular thread cutting tool for rough turning, and then use a comb cutter for finish turning. It is recommended to increase the manufacturing tolerance for the flatness parameter of the thread crest and root of oil pipe threads from ±0.02 mm to ±0.01 mm, and to keep the cumulative pitch tolerance of oil pipe threads within ±0.02 mm, in order to improve the sealing performance of API oil pipe thread connections. In \"Factors Affecting the Sealing Performance of API Tubing Threads,\" the author starts with the profile characteristics and sealing mechanism of API tubing threads, and analyzes the impact of factors such as taper error, pitch error, and profile error on the sealing performance of API tubing thread connections ； The reasons for the occurrence of taper error, pitch error, and thread form error during the machining of oil pipe threads are briefly described. In this article, based on experience in tubing thread processing and quality control, the author will explain the control measures to ensure the sealing performance of API tubing threads, following the processing process for such threads, in order to share insights with colleagues. Reshaping and installation of tubing thread comb cutters: Taking a two-toothed external prism-shaped comb cutter and a two-toothed four-edge internal prism-shaped comb cutter as examples, the control procedures for reshaping and installing tubing thread comb cutters are explained. 1. Regrinding of threading tools: When the threading tool for oil pipes becomes worn to a certain extent or when there are defects in its thread profile, it must be reground before it can be used again. Otherwise, the threads produced on the oil pipes will hardly meet the requirements regarding connection and sealing performance. However, the regrinding of the comb knife should ensure that the shape and dimensions of its cutting edge remain essentially unchanged after grinding. The forming cylindrical surfaces of both the inner and outer comb cutters for oil pipe threads are back tool surfaces. The angle between the rake face and the flank face of the prismatic external threading tool is 82°. The clearance angle of the top cutting edge when mounted on a special external threading tool shank is 6° (for an external threading tool with 8 threads per inch, this angle is 8°) ； The angle between the front and rear cutting surfaces of the in-body comb cutter is 84°, while the clearance angle of the top edge mounted on the dedicated comb cutter shaft is 6°. Based on the calculation of the depth of cut for a prism-shaped comb cutter, it can be seen that, with the back angle remaining unchanged after the cutter is installed, when the front angle of the cutter is ground to +6°, the profile height of a 10-teeth thread increases by 0.023 mm ； When the front cutting edge of the comb knife is ground to +10°, it will increase by 0.047 mm. Therefore, to ensure the correct cross-sectional shape of the comb tool after regrinding and smooth cutting, the following control measures are taken for its regrinding: 1) The inner and outer surfaces of the oil pipe thread on the comb tool must have their cutting edges ground on a tool grinder or a surface grinder ； 2) The grinding angle for the outer comb cutter is 8°+1° (Figure 1a), while that for the inner comb cutter is 2°±30′ (Figure 1b) ； 3) The root surfaces of the inner and outer comb knives after sharpening, as well as the arc radius at the top of the corrective teeth, must have a smooth transition; the sides of the teeth must be straight, with no defects present. 2. Installation of comb knives: Both the internal and external comb knives for oil pipe threads are mounted on dedicated internal and external comb knife rods; the quality of this installation directly affects the machining accuracy of the parts as well as their operational reliability. To address this, the following measures have been taken: 1) Before installation, the cutter shaft, the blade positioning groove, and the blade itself should be cleaned thoroughly. After the blade is pressed in place, a feeler gauge should be used to check whether the blade makes proper contact with the surface of the cutter shaft’s positioning groove. This check should be repeated after machining several parts to ensure reliable blade positioning ； 2) When the inner and outer combing tool rods are installed on the machine tool’s tool holder, a dial indicator must be used to ensure that the alignment plane of the tool rods is parallel to the machine tool’s rotational axis (for inner combing tool rods) or perpendicular to it (for outer combing tool rods), with the error remaining within 0.02 mm ； 3) When machining oil tube threads of small sizes such as 1.050, 1.315, 1.660 TBG or UP TBG, if the issue of the helix angle is not taken into account on the special comb cutter shaft, it is permissible to take certain measures on the comb cutter shaft to give the comb blades a positive cutting edge inclination, thereby reducing the impact of the helix angle on the surface quality of the threads. The specific method is as