Screw thread, do mechanical daily contact, a long time, domestic and foreign, metric, English, straight cone, sealed, unsealed, inside and outside, 55 degrees, 60 degrees.
When we process the thread of the workpiece, there are often different thread standards. In order to deal with different threads, it is usually necessary to purchase different CNC cutting tools. The standard of thread is very messy, but this article has sorted it out!
Screw thread, do mechanical daily contact, a long time, domestic and foreign, metric, English, straight cone, sealed, unsealed, inside and outside, 55 degrees, 60 degrees. In a word, they are often confused. They are checked once from the beginning to the end. I can summarize them here. I hope they can help.
NPT
NPT is an American standard taper pipe thread for general use with a profile angle of 60 degrees
PT thread is British taper thread with 55 ° profile angle, which is most commonly used in sealing. British pipe thread is fine thread, because the depth of coarse thread is large, it will seriously reduce the strength of the outer diameter pipe.
Pf thread is parallel thread for pipe.
G is a 55 degree non threaded sealed pipe thread, belonging to Wyeth thread family. The mark g represents cylindrical thread, and G is the general name of pipe thread. The division of 55 and 60 degrees is functional
ZG is commonly known as pipe cone, that is, the thread is made of a round cone surface. The general water pipe joints are like this, and the old national standard mark is RC
Metric thread is represented by pitch, while American British thread is represented by the number of threads per inch. This is the biggest difference between them. Metric thread is 60 degree equilateral profile, British thread is 55 degree isosceles profile, and American thread is 60 degree. Metric screw thread uses metric unit, American and British screw thread uses English unit.
Pipe thread is mainly used for pipe connection, and its internal and external threads are closely matched, including straight pipe and tapered pipe. The nominal diameter refers to the diameter of the pipe to be connected. Obviously, the major diameter of the thread is larger than the nominal diameter.
1 / 4, 1 / 2, and 1 / 8 are nominal diameters of inch threads in inches.
Inch unified thread
It is widely used in inch system countries. This kind of thread is divided into three series: coarse thread series UNC, fine thread series UNF, ultra fine thread series UNFF, plus a fixed pitch series UN.
Marking method: thread diameter – teeth per inch series code – accuracy grade
Example: coarse tooth series 3 / 8-16unc-2a
Fine tooth series 3 / 8-24unf-2a
Super fine tooth series 3 / 8-32unff-2a
Fixed pitch series 3 / 8-20un-2a
The first digit 3 / 8 represents the outer diameter of thread, and the unit is inch. To convert it into metric unit mm, it needs to multiply 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 the length of 25.4mm); the text codes after the third digit, such as UNC, UNF, UNFF and UN, are the series codes, and the last two digits 2a are the accuracy level.
Transformation of 55 ° cylindrical pipe thread
55 ° cylindrical pipe thread is derived from Inch series, but it is widely used in metric and inch countries for the connection of pipe joints and pipes for conveying liquid, gas and installing wires. However, the codes of different countries are different. The foreign codes in the following table (comparison table) should be converted into Chinese codes. Now the codes of 55 ° cylindrical pipe thread in different countries are listed in the following table.
Country code
China g
Japan g, PF
BSP, BSPP in UK
France G
Germany R (internal thread), K (external thread)
The former Soviet Union
ISO Rp
Transformation of 55 ° taper pipe thread
55 ° tapered pipe thread refers to the thread with a profile angle of 55 ° and a taper of 1:16. This series of thread is widely used in the world. Its code number varies from country to country. See the table below. The foreign codes in the following table are converted into Chinese codes.
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)
Pt, R in Japan
ISO R (external thread), RC (internal thread)
Transformation of 60 ° taper pipe thread
60 ° tapered pipe thread refers to the pipe thread with a profile angle of 60 ° and a thread taper of 1:16. This series of thread is used in China’s machine tool industry, the United States and the former Soviet Union. Its code name, China used to be K, later Z, now NPT. See the table below for the comparison table of thread code.
