Measuring device and method for determining the thread start of a butt and thread with an octagonal thread

Abstract

This invention discloses a measuring device and method for determining the starting point of a trapezoidal thread coupling, belonging to the field of petroleum thread detection technology. The method includes the following steps: Step S1, placing the trapezoidal thread coupling on the measuring device and establishing a three-dimensional coordinate system with the center of the trapezoidal thread coupling end face as the origin; Step S2, rotating the trapezoidal thread coupling so that the distance from the intersection of the first or second bearing surface of the measured thread groove with the thread pitch diameter to the first thread is an integer multiple of the thread pitch; Step S3, determining the plane containing the thread starting point; Step S4, determining the intersection of the line segment L from the coupling end face to the thread starting point and the plane containing the thread starting point as the thread starting point. This invention solves the problems of difficulty in accurately finding the thread starting point, low accuracy, and low efficiency in the prior art, achieving efficient and accurate positioning of the thread starting point of petroleum trapezoidal threads.

Description

Technical Field

[0001] This invention belongs to the field of petroleum thread detection technology, and specifically relates to a measuring device and method for determining the starting point of a trapezoidal thread coupling. Background Technology

[0002] In the petroleum industry, oil pipes are connected by oil thread couplings to form oil well tubing for the extraction and transportation of oil and natural gas. The quality and integrity of the oil threads in the couplings affect the operational safety and efficiency of the petroleum industry. The thread initiation point, the starting position of thread machining (the intersection of the helix and the end face), is crucial for ensuring the reliability and stability of the oil pipe connection. However, during field machining, the accurate location of the thread initiation point is difficult to find due to variations in manufacturers' processing capabilities and the quality of the finished products.

[0003] Currently, methods for measuring the starting point of a thread include manual measurement using a thread starting point confirmation caliper, visual observation, and rotation along the thread line to find the inflection point; and optical methods such as flash gauges or image analyzers to extract or calculate the thread profile features. Since oil pipe manufacturers generally control the thread starting point position within ±1° and require measurement time within 50 seconds, this places high demands on measurement accuracy and efficiency. However, for internal threads, caliper measurement is difficult, and optical methods are limited by factors such as light and angle.

[0004] Therefore, there is an urgent need to develop a measuring device and method for accurately determining the starting point of the thread, so as to effectively solve the problems of insufficient accuracy and low efficiency of the aforementioned measurement results. Summary of the Invention

[0005] One objective of this invention is to provide a measuring device for determining the starting point of a trapezoidal threaded coupling, so as to solve the problems of difficulty in finding the accurate location of the starting point of the thread in the prior art.

[0006] Another objective of this invention is to provide a method for measuring the starting point of a thread in a petroleum-grade trapezoidal thread, in order to solve the problems of low efficiency and low accuracy of manual measurement in the prior art.

[0007] This invention is achieved through the following technical solution:

[0008] A measuring device for determining the starting point of a trapezoidal threaded coupling includes: a software system and a hardware system.

[0009] The hardware system includes a servo control unit, a servo motor, a bracket with guide rails, a turntable for placing couplings, and a star-shaped probe. The star-shaped probe is connected to the bracket and is used to measure thread profile data. The servo control unit is used to control the servo motor to work and transmit data to the software system. The servo motor is used to drive the bracket to move along the guide rails, to drive the turntable to rotate, and to drive the star-shaped probe to move and measure in multiple directions.

[0010] The software system is used to record measurement data of the star-shaped probe, control the motion parameters of the servo motor, and send instructions to the servo control unit in the form of digital signals.

[0011] Furthermore, the star-shaped probe includes five probes, which point in different directions. The first probe points in the positive Z-axis direction, i.e., the vertically downward direction; the second probe points in the positive X-axis direction; the third probe points in the positive Y-axis direction; and the fourth and fifth probes point in the opposite directions to the second and third probes, respectively.

[0012] In another aspect, the present invention provides a method for measuring the starting point of a trapezoidal threaded coupling using the aforementioned measuring device, comprising the following steps:

[0013] Step S1: Place the trapezoidal threaded coupling on the measuring device and establish a three-dimensional coordinate system with the center of the end face of the trapezoidal threaded coupling as the origin of the coordinate system.