follows: when machining internal threads, add shims to the rear end of the tool shank. Within a length of 100 mm, the rear end should be raised by 1.2–1.6 mm relative to the front end; this creates a positive rake angle of 0°41′–0°55′ ； When machining external threads, a shim with a width of 5 mm, a length of 120 mm, and a thickness of 0.3–0.4 mm is placed inside the tool holder in the direction of the tool shank width; this allows for a positive cutting edge inclination of 0°41′–0°55′ (when the tool shank width is 30 mm). Control of tubing thread taper error: The taper error accounts for a small proportion of the cumulative tightening clearance value during tubing thread connections. The corresponding impact on the sealing performance is also minimal. However, due to the change in the position of the tightly connected areas caused by taper errors, this has a significant impact on the stability, reliability, connection strength, and level of connection stress of oil pipe threads. The taper of oil pipe threads is generally manufactured in two steps: first, rough turning is done using an external cylindrical cutter, and then fine turning is carried out using an oil pipe thread finishing tool. To minimize the impact of taper errors on the sealing performance of threaded connections in oil pipes, the following measures have been formulated based on the main causes of significant taper errors during the machining of oil pipe threads: 1) During the machining of oil pipe thread components, the error in the taper angle should be kept within the range of 1°47′24″ (+5′, -3′); moreover, taper errors must be regularly checked and corrected. Due to the accurate measurement of the outer cone, it is possible to first machine the cone shape and then use a sine bar to measure the slope angle; the inner cone surface is inspected by using cone-shaped plug gauges of the same specification and marked with color, with the contact area needing to be greater than 60%. 2) The template holder of the template lathe is inspected every 3 months, while the accuracy of both the template lathe and the CNC lathe is checked every 6 months. 3) After rough turning of the inner and outer cones of the oil pipe thread, leave a single-sided finishing amount of 0.3～0.5 mm to ensure the integrity of the thread profile during fine finishing. To ensure sufficient machining allowance for the internal threads of the oil pipe, the diameter of the pre-formed blank hole in the coupling is specified (Table 1), with a tolerance zone of H14. 4) The straightness of the threaded taper on the oil pipe shall be checked by means of light transmission using a straightedge; when the generatrix is concave in the middle, it must not exceed 0.01 mm, and when the generatrix is convex in the middle, it must not exceed 0.03 mm. Control of thread pitch error in tubing threads: Pitch error accounts for the largest proportion, up to 50%, of the cumulative tightening clearance during tubing thread connections; therefore, pitch error is the most significant factor affecting the sealing performance of API tubing threads. In both template-controlled lathes and CNC lathes, when machining oil pipe threads, the workpiece rotates, while the threading tool (or threading cutter) translates equally along the workpiece’s axis according to the desired pitch parameters. The pitch accuracy of the workpiece is primarily ensured by the motion accuracy of the relevant components of the lathe. Although it is very rare for the pitch error of oil tube thread components to approach or exceed the cumulative pitch tolerance, once this occurs, it has a fatal impact on the sealing performance of the batch of components. To minimize the pitch error of workpieces and ensure the sealing performance of threaded connections in oil pipes, the following control measures were implemented: 1) The pitch tolerance was reduced such that the pitch error between any two threads within a length of L4-g does not exceed 0.02 mm. As a result, the proportion of pitch error in the total cumulative clearance during threaded connection of oil pipes is reduced to 12%. Additionally, the maximum cumulative hand-tightening clearance for oil pipe threads can be decreased from 0.26 mm to 0.146 mm, which is less than the 0.158 mm of clearance that can be eliminated by tightening two full turns using a wrench (for threads with 10 threads per inch). 