May I ask how fine a thread can be called a fine thread? We might as well define it in this way. The so-called coarse thread can be defined as a standard thread; while the fine thread is relative to the coarse thread. Under the same nominal diameter, the number of threads per inch is different, that is, the pitch is different. The pitch of the teeth is large, and the pitch of the fine teeth is small. In other words, for the 1/2-13 and 1/2-20 specifications, the former is coarse and the latter is fine. Therefore, it is expressed as 1/2-13UNC and 1/2-20UNF.
Since there are two different thread specifications for the same nominal diameter, how to choose coarse and fine threads?
Coarse thread
The so-called coarse thread actually refers to the standard thread. Unless otherwise specified, the stainless steel screws and other fasteners we generally buy are all coarse threads.
Coarse threads are characterized by high strength, good interchangeability, and standards for comparison. Generally speaking, coarse threads should be the best choice;
Compared with fine thread, because the pitch is large, the thread rise angle is also large, and the self-locking performance is poor. It is necessary to install anti-loosening washers and self-locking devices in a vibration environment; the advantage is that it is convenient to disassemble and assemble, and the supporting standard parts are complete. , Easily interchangeable;
Coarse threads do not need to be marked with pitch, such as M8, M12-6H, M16-7H, etc., which are mainly used for connecting threads.
Fine thread
Fine thread and coarse thread are just the opposite. They are specified to supplement the special use requirements that coarse thread cannot meet. There are also pitch series for fine thread. The pitch of fine thread is smaller, so it is more conducive to self-locking and anti-loosening. And the number of teeth is more, which can reduce the effect of leakage and achieve the effect of sealing. In some precision occasions, fine-tooth stainless steel screws are more convenient for precise control and adjustment.
The disadvantage is that the tensile value and strength are relatively low, and the thread is easy to damage. It is not recommended to disassemble and assemble multiple times. The matching nuts and other fasteners may be equally accurate, and the size is slightly wrong, which will easily damage the screws and nuts at the same time.
Fine thread is mainly used in metric pipe fittings of hydraulic systems, mechanical transmission parts, thin-walled parts with insufficient strength, internal parts restricted by space, and shafts with high self-locking requirements. When marking fine thread, the pitch must be marked to show the difference from coarse thread.
How to choose coarse and fine threads
Coarse thread and fine thread screws are used for fastening purposes.
Fine thread screws are generally used to lock thin-walled parts and parts with high requirements for vibration resistance. The fine thread has good self-locking performance, so it has strong anti-vibration and anti-loose ability. However, because the thread depth is shallow, the ability to withstand greater tension is worse than that of the coarse thread.
When no anti-loosening measures are taken, the anti-loosening effect of fine thread is better than that of coarse thread. It is generally used for thin-walled parts and parts with higher requirements for anti-vibration.
When used for adjustment, fine thread has more advantages. Disadvantages of fine thread: It is not suitable for application on materials with excessively coarse structure and poor strength. When the tightening force is too large, it is easy to slip.