[0014] Step S2: Rotate the trapezoidal thread coupling so that the distance from the intersection of the first bearing surface or the second bearing surface of the measured thread groove with the thread pitch diameter to the first thread is an integer multiple of the thread pitch.

[0015] Step S3: Determine the plane where the thread start point is located;

[0016] Step S4: Determine the intersection of line segment L from the end face of the coupling to the thread start point and the plane where the thread start point is located as the thread start point.

[0017] Furthermore, in step S1, a three-dimensional coordinate system is established with the center of the trapezoidal threaded coupling end face as the origin of the coordinate system. Specifically, the center of the coupling end face is taken as the origin of the coordinate system, the direction of the coupling thread center axis is taken as the Z-axis direction, the direction towards the coupling tooth groove is taken as the X-axis direction, and the direction horizontal and perpendicular to the X-axis direction is taken as the Y-axis direction.

[0018] Further, rotating the trapezoidal thread coupling so that the distance from the intersection of the measured thread groove's first or second bearing surface and the thread pitch diameter to the first thread is an integer multiple of the thread pitch includes:

[0019] Step S21: Determine the coordinates of any point on the root of the thread. Let the coordinates of this point be (X...d Y d Z d );

[0020] Step S22: Move the coordinates of a point on the root of a thread obtained in step S21 by half the height of the thread profile along the negative or positive X-axis to determine the location of the thread pitch diameter line.

[0021] Step S23: Move the point of the thread pitch diameter line in the negative Z-axis direction to find the coordinates of the intersection of the first or second bearing surface of the thread groove and the thread pitch diameter line. Let the coordinates of this point be (X... m Y d Z m );

[0022] Step S24: Calculate the rotation angle θ of the trapezoidal thread coupling according to formula (1), rotate the trapezoidal thread coupling by angle θ with the central axis of the trapezoidal thread as the rotation center, and return the coupling to the correct position. At this time, the distance from the intersection of the first bearing surface or the second bearing surface of the thread groove and the thread pitch diameter line to the first thread is an integer multiple of the thread pitch.

[0023]

[0024] In the formula, θ is the rotation angle of the coupling, and P is the pitch.

[0025] Further, in step S21, the coordinates of a point on any thread root are determined, and the coordinates of this point are denoted as (X...). d Y d Z d Specifically, it includes the following steps:

[0026] After moving the star-shaped probe a height H along the positive Z-axis from the center of the coupling end face, measure W points evenly distributed within a pitch P in either the positive or negative X-direction. The point with the largest absolute value of the measured X-coordinate is the point on the thread root. Let the coordinates of the point on the thread root be (X... d Y d Z d ).

[0027] Furthermore, the height H ≥ L + P; W ≥ 3.

[0028] Further, in step S3, determining the plane where the thread start point is located includes: determining the position of the first thread; determining the angle α between the thread start point and the positive X-axis; adjusting the angle α of the rotating measuring device turntable to adjust the thread start point and the positive X-axis, at which point the plane where the positive X-axis and positive Y-axis of the first thread position are located is determined as the plane where the thread start point is located.

[0029] Further, determining the position of the first thread includes: moving the star-shaped probe from a position at height H in the negative Z-axis direction by a distance nP. At this point, the plane containing the X and Y directions of the star-shaped probe is the position of the first thread; where n is an integer multiple of the height H and the pitch P.

[0030] Furthermore, the formula for calculating the angle α between the thread starting point and the positive X-axis is as follows:

[0031]

[0032] In the formula, n is an integer multiple of the height H and the pitch P; L is the distance from the end face of the coupling to the starting point of the thread.

[0033] Compared with the prior art, the present invention has the following beneficial effects: The present invention solves the problems of difficulty in finding the accurate position of the thread starting point, low position accuracy, and low efficiency in the prior art, and realizes efficient and accurate positioning of the thread starting point of the oil-deflected trapezoidal thread. Attached Figure Description

[0034] Figure 1 The relationship between the angle, height, and pitch of the trapezoidal threaded coupling is shown.

