Threads are something that mechanical people deal with every day. After a long time, they come across domestic and foreign threads, metric and imperial threads, straight and tapered threads, sealed and non-sealed threads, internal and external threads, 55 degrees and 60 degrees threads. In short, they are often confused. I check them from beginning to end every time I use them. I will summarize them here and hope they will be helpful.
My practice is to print out the commonly used threads and put them on the side of the table. I will check them whenever I need them. After a long time, I will naturally remember them.
NPT is a general-purpose American standard tapered pipe thread with a tooth angle of 60°. PT threads are imperial tapered threads with a tooth angle of 55°, which are most commonly used in sealing. Imperial pipe threads are fine threads, because the coarse thread has a large tooth depth, which will seriously reduce the strength of the outer diameter pipe of the thread. PF threads are parallel threads for pipes. G is a 55-degree non-threaded sealed pipe thread, which belongs to the Whitworth thread family. The mark G represents cylindrical thread. G is the general name of 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 from a conical surface. General water pipe joints are like this. The old national standard is marked as Rc. Metric thread is expressed by pitch, and American and British threads are expressed by the number of threads per inch. This is their biggest difference. Metric thread is 60-degree equilateral tooth type, British thread is isosceles 55-degree tooth type, and American thread is 60 degrees. Metric thread uses metric units, and American and British threads use imperial units. Pipe threads are mainly used for pipe connection. The internal and external threads are tightly matched. There are two types: straight pipes and tapered pipes. The nominal diameter refers to the diameter of the connected pipe. Obviously, the major diameter of the thread is larger than the nominal diameter. 1/4, 1/2, and 1/8 are the nominal diameters of British threads, in inches. 1. Inch unified thread is widely used in inch-based 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 threads per inch series code-precision grade Example: coarse thread series 3/8-16UNC-2A fine thread series 3/8-24UNF-2A extra fine thread series 3/8-32UNFF-2A fixed pitch series 3/8-20UN-2A The first digit 3/8 represents the outer diameter of the thread in inches. To convert to metric units, multiply by 25.4, that is, 3/8×25.4=9.525mm; the second and third digits 16, 24, 32, 20 are the number of threads per inch (the number of threads on a length of 25.4mm); the text codes UNC, UNF, UNFF, and UN after the third digit are the series codes, and the last two digits 2A are the precision grade. 2. Conversion of 55° cylindrical pipe thread 55° cylindrical pipe thread, originated from the inch series, but is widely used in both metric and inch countries. It is used to connect pipe joints and pipes for transporting liquids, gases and installing wires. However, the codes of different countries are different. They should be converted into Chinese codes according to the foreign codes in the table below (comparison table). The 55° cylindrical pipe thread codes of various countries are listed in the table below. Country Code China G Japan G, PF United Kingdom BSP, BSPP France G Germany R (internal thread), K (external thread) Former Soviet Union G, TPУБISO Rp3. Conversion of 55° tapered pipe thread 55° tapered pipe thread refers to a thread with a tooth angle of 55° and a taper of 1:16. This series of threads is widely used in the world. Its codes are different in different countries. See the table below. Convert the foreign codes in the table below into Chinese codes. Country Code China ZG, R (external thread) United Kingdom BSPT, R (external thread), Rc (internal thread) France G (external thread), R (external thread) Germany R (external thread) Japan PT, RISO R (external thread), Rc (internal thread) 4. Conversion of 60° tapered pipe thread 60° tapered pipe thread refers to a pipe thread with a tooth angle of 60° and a thread taper of 1:16. This series of threads is used in my country's machine tool industry and the United States and the former Soviet Union. Its code was previously specified as K in my country, then as Z, and now changed to NPT. The thread code comparison table