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The basic knowledge of inch thread

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The inch thread is the size of the thread marked in the inch system. According to the shape, it is divided into two types: cylindrical and conical; according to the tooth angle, it is divided into two types: 55° and 60°. The 1/4, 1/2, and 1/8 marks in the thread refer to the diameter of the thread size, and the unit is inches. One inch equals 8 points, 1/4 inch equals 2 points, and so on.


1. Strictly speaking, UN, UNR, UNC, UNF, UNS and UNEF are the prescribed codes for thread series, not the abbreviations of certain English phrases:

Unified Screw Threads: Unified Screw Threads (jointly developed by the United Kingdom, Canada and the United States in 948;

UN: A flat root contour is specified, but it is necessary to provide for some threading tool crest wear, hence a rounded root contour cleared beyond the 0.25p flat width of the Basic Profile is optional.)

UNR: In order to reduce the rate of threading tool crest wear and to improve fatigue strength of a flat root thread, the Design Profile of the UNR thread has a non-reversing continuous curved root tangent to the thread flanks at a depth of 0.625 H.)

  UNC: Coarse-Thread Series;

  UNF: Fine-Thread Series;

  UNEF: Extra-Fine-Thread Series;

  UNS: Special pitch series (Selected Combinations and Other Threads of Special Diameters, Pitches, and Lengths of Engagement);

  There are also Constant Pitch Series, 8UN, 12UN, 16UN and so on.


2. Marking method:

The inch system uniform thread is widely used in inch system countries. This type of thread is divided into three series: coarse thread series UNC, fine thread series UNF, extra fine thread series UNFF, plus a fixed pitch series UN.

Marking method: thread diameter—number of teeth per inch series code—precision grade


3. Thread fit level:

Thread fit is 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.

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

  • 1. Class 1A and 1B, very loose tolerance class, which is suitable for the tolerance fit of internal and external threads.

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

  • 3, 3A and 3B grades, screwing to form the tightest fit, suitable for fasteners with tight tolerances, for the key design of safety.

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

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