Country code
China Z (old) NPT (New)
Us NPT
Soviet Union B
Transformation of 55 ° trapezoidal thread
Trapezoidal thread refers to metric trapezoidal thread with 30 ° profile angle. This series of thread is relatively unified at home and abroad, and its code number is also quite consistent. See the following table for the code number of thread
Country code
China t (old) tr (New)
ISO Tr
TR, Germany
Former Soviet tr
Thread classification
According to the different uses of thread, it can be divided into:
International metric thread system
The thread used in China’s national standard CNS. The top of the tooth is flat and easy to turn, while the bottom of the tooth is circular to increase the strength of the thread. The thread angle is 60 degrees and the specification is indicated by M. Metric thread can be divided into coarse thread and fine thread. The expression is as follows: m8x1.25. (M: code, 8: nominal diameter, 1.25: pitch).
American Standard thread
Both the top and the root of the thread are plane, so the strength is better. The thread angle is also 60 degrees, and the specification is expressed by the number of teeth per inch. This kind of thread can be divided into NC, NF and Nef. Such as 1 / 2-10nc. (1 / 2: outer diameter; 10: teeth per inch; NC code).
Unified thread
Developed by the United States, Britain and Canada, it is the commonly used British screw thread.
The thread angle is also 60 degrees, and the specification is expressed by the number of teeth per inch. This kind of thread can be divided into coarse thread (UNC), fine thread (UNF) and ultra fine thread (UNEF). Such as 1 / 2-10unc. (1 / 2: outer diameter; 10: teeth per inch; UNC code)
V-thread (sharp Vthread)
Both the top and the root are pointed, with weak strength and are not often used. The thread angle is 60 degrees.
Whitworth Thread
British national standard thread. The thread angle is 55 degrees and the symbol is “W”. It is suitable for rolling manufacturing. The expression is as W1 / 2-10. (1 / 2: outer diameter; 10: teeth per inch; W code).
Knucklethread
It is the standard thread of DIN. Suitable for the connection of light bulb and rubber tube. The symbol is “Rd”.
Pipe thread
To prevent leakage, the thread is often used for connecting 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 inclined pipe thread code “n.p.t.” with a taper of 1:16, i.e. 3 / 4 inch per foot.
Square thread
The high transmission efficiency is only inferior to that of the ball screw thread, and it can not be adjusted with the nut after wear. Generally used for screw of vice and screw of crane.
Trapezoidal thread
Also known as EKM thread. The transmission efficiency is slightly smaller than that of square thread, but it can be adjusted by nut after wear. The thread angle is 30 degrees in metric system and 29 degrees in English system. It is generally used for the guide screw of lathe. The symbol is “tr”.
Buttress thread
It is also called bevel thread. It is only suitable for single direction transmission. Such as screw jack, pressure machine, etc. The symbol is “bu”.
Ball screw
As the thread with the best transmission efficiency, it is difficult to manufacture and the cost is very high, so it is used in precision machinery. Such as CNC tool machine guide screw.
Representation of British bolts:
LH 2N 5/8 × 3 – 13UNC-2A
(1) LH is the left thread (RH is the right thread, which can be omitted).
(2) 2n double thread.
(3) 5 / 8 inch thread, 5 / 8 “OD.
(4) 3 bolt length 3 “.
(5) 13 threads, 13 teeth per inch.
(6) UNC unified standard thread coarse thread.
(7) Grade 2 fit, external thread (3: tight fit; 2: medium fit; 1: loose fit) a: external thread (can be omitted) B: internal thread
Inch thread:
The size of inch thread is usually expressed by the number of threads per inch of thread length, referred to as “teeth per inch”, which is exactly equal to the reciprocal of pitch. For example, the pitch of a thread with 8 teeth per inch is 1 / 8 inch.
UKO products include a variety of thread cutting tools, including thread inserts and taps.