Thread, dealing with machinery every day, for a long time, domestic and foreign, metric, imperial, straight cone, sealed and unsealed, inner and outer, 55 degrees and 60 degrees. In short, I often get messed up, and I use it to check from beginning to end. I can summarize it here. I hope it helps.
My method is to print out the commonly used ones and put them on the table, and use them to check them at any time. After a long time, they will naturally be remembered.
NPT is a general-purpose American standard taper pipe thread with a profile angle of 60°
The PT thread is an inch taper thread with a profile angle of 55°, which is most commonly used in sealing. The inch pipe thread is a fine thread, because the coarse thread has a large tooth depth, which will seriously reduce the strength of the pipe with the outer diameter of the thread.
PF thread is a parallel thread for pipe.
G is a 55-degree non-thread sealed pipe thread, which belongs to the Wyeth thread family. Marked as G stands for cylindrical thread, G is the general term for pipe thread (Guan), the division of 55 and 60 degrees is functional
ZG is commonly known as pipe cone, that is, the thread is processed by a conical surface. The general water pipe joints are like this, and the old national standard is marked as Rc
Metric thread is expressed by the pitch, American and British thread is expressed by the number of threads per inch. This is the biggest difference between them. Metric thread is 60 degree equilateral thread, inch thread is isosceles 55 degree thread, American thread 60 degrees. Metric units are used for metric threads, and imperial units are used for American and British threads.
Pipe threads are mainly used to connect pipelines. The internal and external threads are closely matched, and there are two types of straight pipes and tapered pipes. The nominal diameter refers to the diameter of the connected pipe, obviously the larger diameter of the thread is larger than the nominal diameter.
1/4, 1/2, 1/8 are the nominal diameters of inch threads, and the unit is inches.
(two)
1. Inch uniform thread
Widely used in inch system countries, this type of thread is divided into three series: coarse thread series UNC, fine thread series UNF, extra fine thread series UNFF, plus a fixed pitch series UN.
Marking method: thread diameter—number of teeth per inch series code—precision grade
Example: Coarse Tooth Series 3/8—16UNC—2A
Fine tooth series 3/8—24UNF—2A
Extra fine teeth series 3/8—32UNFF—2A
Fixed pitch series 3/8—20UN—2A
The first digit 3/8 represents the outer diameter of the thread, the unit is inches, and the conversion to the metric unit mm needs to be multiplied by 25.4, that is, 3/8×25.4=9.525mm; the second and third digits 16, 24, 32, and 20 are The number of teeth per inch (the number of teeth on a length of 25.4mm); the text codes UNC, UNF, UNFF, UN after the third digit are the serial codes, and the last two digits 2A are the accuracy grades.
2. Conversion of 55° cylindrical pipe thread
The 55° cylindrical pipe thread comes from the inch series, but it is widely used in metric and inch countries. It is used to connect pipe joints and pipes for conveying liquids, gases, and installing wires. However, the codes in different countries are different, so you should press The foreign codes in the table (contrast table) are transformed into the codes of our country. Now the codes of 55° cylindrical pipe threads in various countries are listed in the table below.
Country code
China G
Japan G, PF
British BSP, BSPP
French G
Germany R (internal thread), K (external thread)
Former Soviet Union G, TPУБ
ISO Rp
3. Conversion of 55° tapered pipe thread
55° tapered pipe thread means that the thread profile angle is 55° and the thread has a taper of 1:16. This series of thread is widely used in the world, and its code is different from country to country. See the table below. According to the foreign code in the table below, it will be transformed into our country code.
Country code
China ZG, R (external thread)
British BSPT, R (external thread), Rc (internal thread)
France G (external thread), R (external thread)
Germany R (external thread)
Japan PT, R
ISO R (external thread), Rc (internal thread)
4. Conversion of 60° tapered pipe thread
The 60° tapered pipe thread refers to a pipe thread with a tooth profile angle of 60° and a thread taper of 1:16. This series of threads are used in the machine tool industry in my country, the United States and the former Soviet Union. Its code name was designated as K in the past in our country, and later designated as Z, and now it is changed to NPT. See the table below for the comparison table of thread codes.
