What is thread

What is thread?

In machining, the thread is cut on a cylindrical shaft (or inner hole surface) with a cutter or grinding wheel. When the workpiece rotates, the cutter moves a certain distance along the axial direction of the workpiece, and the trace cut by the cutter on the workpiece is the thread. The thread formed on the outer circular surface is called external thread. The thread formed on the inner hole surface is called internal thread. The basis of the thread is the circular shaft surface Generally, if the cross section of the thread is triangle, it is called triangle thread; if the cross section is trapezoid, it is called trapezoid thread; if the cross section is serrated, it is called serrated thread; if the cross section is square, it is called square thread; if the cross section is arc, it is called arc thread, etc.

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Geometric parameters of thread

Main geometric parameters of cylindrical thread

  • ① Outside diameter (major diameter), the imaginary cylinder diameter that coincides with the top of external thread or the bottom of internal thread. The nominal diameter of thread is major diameter.
  • ② Inner diameter (minor diameter), the imaginary diameter of a cylinder that coincides with the base of an external thread or the apex of an internal thread.
  • ③ The generatrix passes through an imaginary cylinder with the same width of protrusion and groove on the tooth profile.
  • ④ Pitch, the axial distance between two points corresponding to the pitch line of adjacent teeth.
  • ⑤ Lead, the axial distance between two points on the same helix.
  • ⑥ The angle between two adjacent teeth on the thread profile.
  • ⑦ The angle between the tangent line of the helix on the pitch diameter cylinder and the plane perpendicular to the thread axis.
  • ⑧ Working height, the distance between the overlapping parts of the two-phase matching thread profile perpendicular to the thread axis, etc. the nominal diameter of the thread is the outer diameter except the inner diameter of the pipe. The thread has been standardized, including metric system (metric system) and English system. The international standard adopts metric system, and China also adopts metric system.

In addition to the pipe thread, the inner diameter of the pipe is the nominal diameter, the other threads are the nominal diameter of the outer diameter. The thread pair whose thread rise angle is less than the friction angle does not rotate under the action of axial force is called self-locking, and its transmission efficiency is low.
Among the cylindrical threads, triangular thread has good self-locking performance. It is divided into coarse thread and fine thread. Generally, coarse thread is used for connection. Fine thread has small pitch, small angle of rise and better self-locking performance. It is commonly used in thin-walled tube of small parts, connection with vibration or variable load, and fine-tuning device.
The thread shape of taper thread is triangle, which mainly depends on the deformation of the thread to ensure the tightness of the thread pair. It is mostly used for pipe fittings.

Formation and structure of thread

  • (1) The formation of thread: a point on the cylindrical surface revolves around the axis of the cylinder at the same speed, and at the same time moves along a straight line at the same speed. The trajectory of the compound motion is the helix.
  • (2) Thread structure: the convex part of the thread is called the top of the tooth, and the groove part is called the bottom of the tooth. In order to prevent the end of the thread from being damaged during the installation, the conical chamfer or spherical rounding is processed at the beginning of the thread. At the end of the thread, there is an end or undercut.

Structural elements of thread

  • (1) Tooth type: triangle, trapezoid, serrated and square.
  • (2) Nominal diameter: refers to the diameter representing the size of the thread, generally refers to the major diameter of the thread. It is indicated by D (external thread) or D (internal thread).
  • (3) Thread number: there are single thread and multi thread. The thread formed along one helix is called single thread; the thread formed along two or more helix is called multi thread. It is indicated by n.
  • (4) Pitch and lead: the axial distance between two points on the pitch diameter line of two adjacent threads is called pitch, which is expressed by P. the axial distance between two points on the pitch diameter line of two adjacent threads on the same helix is called lead, which is expressed by S. it can also be understood as the rising distance of one turn. For single thread, lead and pitch are equal, that is, s = P. for multi thread, s = n ×p.
  • (5) Direction of rotation: the direction of rotation of thread can be divided into left-hand and right-hand. When the thread is rotated clockwise, it is right-hand thread; when the thread is rotated counterclockwise, it is left-hand thread. When the internal and external threads are connected, the above elements must be the same before they can be rotated together.
  • The three elements of thread: profile, diameter and pitch are the most basic elements to determine the thread. The three elements that meet the national standard are called standard thread; the profile that meets the standard, but the diameter or pitch does not meet the standard are called special thread; the profile that does not meet the standard, such as square thread, are called non-standard thread.

Type of thread

Connecting thread: common thread with triangular tooth.
Transmission thread: trapezoidal thread, serrated thread and square thread.