[0035] Figure 2 A schematic diagram of the measuring device for determining the thread start point of a trapezoidal threaded coupling in an embodiment of the present invention is shown;

[0036] Figure 3 This diagram illustrates the three-dimensional coordinate system established in the method for measuring the starting point of a trapezoidal threaded coupling using a measuring device in an embodiment of the present invention.

[0037] The above-mentioned figures include the following reference numerals: 10, hardware system; 11, guide rail; 12, bracket; 13, turntable; 14, star-shaped probe; 20, software system. Detailed Implementation

[0038] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the protection scope of the embodiments of this application.

[0039] In the description of the embodiments of this application, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the embodiments of this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of this application. In addition, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0040] In the description of the embodiments of this application, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a replaceable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in the embodiments of this application based on the specific circumstances.

[0041] To address the problems raised in the background art, a first aspect of the present invention provides a measuring device for determining the thread start point of a trapezoidal threaded coupling, comprising a software system and a hardware system. The hardware system includes a servo control unit, a servo motor, a bracket with guide rails, a turntable for placing the coupling, and a star-shaped probe. The star-shaped probe is connected to the bracket and is used to measure thread profile data. The servo control unit controls the servo motor to operate and transmits data to the software system. The servo motor drives the bracket to move along the guide rails, rotates the turntable, and moves the star-shaped probe in multiple directions for measurement. The software system records the measurement data of the star-shaped probe, controls the motion parameters of the servo motor, and sends instructions to the servo control unit in the form of digital signals.

[0042] In a specific embodiment of the present invention, the star-shaped probe includes five probes, which point in different directions. The first probe points in the positive Z-axis direction, i.e., the vertically downward direction; the second probe points in the positive X-axis direction; the third probe points in the positive Y-axis direction; and the fourth and fifth probes point in the opposite directions to the second and third probes, respectively.

[0043] A second aspect of the present invention provides a method for measuring the starting point of a trapezoidal threaded coupling using the aforementioned apparatus, comprising the following steps:

[0044] Step S1: Place the trapezoidal threaded coupling on the measuring device and establish a three-dimensional coordinate system with the center of the end face of the trapezoidal threaded coupling as the origin of the coordinate system.

[0045] Step S2: Rotate the trapezoidal thread coupling so that the distance from the intersection of the first bearing surface or the second bearing surface of the measured thread groove with the thread pitch diameter to the first thread is an integer multiple of the thread pitch.

[0046] Step S3: Determine the plane where the thread start point is located;

[0047] Step S4: Determine the intersection of line segment L from the end face of the coupling to the thread start point and the plane where the thread start point is located as the thread start point.

[0048] In a specific embodiment of the present invention, the center of the end face of the coupling is taken as the origin of the coordinate system, the direction of the central axis of the coupling thread is taken as the Z-axis direction, the direction towards the groove of the coupling is taken as the X-axis direction, and the direction perpendicular to the X-axis is taken as the Y-axis direction.

[0049] In a specific embodiment of the present invention, a rotating trapezoidal threaded coupling is used to make the distance from the intersection of the measured thread groove's first or second bearing surface and the thread pitch diameter to the first thread a multiple of the thread pitch. The specific steps are as follows:

[0050] Since the couplings are placed at different angles, this embodiment moves the star-shaped probe only in the orthogonal direction to facilitate instrument measurement and calculation. The trapezoidal thread coupling has two bearing surfaces with different thread angles: a first bearing surface with a small angle and a second bearing surface with a large angle. This embodiment takes the intersection of the first bearing surface with the small angle and the thread pitch diameter as an example; its specific location is as follows... Figure 1 As shown, the bearing surface (the side with the small angle) of the tooth groove will have several intersection points PNT_1, PNT_2, ... with the thread pitch diameter. Taking PNT_1 as an example, the oblique line from it to the intersection point of the bearing surface and the thread pitch diameter of the next tooth is a helix. After the helix is ​​unfolded with the axis of the cone as the center, it is a straight line in the Z direction whose height is proportional to the angle. When the thread is machined one revolution, that is, for every 360° rotation of the angle, the height in the Z direction changes by one pitch P.

