Measure system of Fastener

There are two main units system of length measurement in the world today, one is the metric system, and the unit of measurement is meters (m), centimeters (cm), millimeters (mm), etc. The species is the British system, and the unit of measurement is mainly inches, which is equivalent to the market inch of the old system in my country. It is widely used in the United States, Britain and other European and American countries.

  1. Metric measurement: (decimal system)


2. Imperial measurement: (8 system)

1 inch = 8 cents 1 inch = 25.4 mm 3/8,,×25.4 =9.52

3, 1/4,, The following products use the number to indicate their name diameter, such as:

4#, 5#, 6#, 7#, 8#, 10#, 12# thread

  1. Thread is a shape with uniform helical protrusions on the cross section of the solid outer surface or inner surface. According to its structural characteristics and uses, it can be divided into three categories: (1) Ordinary thread: the tooth shape is triangular, which is used to connect or fasten parts. Ordinary threads are divided into two types: coarse thread and fine thread according to the pitch, and the connection strength of fine thread is higher.

(2) Transmission thread: There are trapezoidal, rectangular, saw-shaped and triangular tooth shapes.

(3) Sealing thread: used for sealing connection, mainly pipe thread, tapered thread and tapered pipe thread.

2. Thread matching grade:

The thread fit is the loose or tight size between the screwed threads, and the level of fit is the specified combination of deviations and tolerances acting on the internal and external threads.

(1) For unified inch threads, there are three thread grades for external threads: 1A, 2A and 3A, and three grades for internal threads:

Grades 1B, 2B and 3B are all clearance fits. The higher the rating number, the tighter the fit. In inch threads, the deviation is only specified for grades 1A and 2A, the deviation of grade 3A is zero, and the grade deviation of grades 1A and 2A is equal.

The larger the number of levels, the smaller the tolerance.

Classes 1, 1A and 1B, very loose tolerance classes, which are suitable for tolerance fits of internal and external threads.

Grades 2, 2A and 2B are the most common thread tolerance grades specified for inch series mechanical fasteners.

Grades 3, 3A and 3B, screwed together to form the tightest fit, suitable for fasteners with tight tolerances, for safety-critical designs.

4. For external threads, 1A and 2A grades have a matching tolerance, 3A grades do not. The 1A tolerance is 50% larger than the 2A tolerance and 75% larger than the 3A tolerance. For the internal thread, the 2B tolerance is 30% larger than the 2A tolerance. Class 1B is 50% larger than class 2B and 75% larger than class 3B.

(2) Metric threads, there are three thread grades for external threads: 4h, 6h and 6g, and three thread grades for internal threads: 5H, 6H, 7H. (The Japanese standard thread accuracy grade is divided into three grades: I, II, and III, and is usually grade II.) In the metric thread, 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. as the picture shows:

  1. H is a common tolerance zone position for internal threads. Generally, it is not used as a surface coating, or a very thin phosphating layer is used. The basic deviation of G position is used for special occasions, such as thicker coating, which is rarely used.
  2. g is often used to coat a thin coating of 6-9um. If the product drawing requires a 6h bolt, the thread before plating uses a 6g tolerance zone.
  3. It is best to combine the thread fit into H/g, H/h or G/h. For bolts, nuts and other refined fastener threads, the standard recommends 6H/6g fit (3), thread marking
  4. Main geometric parameters of self-tapping and self-drilling threads: (1) Major diameter/tooth outer diameter (d1), which is the imaginary cylindrical diameter of the coincidence of the thread crests. thread

The major diameter basically represents the nominal diameter of the thread size.

(2), Minor diameter/root diameter (d2): It is the imaginary cylindrical diameter of the coincidence of the thread root.

(3), tooth distance (p): the axial distance between adjacent teeth corresponding to two points on the mid-longitude line. in the UK

per inch (25.4

mm) to indicate the tooth distance.

The following table lists the common specifications of the pitch (metric) and the number of teeth (imperial)

  1. Metric self-tapping:

Specifications S T 1.5 S T

1.9 ST

2.2 ST

2.6 ST

2.9 ST

3.3 ST

3.5 ST

3.9 ST

4.2 ST

4.8 ST

5.5 ST

6.3 ST

8.0 S T


Pitch 0.5 0.6 0.8 0.9 1.1 1.3 1.3 1.3 1.4 1.6 1.8 1.8 2.1 2.1

2. Inch self-tapping:

Specifications 4# 5# 6# 7# 8# 10# 12# 14# Teeth

Number AB teeth 24 20 20 19 18 16 14 14 A teeth 24 20 18 16 15 12 11 10

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