2) Regularly inspect the machine tool to eliminate and reduce errors such as spindle rotation error, guide rail guidance error, and transmission chain error, thereby improving the pitch accuracy during the threading of oil pipes. Specific methods include: ① Adjusting the machine tool spindle and lead screw to eliminate axial play ； ②Reasonably adjust the meshing clearance of the exchange gears, as well as the runout of the spindle, lead screw, and shaft journals of the exchange gears ； ③Install automatic transmission error compensation devices on conventional CNC lathes, etc. 3) Use the pitch gauges for internal and external threads or other measurement methods available in the API oil pipe thread single-item measuring instrument to regularly check whether the pitch error is within the range of 0.02 mm. Control of the tooth profile error in tubing threads: Tubing threads are a very special type of connection thread; in addition to the tooth flanks being the main working surfaces, the tooth peaks and tooth roots also serve as working surfaces, and they similarly have a significant impact on the sealing performance of tubing thread connections. Although the national standards and API Std 5B specify only two types of thread form tolerances (the tolerance for thread form height and the tolerance for thread form angle), and although these two tolerance values appear to be relatively large on the surface, in reality, the errors in the thread form shape and the roundness of the cross-section caused by workpiece clamping and machining methods during the processing of oil pipe threads should also be included. Theoretically, when the profile height of the internal thread on the coupling is at its maximum and the profile height of the external thread on the tubing is at its minimum (with the flatting heights at the thread peaks and threads roots increased or decreased by the same amount), the contact condition between the two when tightly connected is shown in Figure 2. At this point, the hand-tightening clearance between the tip of the internal thread and the root of the external thread decreases from 0.076 mm to a interference value of 0.0005 mm, while the hand-tightening clearance between the tip of the external thread and the root of the internal thread increases to 0.1525 mm. Under such circumstances, if a soft metal coating is not used as a gap filler along with a high-performance sealing grease, leakage will inevitably occur after the oil pipe thread connection machine is tightened. In the processing of oil pipe threads, the thread profile of the workpiece is entirely determined by the profile of the oil pipe thread forming tool. Although the manufacturing tolerances regarding the thread profile angles and the heights at which the thread crests and roots are trimmed for both internal and external oil pipe thread tools are strictly controlled, the aforementioned extreme situations can still occur if the oil pipe thread forming tool is used improperly or if the thread profile fails to be fully formed during processing. 1. Tooth profile error during rough turning and finish turning using a threading tool
To ensure that the tooth profile of the workpiece matches that of the threading tool exactly, radial feed is employed during the machining of oil pipe threads. The pitch parameter is used as the feed rate. Based on the thread dimensions, the machinability of the material, and the rigidity of the part, an appropriate cutting speed and number of passes are selected. In production practice, due to the high rigidity of the cemented carbide prismatic comb cutter and the high strength of its specialized cutter shaft, some operators use higher cutting speeds when machining the external threads of oil pipes, managing to finish threading after just 3 to 4 passes. Although this processing method yields a better surface roughness of the threads and high production efficiency, it has the following disadvantages: 1) The cutting load on the tool tip is high, which makes it prone to tool damage and edge failure, thereby reducing the service life of the combing tool ； 2) The wear of the combing blade accelerates, and the wear is uneven – more wear occurs at the tip and on the sides of the blade, while less wear occurs at its bottom. After operating for a period of time, the workpiece ends up in the extreme condition mentioned earlier, resulting in shallow teeth on the workpiece, which affects the sealing performance of the connection ； 3) Since oil tube thread components are generally hollow, thin-walled parts, and the clearance angle of the combing tool is small while its rake angle is essentially zero, machining with large cutting amounts can easily cause the workpiece to deform due to compression, lead to tool deflection, alter the correct shape of the thread profile, result in incomplete commingling of the thread teeth in certain areas, or increase the circularity error of the cross-section. 2. Tooth profile errors in rough turning with a regular thread turning tool and finish turning with a comb tool. The author believes that the correct processing method is to first perform rough turning using a regular thread turning tool, and then finish turning using a comb tool, as shown in Figure 3. The advantages of this machining method are: ① During rough turning, the cutting tool can use larger rake and clearance angles, allowing for smoother cutting and reducing the risk of deformation of the workpiece due to compression ； ②It reduces the cutting load on the comb blade, results in more even wear of the blade, and extends its service life. The author measured the parameters of a 10-tooth external broach before and after machining. After machining 35 fixed valve cover parts using the aforementioned