[0051] During measurement, the star-shaped probe descends a distance H from the coupling end face (i.e., in the positive Z-axis direction of the coordinate system), and then measures the profile data in the X+ (positive X-axis direction). However, due to the different horizontal orientations of the coupling, H may not be an integer multiple of the pitch from PNT to the coupling end face. To correct its orientation, the star-shaped probe is used to measure W points evenly distributed within a pitch P height in the positive X-direction. The point with the largest X-coordinate obtained at this time is the point on the thread root. Let the coordinates of the point on the thread root be (X... d Y d Z dFrom this point, backtrack half the tooth profile height in the X- direction to find the pitch diameter cone, then find the PNT in the Z- direction from this point. Let the position of this point be (X). m Y d Z m ), then according to Figure 1 The deviation of angle θ can be obtained by rotating the trapezoidal thread coupling by an angle θ with the central axis of the trapezoidal thread as the rotation center, and returning the coupling to the correct position. At this time, the distance from the intersection of the first bearing surface of the thread groove and the thread pitch diameter line to the first thread is an integer multiple of the pitch.

[0052]

[0053] In the formula, θ is the rotation angle of the coupling, and P is the pitch.

[0054] In a specific embodiment of the present invention, determining the plane where the thread starting point is located includes the following steps:

[0055] The thread starting point is the cutting position of the machining lathe. The distance from the coupling end face to the thread starting point is a fixed value designed according to the thread specification and type. Let the distance from the coupling end face to the starting point be L. Move the star probe from the previously determined height H position towards the Z- direction by a distance nP, where n is a positive natural number and nP is less than H. At this time, the position of the star probe is the plane where X and Y are located, which is the position of the first thread. The distance from the first thread in the plane where X and Y are located to the coupling end face is H-nP. Since the height dropped by one thread per revolution is one pitch P, the formula for calculating the angle α between the X+ direction and the thread starting point is formula (2). Adjust the angle α of the rotating measuring device turntable to the thread starting point and the positive X-axis direction. At this time, the plane where the positive X-axis direction and the positive Y-axis direction where the first thread is located is determined as the plane where the thread starting point is located.

[0056]

[0057] In the formula, n is an integer multiple of the height H and the pitch P; L is the distance from the end face of the coupling to the starting point of the thread.

[0058] In some specific embodiments of the present invention, the height H ≥ L + P; W ≥ 3.

[0059] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0060] Example

[0061] A measuring device for determining the thread start point of a trapezoidal threaded coupling, such as... Figure 2As shown, the system includes a hardware system 10 and a software system 20. The hardware system 10 includes a servo control unit, a servo motor, a bracket 12 with a guide rail 11, a turntable 13 for placing couplings, and a star-shaped probe 14. The star-shaped probe 14 is connected to the bracket 12 and is used to measure thread profile data. The servo control unit is used to control the servo motor and transmit data to the software system 20. The servo motor is used to move the bracket 12 along the guide rail 11, rotate the turntable 13, and move the star-shaped probe 14 in multiple directions for measurement. The software system 20 is used to record the measurement data of the star-shaped probe 14, control the motion parameters of the servo motor (such as moving speed, moving position, etc.), and send instructions to the servo control unit in the form of digital signals.

[0062] The star-shaped probe 14 includes five probes, which point in different directions. The first probe points in the positive Z-axis direction, i.e., the vertical downward direction. The second probe points in the positive X-axis direction. The third probe points in the positive Y-axis direction. The fourth and fifth probes point in the opposite directions to the second and third probes, respectively.

[0063] The working process of the measuring device is as follows: the software system 20 sends the instructions of the motion parameters set by the measuring device to the servo control unit in the form of digital signals. The servo control unit controls the servo motor to work. The servo motor controls the movement of the guide rail 11 and the rotation of the turntable 13, while driving the star-shaped probe 14 to measure in different directions, so that the star-shaped probe 14 can accurately locate the thread starting point of the trapezoidal thread coupling.

