A brief introduction to the thread of standard parts

The so-called bolt thread refers to a uniform helix convex shape on the outer surface or inner surface of the bolt shaft.

1. According to the structural characteristics and uses of the thread, it can be divided into three categories:

(1) Ordinary thread: this thread has a triangular profile and is used to connect or fasten parts. Ordinary threads are divided into coarse thread and fine thread according to the pitch, and the connection strength of fine thread is higher.

(2) Transmission thread: The thread shape of this thread includes trapezoid, rectangular, saw-shaped and triangular.

(3) Sealed thread: This kind of thread is mainly used for sealing connection, mainly pipe thread, taper thread and taper pipe thread.

2. Thread matching grade:

Thread fit refers to the size of looseness or tightness between screwed threads, and the level of fit is the specified combination of deviations and tolerances acting on internal and external threads.

(1) For unified inch threads, there are three thread grades for external threads: 1A, 2A and 3A, and there are three grades for internal threads:Class 1B, 2B and 3B are all clearance fits. The higher the level number, the tighter the fit. In the inch thread, the deviation is only specified for 1A and 2A grades, the deviation of 3A grade is zero, and the grade deviations of 1A and 2A are equal.

(2) Metric thread: There are three thread grades for external thread: 4h, 6h and 6g, and there are three thread grades for internal thread: 5H, 6H, 7H. (Japanese standard thread accuracy grades are divided into three grades: I, II, and III. Normally, it is grade II.) In metric threads, the basic deviation of H and h is zero. The basic deviation of G is positive, and the basic deviation of e, f, and g is negative.

1. H is the commonly used tolerance zone position for internal threads, and is generally not used as a surface coating, or a very thin phosphate layer is used. The basic deviation of G position is used in special occasions, such as thicker plating, which is generally rarely used.

2. g is commonly used to plate a thin coating of 6-9um. For example, the product drawing requires a 6h bolt, and the thread before plating adopts a tolerance zone of 6g.

3. The thread fit is best combined into H/g, H/h or G/h. For refined fastener threads such as bolts and nuts, the standard recommends using 6H/6g fit.