The British UNF thread series is introduced as follows!

Thread Content

] I. Strictly speaking, UN, UNR, UNC, UNF, UNS, and UNEF are the designated codes for screw thread series, rather than abbreviations of certain English phrases: Unified Screw Threads: These were jointly established in 1948 by the United Kingdom, Canada, and the United States； 　　UN: For external threads, the root contour can be flat or rounded. A flat root contour is specified, but it is necessary to account for some wear on the cutting edge of the threading tool; therefore, it is optional to use a rounded root contour that extends beyond the 0.25p flat width of the basic profile.
UNR: For external threads, the root contour must be rounded. In order to reduce wear on the cutting edge of the threading tool and to enhance the fatigue resistance of threads with a flat root, the design profile of UNR threads features a continuous curved root that is 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 is also the Constant Pitch Series, which includes 8UN, 12UN, 16UN, and so on; all of them cannot be listed here one by one. II. Marking method: Unified threads in the inch system are widely used in this system. These threads are divided into three series: the coarse-thread series UNC, the fine-thread series UNF, the extra-fine-thread series UNFF, plus a constant-pitch series UN. 　　Notation method: Thread diameter—TPI Series code—Precision grade. Example: Coarse thread series 3/8—16 UNC—2A; Fine thread series 3/8—24 UNF—2A; Extra-fine thread series 3/8—32 UNFF—2A; Constant-pitch series 3/8—20 UN—2A. The first digit, 3/8, indicates the outer diameter of the thread in inches; to convert it to millimeters, multiply by 25.4, i.e., 3/8×25.4=9.525 mm ； The second and third digits, 16, 24, 32, 20, represent the number of teeth per inch (the number of teeth over a length of 25.4 mm) ； The code letters after the third one, UNC, UNF, UNFF, and UN, are series codes, while the last two digits, 2A, indicate the precision grade. 　　 III. Thread fit grade: The thread fit refers to the degree of looseness or tightness between mating threads, and the fit grade is a specified combination of tolerances and deviations applied to the internal and external threads. 　　For unified British thread standards, there are three thread grades for external threads: 1A, 2A, and 3A; there are three grades for internal threads: 1B, 2B, and 3B. All of these are clearance fits. The higher the grade number, the tighter the fit. In imperial threads, tolerances are specified only for grades 1A and 2A; the tolerance for grade 3A is zero, and the grade tolerances for 1A and 2A are equal. 　　1, 1A, and 1B grades are very loose tolerance grades, suitable for the tolerance fits of internal and external threads. 　　Grades 2, 2A, and 2B are the most commonly specified thread tolerance grades in the British series of mechanical fasteners. 　　Levels 3, 3A, and 3B: they achieve the tightest fit through threading, and are suitable for fasteners with tight tolerances, used in critical designs where safety is important. 　　4. For external threads, grades 1A and 2A have a fit tolerance, while grade 3A does not. The tolerance of grade 1A is 50% greater than that of grade 2A, and 75% greater than that of grade 3A. For internal threads, the tolerance of grade 2B is 30% greater than that of grade 2A. Grade 1B is 50% larger than Grade 2B, and 75% larger than Grade 3B.