method, the height at which the tops of the broach teeth were flattened remained essentially unchanged; meanwhile, the height at which the bottoms of the teeth were flattened increased by 0.009 mm. This indicates that the wear on the top and sides of the comb cutter teeth is relatively uniform; the bottom of the comb cutter teeth experiences less wear due to the reduced combing amount after rough turning of the taper. When using a regular thread turning tool for rough turning and a combing tool for fine turning of oil pipe threads, it is important first to ensure that the longitudinal positions of the combing tool and the rough turning tool coincide, so as to enable proper combing on both sides of each thread. Special attention should be paid to the internal threads of the fittings, as it is difficult to observe and make adjustments there ； Secondly, pay attention to the tip width and cutting depth of the roughing tool to ensure that the root of the oil pipe threads is smoothly rounded and intact, while trying to make the finishing allowance as uniform as possible. When the allowance left on one side after turning a 10-thread-per-inch oil pipe thread taper is 0.3 mm, the tip width of the roughing tool can be controlled within 0.2–0.3 mm, and the depth of cut can be maintained at 1.2–1.3 mm, as shown in Figure 3. 3. Control measures to reduce tooth profile error. Theoretically, pitch error is the main factor affecting the sealing performance of API tubing threads; however, in practical production, situations where the pitch error of tubing threads falls within the cumulative pitch tolerance are very rare. Therefore, it is necessary to take corresponding measures from various aspects to effectively control the profile error of tubing threads, so that the profile of these threads meets the ideal conditions as much as possible. This is the best way to improve the machining quality of API tubing threads and ensure their sealing performance in connections. To minimize the profile error of oil pipe threads, in addition to controlling it in terms of re-grinding and installation of the broach as described in this paper, the following measures should also be taken. 1) Rational clamping and improving the stiffness of the workpiece. Tube threaded parts are generally hollow and thin-walled components. During installation, special jaws should be used (Figure 4). These jaws are made of 45# steel, with a hardness of 28–35 HRC. Additionally, the cylindricity of the portion where the workpiece is clamped must be maintained within 0.03 mm. The clamping surfaces of the jaws should be periodically trimmed to ensure good contact with the workpiece, thereby distributing the clamping force evenly and minimizing any deformation of the workpiece due to clamping ； When the workpiece has a large overhang or low stiffness, process measures such as end face centers and smooth surface centers are employed to improve the workpiece’s stiffness and reduce deformation due to stress. 2) After the taper of the oil pipe thread has been turned, rough turning should be performed first using a regular threading tool to reduce the load on the finishing cutter and ensure the correct thread profile. For threads with 10 threads per inch, the rough turning should be carried out at least 5 times; for threads with 8 threads per inch, it should be done at least 6 times. After rough turning, the allowance for finishing on the flanks and root of the oil pipe threads should be maintained at 0.3–0.5 mm. 3) After rough turning of the oil pipe thread, fine turning is carried out using a thread comb tool; the fine turning pass should be performed at least 3 times. The starting position of the comb tool should be at least 15 mm away from the starting point. The crest, flanks, and root of the oil pipe thread must be finely turned to ensure that the thread profile is complete and correct. During processing, it should be ensured that the amount of material removed from the bottom of the tooth on the right side of the threading tool’s calibration tooth to the top of the workpiece’s tooth does not exceed 0.02 mm (Figure 3). In this figure, δ represents the difference between the height of the flattened bottom on the right side and that on the left side of the calibration tooth; its value is PK/2. 4) To ensure that the tooth profile parameters of the thread comb tool do not change significantly during processing, it is stipulated that processing must be halted after 50 pieces have been precision-turned with the outer comb tool or 30 pieces with the inner comb tool; or if defects are detected in the tooth profile of the tool, it must be resharpened before it can be used again. There are two methods for quality inspection of tubing threads as specified in API Std 5B: First, all threads must be checked against API-approved comparison gauges in accordance with the measurement requirements in Chapter 3 ； Second, except for pipe threads that are finer than 11.5 teeth per inch, the thread parameters of all threads shall be inspected in accordance with Chapter 6. Both