is shown in the table below. Country Code China Z (old) NPT (new) United States NPT Soviet Union B5. Conversion of 55° trapezoidal thread Trapezoidal thread refers to a metric trapezoidal thread with a tooth angle of 30°. This series of threads is relatively unified at home and abroad, and its code is also quite consistent. The thread code is shown below. Country Code China T (old) Tr (new) ISO Tr Germany Tr Former Soviet Union Tr Thread classification According to the different uses of threads, it can be divided into: 1. International Metric Thread System (International Metric Thread System) Threads adopted by my country's national standard CNS. The top of the thread is flat, easy to turn, and the bottom of the thread is arc-shaped to increase the strength of the thread. The thread angle is 60 degrees, and the specification is represented by M. Metric threads can be divided into coarse and fine pitch. The representation method is such as M8x1.25. (M: code, 8: nominal diameter, 1.25: pitch). 2. American Standard Thread (American Standard Thread) The top and root of the thread are both flat, and the strength is better. The thread angle is also 60 degrees, and the specification is represented by the number of teeth per inch. This type of thread can be divided into three levels: coarse pitch (NC); fine pitch (NF); extra fine pitch (NEF). The representation is as follows: 1/2-10NC. (1/2: outer diameter; 10: number of threads per inch; NC code). 3. Unified Thread is jointly formulated by the United States, the United Kingdom, and Canada, and is the most commonly used British thread. The thread angle is also 60 degrees, and the specification is expressed in the number of threads per inch. This type of thread can be divided into coarse thread (UNC); fine thread (UNF); extra fine thread (UNEF). The representation is as follows: 1/2-10UNC. (1/2: outer diameter; 10: number of threads per inch; UNC code) 4. V-shaped thread (Sharp VThread) The top and root are both pointed, the strength is weak, and it is also not often used. The thread angle is 60 degrees. 5. Whitworth Thread is the thread adopted by the British national standard. The thread angle is 55 degrees, and the symbol is "W". It is suitable for rolling manufacturing. The representation is as follows: W1/2-10. (1/2: outer diameter; 10: number of threads per inch; W code). 6. Knuckle Thread is a standard thread set by German DIN. It is suitable for connecting light bulbs and rubber hoses. The symbol is "Rd". 7. Pipe Thread is a thread used to prevent leakage and is often used to connect gas or liquid pipes. The thread angle is 55 degrees. It can be divided into straight pipe threads with codes "P.S., N.P.S." and oblique pipe threads with code "N.P.T.". Its taper is 1:16, that is, 3/4 inch per foot. 8. Square Thread has a high transmission efficiency, second only to ball thread, but it cannot be adjusted with a nut after wear, which is its disadvantage. It is generally used for vise screws and crane threads. 9. Trapezoidal Thread is also called Acme Thread. The transmission efficiency is slightly lower than that of square thread, but it can be adjusted with a nut after wear. The metric thread angle is 30 degrees and the imperial thread angle is 29 degrees. It is generally used for the lead screw of a lathe. The symbol is "Tr". 10. Buttress thread, also known as bevel thread, is only suitable for single-direction transmission. Such as screw jacks, presses, etc. The symbol is "Bu". 11. Ball thread is the thread with the best transmission efficiency. It is difficult to manufacture and has a very high cost. It is used in precision machinery. Such as the lead screw of a CNC machine tool. The representation of imperial bolts: LH 2N 5/8 × 3 - 13UNC-2A (1) LH is a left-hand thread (RH is a right-hand thread, which can be omitted). (2) 2N double-thread thread. (3) 5/8 imperial thread, outer diameter 5/8". (4) 3 bolt length 3". (5) 13 thread number of teeth per inch 13 teeth. (6) UNC unified standard thread coarse pitch. (7) 2-stage fit, external thread (3: tight fit; 2: medium fit; 1: loose fit) A: external thread (can be omitted) B: internal thread Imperial thread: The size of an imperial thread is usually expressed by the number of threads per inch of thread length, referred to as "threads per inch", which is exactly the reciprocal of the pitch. For example, a thread with 8 threads per inch has a pitch of 1/8 inch.