Structure classification

According to its cross-section shape (tooth type), thread can be divided into triangular thread, rectangular thread, trapezoidal thread and serrated thread. Among them, triangular thread is mainly used for connection (see thread connection), while rectangular, trapezoidal and serrated thread are mainly used for transmission. The thread distributed on the outer surface of the matrix is called external thread, while the thread on the inner surface of the matrix is called internal thread. The thread formed on the cylinder matrix is called internal thread Cylindrical thread, the thread formed on the conical matrix is called conical thread. According to the direction of helix, the thread can be divided into left-hand and right-hand, generally right-hand thread. The thread can be divided into single line and multi line, and the connection is mostly single line. When used for transmission, it requires fast advance or high efficiency, and double line or multi line is adopted, but generally no more than 4 lines.
Triangle thread is mainly used for connection, rectangular, trapezoidal and serrated thread is mainly used for transmission; according to the direction of helix, it can be divided into left-hand thread and right-hand thread, generally right-hand thread; according to the number of helix, it can be divided into single thread, double thread and multi thread; the connection is mostly single thread, and the transmission is mainly double thread or multi thread; according to the size of teeth, it can be divided into coarse thread and fine thread It can be divided into fastening thread, pipe thread, transmission thread, special thread and so on.
Among the cylindrical threads, triangular thread has good self-locking performance. It is divided into coarse thread and fine thread. Generally, coarse thread is used for connection. Fine thread has small pitch, small angle of rise and better self-locking performance. It is commonly used in thin-walled tube of small parts, connection with vibration or variable load and fine-tuning device. Pipe thread is used for tight connection of pipe fittings. Rectangular thread has high efficiency, but it is not easy to grind, and its internal and external parts are not easy to be ground It is difficult to center the thread, so it is often replaced by trapezoidal thread. The working edge of serrated thread is close to the rectangular straight edge, which is mostly used to bear one-way axial force.
Conical thread is triangle in shape, which is mainly used for pipe fitting because of its tight thread pair ensured by tooth deformation.
According to the sealing is divided into sealing thread and non sealing thread.

Method of processing thread


Method of processing thread directly with mold.


The processing method of making the workpiece produce plastic deformation by forming rolling die to obtain thread. Thread rolling is generally in thread rolling machine. Thread rubbing machine or is carried out on the automatic lathe with automatic opening and closing thread rolling head, which is suitable for mass production of external threads of standard fasteners and other thread connection parts. The outer diameter of the rolling thread generally does not exceed 25mm and the length is not more than 100mm. The thread accuracy can reach Grade 2 (GB197-63). The diameter of the blank is roughly equal to the middle diameter of the processed thread. Rolling generally can not process the internal thread, but for the workpiece with soft material, the internal thread can be cold extruded with a non-groove extrusion tap (the maximum diameter can reach about 30mm), and the working principle is similar to tapping. When the internal thread is cold extruded, the required torque is about 1 time larger than the tapping wire, and the processing precision and surface quality are slightly higher than the tapping wire.
The advantages of thread rolling are: the surface roughness is less than turning, milling and grinding; The surface of the thread after rolling can improve the strength and the utilization rate of hardness materials due to cold work hardening; The productivity is doubled compared with the cutting process, and easy to realize automation; Rolling mold has a long life. But the rolling thread requires the hardness of the work-piece material not exceeding HRC40; It requires high precision of the Blank’s dimension; It also requires high precision and hardness of the rolling mould, and it is relatively difficult to manufacture the mould; not suitable for rolling asymmetric thread.
According to the difference of rolling mold, thread rolling can be divided into two categories: rubbing wire and rolling wire.
Wire rubbing two thread thread thread rubbing plates are staggered with the relative arrangement of 1/2 thread pitch. The static plate is fixed and the moving plate is in reciprocating linear motion parallel with the static plate. When the workpiece is sent between the two plates, the moving plate rolls the workpiece forward to make its surface plastically deformed into a thread.
There are 3 kinds of rolling, such as radial rolling, tangential rolling and rolling head rolling.

  • Radial rolling wire: 2 (or 3) thread thread roller wheels are installed on axes parallel to each other, the workpiece is placed on the support between the two wheels, and the two wheels are rotated in the same direction and at the same speed, one of them also makes a radial feed movement. The rotating surface of the workpiece is squeezed by the radial direction to form a thread under the driving of the roller wheel. For some lead screws with low precision requirements, similar methods can also be adopted for rolling forming.
  • Tangential rolling wire: It is also called planetary rolling wire. The rolling tool consists of one rotating central rolling wire wheel and three fixed curved wire plates. When rolling, the workpiece can be continuously delivered, so the productivity is higher than that of rubbing and radial rolling.
  • Rolling head rolling wire: it is carried out on an automatic lathe, usually used to process short threads on workpieces. There are 3 to 4 wire roller wheels uniformly distributed around the workpiece in the rolling head. When rolling the wire, the workpiece rotates the rolling head to feed in the axial direction to roll the workpiece out of the thread.