the old standard YB239-63 and the new national standard GB9253.3-88 specify comprehensive measurement criteria for thread ring gauges and plug gauges; for other thread parameters, only tolerance values are listed. The comprehensive measurement of tubing threads using thread ring gauges and plug gauges essentially involves checking the tightness of the thread fit, so as to ensure that after the tubing is tightened to the coupling machine, no incomplete parts of the tubing’s external threads are exposed or screwed in too far. At present, the only way to ultimately determine whether the oil pipe threads are tightly connected and meet the usage requirements is to conduct a hydrostatic test. In light of the special requirements associated with the machining of tubing threads, and on the basis of effective quality management throughout the entire machining process, the author has developed a comprehensive set of quality inspection procedures for API tubing thread components (excluding those items that are already specified in the standards, such as the comprehensive measurement using ring gauges and plug gauges). The main contents are as follows: 1) The complete tooth profile surface of the tubing threads must be free from defects such as dents, damage, or tool marks; the tolerances related to the taper of the tubing threads, the straightness of the thread profile, and the cumulative pitch are verified in accordance with the aforementioned control measures. 2) The sealing performance of the oil pipe threads is tested using a hydrostatic test. The requirements for this test are as follows: ① Before the test, check that there are no burrs or sharp edges on the oil pipe threads; clean them thoroughly, and then apply a thin layer of high-quality thread sealant to the smaller end of the external threads as well as over the entire length of the threads ； ②The test pressure is 30 MPa; it should be tightened using a friction clamp by one person, and the pressure should be maintained for 8 minutes without any leakage ； ③During pressure testing, a method of mutually testing connected internal and external threaded components is employed. After being used 5 times, the threaded portions of the oil pipes in the pressure testing tools must be repaired. 3) For the first part processed after adjusting the lathe and template holder, as well as re-grinding or reinstalling the comb cutter, a pressure test must be conducted. Only after the part passes the pressure test can mass production commence. 4) The sampling numbers for pressure testing of oil pipe thread parts manufactured in batches are shown in Table 2. Parts that fail the quality specifications must be fully repaired before they can be included in the batch to be accepted; batches that are rejected must undergo 100% comprehensive inspection, and those that can be repaired must be repaired to meet the required standards before they can be accepted. All oil tube thread parts with doubts regarding the quality of certain tooth profiles can be tested. 5) For oil pipe thread parts in the trial production of new products, when no pressure testing tools are available, it is permissible to machine them using the same machining systems such as lathes and cutting tools as those used for other oil pipe thread parts. If those other parts pass the pressure testing, then the aforementioned parts can be exempted from testing. Insights and suggestions: In the past, since oilfield users were allowed to wrap PTFE sealing tape around the tubing thread connections of pumping pumps, problems with the sealing performance of these threads were rare. Since the 1990s, with the deepening of reform and opening up as well as changes in oil field operations, oil field users have placed increasingly strict requirements on the threaded connections used for pumping pumps. It is not allowed to use sealing tape or sealant; only a small amount of SF thread sealant may be applied. Moreover, the connections must be tightened using friction pliers by one person, and they must withstand a pressure test of 30 MPa without any leakage, while also remaining undamaged upon disassembly. Under such circumstances, based on the actual conditions in the factory, the author used ordinary lathe machines and simple CNC lathes, without employing any specialized API oil pipe thread measuring instruments (but instead creating a pitch measurement tool with a precision of 0.02 mm), to produce threads that met API standards and satisfied the pressure testing requirements of oil field users. By addressing the issue of sealing performance in oil tube thread connections, we realized that the processing of API oil tube threads is a systematic process; every aspect, from the operators to the machine equipment, from testing methods to the use of cutting tools and fixtures, is crucial. Any problem in any of these aspects can affect the sealing performance of the oil tube thread connections. Therefore, it is necessary to provide regular training for the personnel involved in the processing and testing of tubing threads on aspects