Inch thread details

I. Strictly speaking, UN, UNR, UNC, UNF, UNS, and UNEF are the code numbers for the thread series, not the abbreviation for some English phrases:
Unified thread: Unified Screw Threads (developed by United Kingdom, Canada, and 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 bypass 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 hollow curved root Tangent to the thread flanks at a depth of 0.625 H.)
UNC: Coarse-Thread Series (Coarse-Thread Series);
UNF: Fine-Thread Series (Fine-Thread Series);
UNEF: Extra-Fine-Thread Series;
UNS: Selected Combinations and Other Threads of Special Diameters, Pitches, and Lengths of Engagement;
There is also the Constant Pitch Series, 8UN, 12UN, 16UN, etc., which are not listed one by one.

Second, the labeling method:
The inch-in-one thread is widely used in inch-size countries. The threads are divided into three series: coarse teeth series UNC, fine teeth series UNF, special fine teeth series UNFF, plus a fixed pitch series UN.
Marking Method: Thread Diameter - Number of Teeth per inch Series Code - Accuracy Level Example: Coarse Series 3/8-16 UNC-2A
Fine Tooth Series 3/8-24 UNF-2A
Special 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 in inches, and the conversion to metric mm is multiplied by 25.4, ie 3/8×25.4 = 9.525 mm; the second and third digits 16, 24, 32, 20 are The number of teeth per inch (the number of teeth in 25.4 mm length); the third character after the code UNC, UNF, UNFF, UN series code, the last two 2A accuracy class.
Thread fit level:

Third, the thread fit is loose or tight size between the screw threads, the level of coordination is a combination of tolerances and tolerances on the internal and external threads.
(1) For the unified British thread, the external thread has three thread grades: 1A, 2A and 3A, and the internal thread has three grades:
Classes 1B, 2B and 3B are all clearance fits. The higher the rating number, the tighter the fit. In inch threads, the deviations are only specified in levels 1A and 2A, the deviation in levels 3A is zero, and the level deviations in levels 1A and 2A are equal.
Grades 1, 1A and 1B, very loose tolerance grades, apply to the tolerance fit of internal and external threads.
Classes 2, 2A and 2B are the most common thread tolerance classes specified for imperial series mechanical fasteners.
Grades 3, 3A, and 3B, which screw together to form the tightest fit, are suitable for fasteners with tight tolerances for critical safety designs.
4, external threads, 1A and 2A have a matching tolerance, 3A class does not. Class 1A tolerances are 50% larger than Class 2A tolerances and 75% larger than Class 3A. For internal threads, Class 2B tolerances are 30% greater than 2A tolerances. Class 1B is 50% larger than Class 2B and 75% larger than Class 3B.