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Choice of coarse and fine thread

Choice of coarse and fine thread

What is a fine thread? What is a coarse thread? How to choose coarse and fine thread?

How fine a thread is called a fine thread? Let's define it this way. The so-called coarse thread can be defined as a standard thread; While the fine thread is relative to the coarse tooth, in the same nominal diameter, the number of teeth per inch is different, that is, the pitch is different, the coarse tooth pitch is large, and the fine tooth pitch is small. In other words, for both sizes 1/2 to 13 and 1/2 to 20, the former is coarse and the latter is fine. So it's 1/2 minus 13UNC and 1/2 minus 20UNF.

Since there are two different thread specifications for the same nominal diameter, how to choose the coarse thread and the fine thread?

Coarse thread

Definition: The so-called rough teeth are actually refers to the standard thread, in the absence of special instructions, we generally buy stainless steel screws and other fasteners are rough teeth.
The characteristics of coarse thread: it has high strength, good interchangeability, and the standard can be compared. Generally speaking, coarse thread should be the best choice.
Ratio with fine thread: due to the large pitch, the thread Angle is also large, self-locking is poor, in the vibration environment need to add anti-loose washer, self-locking device; The advantages are convenient disassembly and installation, complete standard parts, easy to exchange;
Note: When marking coarse thread, it is not necessary to mark the pitch, such as M8, M12-6h, M16-7h, etc., which are mainly used for connecting thread.

Fine thread

Definition: fine teeth and coarse teeth are on the contrary, in order to supplement the coarse teeth thread can not meet the special use requirements and specified, fine teeth thread also has pitch series, fine teeth pitch is smaller, so it is more conducive to self-locking, anti-loose, and the number of teeth, can reduce the effect of leakage, to achieve the effect of sealing. In some precision occasions, fine tooth stainless steel screw more convenient precise control, adjustment.
Disadvantages: the tensile value, strength is lower than the rough teeth, and the thread is easy to damage, it is not recommended to disassemble multiple times, the matching nut and other fasteners may be as accurate, the size of a little error, easy to screw nut damage.
Application: Fine thread is mainly used in the metric pipe fittings of hydraulic system, mechanical transmission parts, thin wall parts with insufficient strength, machine parts limited by space and shaft with higher self-locking requirements, etc. The pitch of fine thread must be marked to distinguish it from coarse thread.

How to choose coarse and fine thread?

Coarse thread and fine thread screw are used for fastening.
Fine tooth screw is generally used to lock thin wall parts and parts with high requirements for vibration proof. The self-locking performance of fine thread is good, so it has a strong anti-vibration and anti-loose ability. However, because of the shallow depth of the thread, the ability to bear greater tension is worse than that of coarse thread.
In the absence of anti-loosening measures, the anti-loosening effect of fine thread is better than that of coarse thread, which is generally used for thin-wall parts and parts with high requirements for anti-vibration.
When used for adjustment, fine thread has more advantages. Disadvantages of fine thread: it is not suitable for material with excessive thickness and poor strength. When the tightening force is too large, it is easy to slip.

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