such as API tubing thread standards, connection characteristics, sealing mechanisms, influencing factors, control measures, and testing methods. This helps to enhance everyone’s quality awareness and sense of responsibility, ensuring that all pay attention to the quality of tubing thread processing, thereby enabling quality control and assurance throughout the entire production process of API tubing threads. To minimize the impact of manufacturing tolerances of tubing thread taps on the sealing performance of tubing thread connections, it is recommended to reduce the manufacturing tolerance for the flattening height of the crests and roots of both internal and external tubing thread taps from ±0.02 mm to ±0.01 mm. Based on the cooperation between Hubei State-Owned Factory No. 388 and Chengdu Tool Research Institute, reducing this tolerance by 50% results in a cost increase of less than 30%. In cases of mass production, the cost increase would be 15%–20%. As a result, the effects are very noticeable: it not only reduces the cumulative tightening gap in the threaded connections of the oil pipes, thereby improving the sealing performance, but also extends the service life of the comb knives.
To ensure that the tooth profile of the workpiece matches that of the threading tool exactly, radial feed is employed during the machining of oil pipe threads. The pitch parameter is used as the feed rate. Based on the thread dimensions, the machinability of the material, and the rigidity of the part, an appropriate cutting speed and number of passes are selected. In production practice, due to the high rigidity of the cemented carbide prismatic comb cutter and the high strength of its specialized cutter shaft, some operators use higher cutting speeds when machining the external threads of oil pipes, managing to finish threading after just 3 to 4 passes. Although this processing method yields a better surface roughness of the threads and high production efficiency, it has the following disadvantages: 1) The cutting load on the tool tip is high, which makes it prone to tool damage and edge failure, thereby reducing the service life of the combing tool ； 2) The wear of the combing blade accelerates, and the wear is uneven – more wear occurs at the tip and on the sides of the blade, while less wear occurs at its bottom. After operating for a period of time, the workpiece ends up in the extreme condition mentioned earlier, resulting in shallow teeth on the workpiece, which affects the sealing performance of the connection ； 3) Since oil tube thread components are generally hollow, thin-walled parts, and the clearance angle of the combing tool is small while its rake angle is essentially zero, machining with large cutting amounts can easily cause the workpiece to deform due to compression, lead to tool deflection, alter the correct shape of the thread profile, result in incomplete commingling of the thread teeth in certain areas, or increase the circularity error of the cross-section. 2. Tooth profile errors in rough turning with a regular thread turning tool and finish turning with a comb tool. The author believes that the correct processing method is to first perform rough turning using a regular thread turning tool, and then finish turning using a comb tool, as shown in Figure 3. The advantages of this machining method are: ① During rough turning, the cutting tool can use larger rake and clearance angles, allowing for smoother cutting and reducing the risk of deformation of the workpiece due to compression ； ②It reduces the cutting load on the comb blade, results in more even wear of the blade, and extends its service life. The author measured the parameters of a 10-tooth external broach before and after machining. After machining 35 fixed valve cover parts using the aforementioned method, the height at which the tops of the broach teeth were flattened remained essentially unchanged; meanwhile, the height at which the bottoms of the teeth were flattened increased by 0.009 mm. This indicates that the wear on the top and sides of the comb cutter teeth is relatively uniform; the bottom of the comb cutter teeth experiences less wear due to the reduced combing amount after rough turning of the taper. When using a regular thread turning tool for rough turning and a combing tool for fine turning of oil pipe threads, it is important first to ensure that the longitudinal positions of the combing tool and the rough turning tool coincide, so as to enable proper combing on both sides of each thread. Special attention should be paid to the internal threads of the fittings, as it is difficult to observe and make adjustments there ； Secondly, pay attention to the tip width and cutting depth of the roughing tool to ensure that the root of the oil pipe threads is smoothly rounded and intact, while trying to make the finishing allowance as uniform as possible. When the allowance left on one side after turning a 10-thread-per-inch oil pipe thread taper is 0.3 mm, the tip width of the roughing tool can be controlled within 0.2–0.3 mm, and the depth of cut can be maintained at 1.2–1.3 mm, as shown in Figure 3. 