[0064] For a 4-1 / 2 trapezoidal thread coupling, the thread pitch P of the coupling is 5.08 mm, the distance L from the coupling end face to the starting point is 8.45 mm, and the thread height is 1.575 mm. The thread starting point of the above-mentioned trapezoidal thread coupling is measured using the aforementioned device of the present invention, and the measurement method is implemented according to the following steps:

[0065] (1) Place the 4-1 / 2 eccentric trapezoidal thread coupling on the turntable of the measuring device, and establish a three-dimensional coordinate system with the center of the eccentric trapezoidal thread end face as the origin of the coordinate system. The coordinate diagram is shown below. Figure 3 As shown, the center of the coupling end face is the origin of the coordinate system, the direction downward of the coupling thread center axis is the positive Z-axis, the direction towards the coupling tooth groove is the X-axis, and the direction horizontal and perpendicular to the X-axis is the Y-axis; the servo motor moves the guide rail and the star-shaped probe to the center of the coupling end face, and the measurement coordinates of the star-shaped probe are (0, 0, 0).

[0066] (2) Lower the star-shaped probe from the coupling end face (i.e., along the positive Z-axis of the coordinate system) to a height H of 55.135 mm. Distribute the star-shaped probe evenly to measure four points within a pitch P along the positive X-axis. The point with the largest X value is the point on the thread root, with coordinates (57.351, 0, 42.113) in mm. From this point, back half the thread height in the X- direction to find the thread pitch diameter. Then, from this point, find the nearest PNT point in the Z- direction, with coordinates (56.566, 0, 41.072) in mm. Then, according to the formula... Calculate θ, which is 73.77°. Finally, control the turntable to rotate 73.77° clockwise around the central axis of the trapezoidal thread to return to the coupling position. At this time, the distance from the intersection of the first bearing surface of the thread groove and the thread pitch diameter to the first thread is an integer multiple of the thread pitch.

[0067] (3) Determine the plane where the thread start point is located: The distance L from the end face of the thread of this specification coupling to the start point is 8.45mm. According to the integer multiple relationship between H and P, n in formula (2) is 10. Move the probe nP = 50.80mm in the Z-direction. At this time, the distance from the first thread of the plane where the positive X-axis and positive Y-axis are located to the end face of the coupling is 4.335mm. Combined with the formula Calculate α as -291.61°, control the turntable to rotate counterclockwise by 68.39°, and at this time, determine the plane where the positive X-axis and positive Y-axis of the first thread position are located as the plane where the thread starts.

[0068] (4) Lower the probe from the end face to the Z+ direction by a height L. At this time, the intersection of the line segment L from the end face of the coupling to the thread start point and the plane where the thread start point is located is determined as the thread start point.

[0069] The embodiments described above are merely illustrative of implementation methods of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. The present invention can also be implemented in other specific ways or forms without departing from its spirit or essential characteristics. Therefore, the described embodiments should be considered illustrative rather than limiting in any respect. The scope of the present invention should be defined by the appended claims, and any variations equivalent to the intent and scope of the claims should also be included within the scope of the present invention.

Claims

1. A measuring device for determining the thread starting point of a trapezoidal threaded coupling, characterized in that, Including software systems and hardware systems, The hardware system includes a servo control unit, a servo motor, a bracket with guide rails, a turntable for placing couplings, and a star-shaped probe. The star-shaped probe is connected to the bracket and is used to measure thread profile data. The servo control unit is used to control the servo motor to work and transmit data to the software system. The servo motor is used to drive the bracket to move along the guide rails, to drive the turntable to rotate, and to drive the star-shaped probe to move and measure in multiple directions. The software system is used to record measurement data of the star-shaped probe, control the motion parameters of the servo motor, and send instructions to the servo control unit in the form of digital signals.

2. The measuring device for determining the thread starting point of a trapezoidal threaded coupling according to claim 1, characterized in that, The star-shaped probe includes five probes, which point in different directions. The first probe points in the positive Z-axis direction, i.e., the vertical downward direction; the second probe points in the positive X-axis direction; the third probe points in the positive Y-axis direction; and the fourth and fifth probes point in the opposite directions to the second and third probes, respectively.

