### 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.

• (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).
• (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.

### Structure classification

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.

### Mould

Method of processing thread directly with mold.

### Rolling

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.

### Cutting

• 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.

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.