3. Control measures to reduce tooth profile error. Theoretically, pitch error is the main factor affecting the sealing performance of API tubing threads; however, in practical production, situations where the pitch error of tubing threads falls within the cumulative pitch tolerance are very rare. Therefore, it is necessary to take corresponding measures from various aspects to effectively control the profile error of tubing threads, so that the profile of these threads meets the ideal conditions as much as possible. This is the best way to improve the machining quality of API tubing threads and ensure their sealing performance in connections. To minimize the profile error of oil pipe threads, in addition to controlling it in terms of re-grinding and installation of the broach as described in this paper, the following measures should also be taken. 1) Rational clamping and improving the stiffness of the workpiece. Tube threaded parts are generally hollow and thin-walled components. During installation, special jaws should be used (Figure 4). These jaws are made of 45# steel, with a hardness of 28–35 HRC. Additionally, the cylindricity of the portion where the workpiece is clamped must be maintained within 0.03 mm. The clamping surfaces of the jaws should be periodically trimmed to ensure good contact with the workpiece, thereby distributing the clamping force evenly and minimizing any deformation of the workpiece due to clamping ； When the workpiece has a large overhang or low stiffness, process measures such as end face centers and smooth surface centers are employed to improve the workpiece’s stiffness and reduce deformation due to stress. 2) After the taper of the oil pipe thread has been turned, rough turning should be performed first using a regular threading tool to reduce the load on the finishing cutter and ensure the correct thread profile. For threads with 10 threads per inch, the rough turning should be carried out at least 5 times; for threads with 8 threads per inch, it should be done at least 6 times. After rough turning, the allowance for finishing on the flanks and root of the oil pipe threads should be maintained at 0.3–0.5 mm. 3) After rough turning of the oil pipe thread, fine turning is carried out using a thread comb tool; the fine turning pass should be performed at least 3 times. The starting position of the comb tool should be at least 15 mm away from the starting point. The crest, flanks, and root of the oil pipe thread must be finely turned to ensure that the thread profile is complete and correct. During processing, it should be ensured that the amount of material removed from the bottom of the tooth on the right side of the threading tool’s calibration tooth to the top of the workpiece’s tooth does not exceed 0.02 mm (Figure 3). In this figure, δ represents the difference between the height of the flattened bottom on the right side and that on the left side of the calibration tooth; its value is PK/2. 4) To ensure that the tooth profile parameters of the thread comb tool do not change significantly during processing, it is stipulated that processing must be halted after 50 pieces have been precision-turned with the outer comb tool or 30 pieces with the inner comb tool; or if defects are detected in the tooth profile of the tool, it must be resharpened before it can be used again. There are two methods for quality inspection of tubing threads as specified in API Std 5B: First, all threads must be checked against API-approved comparison gauges in accordance with the measurement requirements in Chapter 3 ； Second, except for pipe threads that are finer than 11.5 teeth per inch, the thread parameters of all threads shall be inspected in accordance with Chapter 6. Both the old standard YB239-63 and the new national standard GB9253.3-88 specify comprehensive measurement criteria for thread ring gauges and plug gauges; for other thread parameters, only tolerance values are listed. The comprehensive measurement of tubing threads using thread ring gauges and plug gauges essentially involves checking the tightness of the thread fit, so as to ensure that after the tubing is tightened to the coupling machine, no incomplete parts of the tubing’s external threads are exposed or screwed in too far. At present, the only way to ultimately determine whether the oil pipe threads are tightly connected and meet the usage requirements is to conduct a hydrostatic test. In light of the special requirements associated with the machining of tubing threads, and on the basis of effective quality management throughout the entire machining process, the author has developed a comprehensive set of quality inspection procedures for API tubing thread components (excluding those items that are already specified in the standards, such as the comprehensive measurement using ring gauges and plug gauges). The main contents are as follows: 1) The complete tooth profile surface of the tubing threads must be free from defects such as dents, damage, or tool marks; the tolerances related to the taper of the tubing threads, the straightness of the thread profile, and the cumulative pitch are verified in accordance with the aforementioned control measures. 