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Detailed Description of Inch Thread

 
1. Strictly speaking, UN, UNR, UNC, UNF, UNS and UNEF are the prescribed codes of the thread series, not abbreviations of certain English phrases:
 
Unified thread: Unified Screw Threads (co-developed by the United Kingdom, Canada and the United States in 1948);
 
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: The root of the external thread design tooth shape must be round (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 (Constant Pitch Series), there are 8UN, 12UN, 16UN, etc., not listed one by one
 
2. Annotation method:
 
Inch uniform threads are widely used in inch countries. These threads are divided into three series: coarse tooth series UNC, fine tooth series UNF, extra fine tooth series UNFF, plus a fixed pitch series UN.
 
Marking method: thread diameter—number of teeth per inch series code—accuracy class
Example: Coarse teeth series 3/8—16 UNC—2A
Fine Teeth Series 3/8—24 UNF—2A
Extra fine teeth series 3/8—32 UNFF—2A
Fixed pitch series 3/8—20 UN—2A
The first digit 3/8 represents the outer diameter of the thread, the unit is inch, converted to the metric unit mm multiplied by 25.4, that is, 3/8 × 25.4 = 9.525mm; the second and third digits 16, 24, 32, 20 are Number of teeth per inch (number of teeth on 25.4mm length); the character codes UNC, UNF, UNFF, UN after the third digit are series codes, and the last two digits 2A are accuracy grades.
 
Thread fit level:
 
The thread fit is the loose or tight size between the screw threads, and the fit level is the prescribed combination of deviation and tolerance on the internal and external threads.
 
1. For uniform inch threads, there are three thread grades for external threads: 1A, 2A and 3A, and three grades for internal threads:
 
Levels 1B, 2B and 3B are all clearance fits. The higher the level number, the tighter the fit. In the inch thread, the deviation only stipulates the 1A and 2A grades, the deviation of the 3A grade is zero, and the grade deviations of the 1A and 2A grades are equal.
 
1. Class 1A and 1B, very loose tolerance class, which is suitable for tolerance matching of internal and external threads.
2. Grades 2A and 2B are the most common thread tolerance grades specified by the British series of mechanical fasteners.
3. Grades 3A and 3B, screwed together to form the tightest fit, suitable for fasteners with tight tolerances, used for critical design of safety.
4. For external threads, there is a fit tolerance for grades 1A and 2A, but not for grade 3A. Class 1A tolerances are 50% larger than Class 2A tolerances and 75% larger than Class 3A. For internal threads, Class 2B tolerances are 30% larger than 2A tolerances. Grade 1B is 50% larger than Grade 2B and 75% larger than Grade 3B.
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