Precautions for thread turning: considering the expansion of thread processing teeth, the large diameter of the external thread (nominal diameter d generally should be 0.2~0.4mm smaller than the basic size (about 0.13P), ensure that there is a width of 0.125P (P is the pitch of the thread) behind the thread, and ensure that the diameter of the bottom hole of the thread in the bore is nominal diameter -P .. Thread cutting should be paid attention to setting enough promoting-speed feeding blade section 1 and slowing-off blade section 2 at both ends to get rid of the non-standard thread thread with pitch which appears because of variable speed on both ends. Similarly, in the thread cutting process, the feed speed adjustment function and feed pause function are invalid; If the feed pause button is pressed at this time, the cutter will not stop until the thread is processed. The feed amount of thread processing can refer to the thread bottom diameter, that is, the final feed position of the thread knife. Thread diameter: large diameter-1.2times pitch; The feed amount of thread processing should be continuously reduced, and the specific feed amount should be selected according to the tool and workpiece material, but the last time should not be less than 0.1mm. After the thread processing is completed, measures can be taken to judge the thread quality in time by observing the thread shape. But it should be noted that when the thread is not pointed at the top of the thread, increasing the amount of the knife will increase the increase of the diameter of the thread. It depends on the plasticity of the material. When the thread is sharpened, increase the cutting amount of the knife to reduce the large diameter proportionally. According to this feature, the cutting amount of the thread should be correctly treated to prevent scrapping. For general Standard threads, threaded ring gauge or plug gauge is used for measurement. When measuring the external thread, if the thread “passes through the end” ring gauge (general gauge) It just rotates, and if the “stop end” ring gauge (stop gauge) cannot screw in, it means that the processed thread meets the requirements, otherwise it is not qualified. When measuring internal thread, use thread plug gauge to measure in the same way. In addition to the thread ring gauge or plug gauge, other measuring tools can also be used to measure the middle diameter of the thread with thread micrometer, etc.

  • Thread bottom hole D drill = D -P( D is the thread diameter, P is the thread pitch)

Determine the purpose: when tapping the thread, it will not be due to the extrusion, so that the thread will not bite between the top and the bottom of the thread (This phenomenon also exists when tapping with the tap) to ensure an effective working height, etc.

  • Bottom hole depth H drill = h effective +0.7D h is the effective depth of the effective thread, D is the diameter of the thread)

Determining the purpose: the main purpose is to use the tap to tap the thread when the thread is not through the thread. The cutting part of the tap has a cone angle, and the end cannot cut out a complete tooth shape, so the drilling depth is greater than the effective depth of the thread.
M8 has coarse teeth and fine teeth:

  • 1. Coarse teeth M8, pitch P = 1.25, coarse teeth notation: M8 (omitted pitch);
  • 2. Fine teeth M8, pitch P = 1, fine teeth notation: M8x1.

Thread seal:

Any plane can not be completely in close contact, the need for leak-proof seal, the traditional method is to use rubber, asbestos, metal and other gaskets, but due to aging or corrosion will quickly leak. The anaerobic adhesive is used to replace the solid gasket, which can be closely contacted after curing, making the sealing more durable. Matrelay Super lubricant is a kind of special inert material with multiple uses, which is used for the sealing of the thread pipe joint and thread plug, the sealing of the flange plate coordination surface, the sealing of the joint surface of mechanical box, etc, all have good leak-proof effect.
Super lubricant is mainly used to reduce intermetallic contact. As a thread sealing compound, the product forms a contact surface between the external thread and the internal thread, which can protect the joint from friction and wear, and can withstand 1407kg/cm 2 pressure, even wear, corrosion or wrong machined thread surface. The product is also an excellent gearbox additive that can form a layer of film on the internal parts. Thus reducing friction, gear noise and leakage. It also significantly reduces torque stress and meets the demand of power decompression. It can be used on the gasket surface or as a filler to prevent fluid leakage by sealing. It can be applied at 316°C. The product can be constructed in stainless steel, aluminum, iron, barium, glass fiber, plastic and will not be affected by acid, alkali or ordinary solvent.

Thread measurement

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  • 1. For general standard threads, threaded ring gauge or plug gauge is used for measurement.
  • 2. Thread micrometer is used to measure the middle diameter of thread. Dosage the method of needle measuring the middle diameter of thread is called three-needle measuring method.
  • 3. Tooth thickness scale caliper is used to measure trapezoidal thread middle diameter tooth thickness and worm joint diameter tooth thickness.
  • 4. Special measuring tools and instruments for other parameters.

Source: China Thread Fittings Manufacturer – Yaang Pipe Industry (www.steeljrv.com)

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