2) The sealing performance of the oil pipe threads is tested using a hydrostatic test. The requirements for this test are as follows: ① Before the test, check that there are no burrs or sharp edges on the oil pipe threads; clean them thoroughly, and then apply a thin layer of high-quality thread sealant to the smaller end of the external threads as well as over the entire length of the threads ； ②The test pressure is 30 MPa; it should be tightened using a friction clamp by one person, and the pressure should be maintained for 8 minutes without any leakage ； ③During pressure testing, a method of mutually testing connected internal and external threaded components is employed. After being used 5 times, the threaded portions of the oil pipes in the pressure testing tools must be repaired. 3) For the first part processed after adjusting the lathe and template holder, as well as re-grinding or reinstalling the comb cutter, a pressure test must be conducted. Only after the part passes the pressure test can mass production commence. 4) The sampling numbers for pressure testing of oil pipe thread parts manufactured in batches are shown in Table 2. Parts that fail the quality specifications must be fully repaired before they can be included in the batch to be accepted; batches that are rejected must undergo 100% comprehensive inspection, and those that can be repaired must be repaired to meet the required standards before they can be accepted. All oil tube thread parts with doubts regarding the quality of certain tooth profiles can be tested. 5) For oil pipe thread parts in the trial production of new products, when no pressure testing tools are available, it is permissible to machine them using the same machining systems such as lathes and cutting tools as those used for other oil pipe thread parts. If those other parts pass the pressure testing, then the aforementioned parts can be exempted from testing. Insights and suggestions: In the past, since oilfield users were allowed to wrap PTFE sealing tape around the tubing thread connections of pumping pumps, problems with the sealing performance of these threads were rare. Since the 1990s, with the deepening of reform and opening up as well as changes in oil field operations, oil field users have placed increasingly strict requirements on the threaded connections used for pumping pumps. It is not allowed to use sealing tape or sealant; only a small amount of SF thread sealant may be applied. Moreover, the connections must be tightened using friction pliers by one person, and they must withstand a pressure test of 30 MPa without any leakage, while also remaining undamaged upon disassembly. Under such circumstances, based on the actual conditions in the factory, the author used ordinary lathe machines and simple CNC lathes, without employing any specialized API oil pipe thread measuring instruments (but instead creating a pitch measurement tool with a precision of 0.02 mm), to produce threads that met API standards and satisfied the pressure testing requirements of oil field users. By addressing the issue of sealing performance in oil tube thread connections, we realized that the processing of API oil tube threads is a systematic process; every aspect, from the operators to the machine equipment, from testing methods to the use of cutting tools and fixtures, is crucial. Any problem in any of these aspects can affect the sealing performance of the oil tube thread connections. Therefore, it is necessary to provide regular training for the personnel involved in the processing and testing of tubing threads on aspects such as API tubing thread standards, connection characteristics, sealing mechanisms, influencing factors, control measures, and testing methods. This helps to enhance everyone’s quality awareness and sense of responsibility, ensuring that all pay attention to the quality of tubing thread processing, thereby enabling quality control and assurance throughout the entire production process of API tubing threads. To minimize the impact of manufacturing tolerances of tubing thread taps on the sealing performance of tubing thread connections, it is recommended to reduce the manufacturing tolerance for the flattening height of the crests and roots of both internal and external tubing thread taps from ±0.02 mm to ±0.01 mm. Based on the cooperation between Hubei State-Owned Factory No. 388 and Chengdu Tool Research Institute, reducing this tolerance by 50% results in a cost increase of less than 30%. In cases of mass production, the cost increase would be 15%–20%. As a result, the effects are very noticeable: it not only reduces the cumulative tightening gap in the threaded connections of the oil pipes, thereby improving the sealing performance, but also extends the service life of the comb knives.
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