Inch uniform thread is widely used in inch countries. This type of thread is divided into three series: coarse tooth series UNC, fine tooth series UNF, extra fine tooth series UNFF, plus a fixed pitch series UN.
Labeling method: Thread diameter - number of teeth per inch Series code - accuracy level
Example: Coarse tooth series 3/8-16 UNC-2A
Fine tooth series 3/8-24 UNF-2A
Extra fine tooth series 3/8—32 UNFF—2A
Fixed pitch series 3/8-20 UN-2A
The first digit 3/8 indicates the outer diameter of the thread, in inches, converted to metric unit mm multiplied by 25.4, ie 3/8×25.4=9.525mm; the second and third digits 16, 24, 32, 20 are The number of teeth per inch (the number of teeth in the length of 25.4mm); the character code UNC, UNF, UNFF, UN after the third digit is the serial number, and the last two digits 2A are the precision grades.
Thread matching level:
The threaded fit is the loose or tight size between the threaded threads, and the level of fit is the specified combination of deviations and tolerances on the inner and outer threads.
1. For uniform inch thread, the external thread has three thread grades: 1A, 2A and 3A, and the internal thread has three grades:
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 specifies the 1A and 2A levels, the 3A level deviation is zero, and the 1A and 2A level deviations are equal.
1. 1A and 1B grade, very loose tolerance class, which is suitable for tolerance fit of internal and external threads.
2. 2A and 2B, is the most common thread tolerance class specified by the British series of mechanical fasteners.
3. 3A and 3B, screwing to form the tightest fit, suitable for tight tolerance fasteners, for critical design of safety.
4. For external threads, 1A and 2A have a tolerance, and 3A does not. The 1A tolerance is 50% larger than the 2A tolerance and 75% larger than the 3A. For the internal thread, the 2B tolerance is 30% larger than the 2A tolerance. Level 1B is 50% larger than 2B and 75% larger than 3B.