Conversion Method Between Metric and Unified Inch Threads

A thread is a spiral structure used to connect bolts and nuts. In different countries and regions, different thread standards are used. Imperial and metric are the two more common standards.
In the British standard, the common thread specifications are UNC and UNF. UNC is the abbreviation of Unified National Coarse, which refers to the unified national coarse thread; UNF is the abbreviation of Unified National Fine, which refers to the unified national fine thread. For example, UNC 1/4-20 means a unified national coarse thread with a pitch of 20 TPI (threads per inch) and a diameter of 1/4 inch.

Inch system unified threads are 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 threads 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 represents the outer diameter of the thread in inches. To convert to the metric unit mm, multiply by 25.4, that is, 3/8×25.4=9.525mm; the second and third digits 16, 24, 32, and 20 are The number of teeth per inch (the number of teeth on a length of 25.4mm); the text codes after the third digit, UNC, UNF, UNFF, UN, are the series codes, and the last two digits, 2А, are the accuracy level.

It is stipulated in the unified thread standard that a nominal size can have different pitches. For example, 1/2 has 13 teeth, 16 teeth, 20 teeth, and 32 teeth. The standard stipulates that 13 teeth are coarse teeth and 20 teeth are fine teeth. 28 teeth are ultra-fine teeth; 16 teeth and 32 teeth are both called constant pitch, which is no different from coarse teeth and fine teeth.