Country code
China Z (old) NPT (new)
U.S. NPT
Soviet Union B
5. Conversion of 55° trapezoidal thread
Trapezoidal thread refers to a metric trapezoidal thread with a profile angle of 30°. This series of threads are relatively uniform at home and abroad, and their codes are also quite consistent. See below for the thread codes
Country code
China T (old) Tr (new)
ISO Tr
Germany Tr
Former Soviet Union Tr
Three, thread classification
According to different uses of threads, it can be divided into:
1. International Metric Thread System
The thread used in my country's national standard CNS. The top of the tooth is flat, which is easy to turn, and the bottom of the tooth is arc-shaped to increase the thread strength. The thread angle is 60 degrees, and the specification is represented by M. Metric thread can be divided into two types: coarse thread and fine thread. The notation is M8x1.25. (M: code, 8: nominal diameter, 1.25: pitch).
2. American Standard Thread
Both the top and root of the thread are flat and the strength is better. The thread angle is also 60 degrees, and the specification is expressed by a few teeth per inch. This kind of thread can be divided into three grades: coarse thread (NC); fine thread (NF); extra fine thread (NEF). The notation is like 1/2-10NC. (1/2: outer diameter; 10: number of teeth per inch; NC code).
3. Unified Thread (UnifiedThread)
Jointly formulated by the United States, Britain, and Canada, it is the commonly used inch thread at present.
The thread angle is also 60 degrees, and the specification is expressed by a few teeth per inch. This kind of thread can be divided into coarse thread (UNC); fine thread (UNF); extra fine thread (UNEF). The notation is like 1/2-10UNC. (1/2: outer diameter; 10: number of teeth per inch; UNC code)
4. Sharp VThread
The top and roots are both pointed, weak in strength, and not often used. The thread angle is 60 degrees.
5. WhitworthThread (WhitworthThread)
Thread adopted by British National Standard. The thread angle is 55 degrees, and the symbol is "W". Suitable for manufacturing by rolling method. The notation is like W1/2-10. (1/2: outer diameter; 10: number of teeth per inch; W code).
6. Round thread (KnuckleThread)
It is the standard thread specified by German DIN. Suitable for the connection of bulbs and rubber tubes. The symbol is "Rd".
7. Pipe thread (PipeThread)
Threads used to prevent leakage are often used to connect gas or liquid pipe fittings. The thread angle is 55 degrees, which can be divided into straight pipe thread code "P.S., N.P.S." and oblique pipe thread code "N.P.T.", the taper is 1:16, that is, 3/4 inch per foot.
8. Square thread (SquareThread)
The transmission efficiency is high, second only to the ball thread, and it cannot be adjusted with a nut after wear, which is a disadvantage. Generally used for the screw of the vise and the screw of the crane.
9. Trapezoidal thread (TrapezoidalThread)
Also known as Akem thread. The transmission efficiency is slightly lower than that of the square thread, but it can be adjusted with a nut after wear. The metric thread angle is 30 degrees, and the inch thread angle is 29 degrees. Generally used for the lead screw of a lathe. The symbol is "Tr".
10. ButtressThread
Also called oblique thread, it is only suitable for unidirectional transmission. Such as screw jacks, presses, etc. The symbol is "Bu".
11. Ball thread
The thread with the best transmission efficiency is difficult to manufacture and the cost is extremely high. It is used in precision machinery. Such as the lead screw of a CNC machine tool.
Representation of inch bolts:
LH 2N 5/8 × 3-13UNC-2A
(1) LH is left-hand thread (RH is right-hand thread, which can be omitted).
(2) 2N double thread thread.
(3) 5/8 inch thread, outer diameter 5/8”.
(4) 3 Bolt length is 3".
(5) 13 thread counts 13 teeth per inch.
(6) UNC unified standard thread coarse thread.
(7) Level 2 fit, external thread (3: tight fit; 2: middle fit; 1: loose fit) A: external thread (can be omitted) B: internal thread
Inch thread:
The size of the inch thread is usually expressed by the number of threads per inch of length on the thread, referred to as "number of threads per inch", which is exactly equal to the reciprocal of the thread pitch. For example, a thread with 8 teeth per inch has a pitch of 1/8 inch.
