May I ask how fine the thread can be called fine thread? We might as well define it like this. 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 teeth per inch is different, that is, the pitch is different. The pitch of the tooth is large, while the pitch of the fine tooth is small. That is to say, for the two specifications of 1/2-13 and 1/2-20, the former is coarse and the latter is fine. Therefore 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. Without special instructions, the stainless steel screws and other fasteners we generally buy are all coarse threads. Coarse thread is characterized by high strength, good interchangeability, and there are standards for comparison. Generally speaking, coarse thread should be the best choice; compared with fine thread, due to the large pitch, the thread lead angle is also large. The self-locking property is poor, and anti-loosening washers, self-locking devices, etc. need to be installed in vibration environments; the advantage is that it is easy to disassemble and assemble, and the matching standard parts are complete and easy to interchange; when marking coarse threads, it is not necessary to mark the pitch, such as M8, M12-6H, M16-7H, etc., are mainly used for connecting threads.
fine thread
The fine thread and the coarse thread are just the opposite, and are stipulated to supplement the special requirements that the coarse thread cannot meet. The fine thread also has a pitch series, and the pitch of the fine thread is smaller, so its characteristic is that 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 precise occasions, fine-toothed stainless steel screws are more convenient for precise control and adjustment.
The disadvantage is that the tensile value and strength are lower than those of coarse teeth, and the thread is easy to be damaged. It is not recommended to disassemble and assemble it many times. The fasteners such as nuts and other supporting parts may be equally accurate, and the size is slightly wrong, which is easy to damage the screw and nut at the same time. Mainly used in metric pipe fittings of hydraulic systems, mechanical transmission parts, thin-walled parts with insufficient strength, internal parts limited by space and shafts with high self-locking requirements, etc. The thread pitch must be marked when marking the fine thread to show the difference from the coarse thread.
How to choose coarse thread and fine thread
Coarse-thread and fine-thread screws are used for fastening purposes, and fine-thread screws are generally used to lock thin-walled parts and parts that require high vibration resistance. The self-locking performance of the fine thread is good, so the ability of anti-vibration and anti-loosening is strong, but due to the shallow thread depth, the ability to withstand greater pulling force is worse than that of the coarse thread. When anti-loosening measures are not taken, the anti-loosening effect of fine thread is better than that of coarse thread, and it is generally used for thin-walled parts and parts with relatively high 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 thick tissue and poor strength. When the tightening force is too large, it is easy to slip.