3. A method for measuring the starting point of a trapezoidal threaded coupling using the measuring device described in claim 1 or 2, characterized in that, Includes the following steps: Step S1: Place the trapezoidal threaded coupling on the measuring device and establish a three-dimensional coordinate system with the center of the end face of the trapezoidal threaded coupling as the origin of the coordinate system. Step S2: Rotate the trapezoidal thread coupling so that the distance from the intersection of the first bearing surface or the second bearing surface of the measured thread groove with the thread pitch diameter to the first thread is an integer multiple of the thread pitch. Step S3: Determine the plane where the thread start point is located; Step S4: Determine the intersection of line segment L from the end face of the coupling to the thread start point and the plane where the thread start point is located as the thread start point.

4. The method according to claim 3, characterized in that, In step S1, a three-dimensional coordinate system is established with the center of the trapezoidal threaded coupling end face as the origin of the coordinate system, specifically as follows: The origin of the coordinate system is the center of the end face of the coupling. The direction of the central axis of the coupling thread is the Z-axis, the direction towards the groove of the coupling is the X-axis, and the direction perpendicular to the X-axis is the Y-axis. The positive direction of the Z-axis is downwards from the center axis of the coupling thread.

5. The method according to claim 4, characterized in that, A rotating trapezoidal thread coupling is used to ensure that the distance from the intersection of the measured thread groove's first or second bearing surface and the thread pitch diameter to the first thread is an integer multiple of the thread pitch, including: Step S21: Determine the coordinates of any point on the root of the thread. Let the coordinates of this point be (X... d Y d Z d ); Step S22: Move the coordinates of a point on the root of a thread obtained in step S21 by half the height of the thread profile along the negative or positive X-axis to determine the location of the thread pitch diameter line. Step S23: Move the point of the thread pitch diameter line in the negative Z-axis direction to find the coordinates of the intersection of the first or second bearing surface of the thread groove and the thread pitch diameter line. Let the coordinates of this point be (X... m Y d Z m ); Step S24: Calculate the rotation angle θ of the trapezoidal thread coupling according to formula (1), rotate the trapezoidal thread coupling by angle θ with the central axis of the trapezoidal thread as the rotation center, and return the coupling to the correct position. At this time, the distance from the intersection of the first bearing surface or the second bearing surface of the thread groove and the thread pitch diameter line to the first thread is an integer multiple of the thread pitch. In the formula, θ is the rotation angle of the coupling, and P is the pitch.

6. The method according to claim 5, characterized in that, In step S21, determine the coordinates of a point on any thread root, and let the coordinates of this point be (X... d Y d Z d Specifically, it includes the following steps: After moving the star-shaped probe a height H along the positive Z-axis from the center of the coupling end face, measure W points evenly distributed within a pitch P in either the positive or negative X-direction. The point with the largest absolute value of the measured X-coordinate is the point on the thread root. Let the coordinates of the point on the thread root be (X... d Y d Z d ).

7. The method according to claim 6, characterized in that, Height H≥L+P; W≥3.

8. The method according to claim 6, characterized in that, In step S3, determining the plane containing the thread start point includes: Determine the position of the first thread; Determine the angle α between the starting point of the thread and the positive X-axis; The rotation measuring device's turntable angle α is adjusted to align the thread starting point with the positive X-axis direction. At this point, the plane containing the positive X-axis direction and the positive Y-axis direction where the first thread is located is defined as the plane where the thread starting point is located.

9. The method according to claim 8, characterized in that, Determining the position of the first thread includes: Move the star-shaped probe from a position at height H in the negative Z-axis direction by a distance nP. The plane containing the X and Y directions of the star-shaped probe at this point is the position of the first thread; where n is an integer multiple of the height H and the pitch P.

10. The method according to claim 8, characterized in that, The formula for calculating the angle α between the starting point of the thread and the positive X-axis is: In the formula, n is an integer multiple of the height H and the pitch P; L is the distance from the end face of the coupling to the starting point of the thread.

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