Uniform Inch Thread

Unified British threads include coarse (Unified National Coarse, UNC), Fine teeth (Unified National Fine, UNF), Extremely Fine Teeth (Unified National Extra-Fine, UNEF), and Diamond teeth 4UN, 6UN, 8UN, 12UN, 16UN, 20UN, 28UN, 32UN. The mountain shape is an acute angle, and the angle of the mountain is 60°. The unified representation method of inch threads is not to directly indicate the "pitch" like metric threads, but to express "the number of threads per inch".

Thread representation

[Name diameter] - [Number of teeth per inch] [Tooth shape]

Example

¼ - 20 UNC

⁵⁄₈ - 12 UN

Thread Specifications

Thread Angle: 60° (Unified Inch)
Standard: ANSI/ASME B1.1
Series: UNC, UNF, UNEF, 4UN-32UN
Application: General purpose fasteners

ANSI / ASME B1.1 Unified Inch Thread Series

Unified Thread Specifications

Unit mm

Name of Thread	Threads per Inch	Pitch P	Major Diameter D, d	Median Diameter D₂, d₂	Minor Diameter D₁, d₁

FAQ

What is the difference between UNC, UNF, and UNEF threads?
UNC (Unified National Coarse) has the largest pitch for quick assembly, UNF (Unified National Fine) has smaller pitch for better holding power, and UNEF (Unified National Extra Fine) has the smallest pitch for maximum precision and vibration resistance.
How do I read unified thread designations like "¼-20 UNC"?
The first number (¼) is the major diameter in inches, "20" is the threads per inch (TPI), and "UNC" indicates the thread series. This tells you the size, pitch, and type in the American unified system.
What do the constant pitch series (4UN, 6UN, 8UN, etc.) mean?
These are constant pitch series where the number indicates threads per inch regardless of diameter. For example, 8UN means 8 threads per inch for all diameters in that series, providing consistent pitch across different sizes.
Why do inch threads use TPI instead of pitch like metric threads?
TPI (threads per inch) is traditional in the imperial system and directly relates to the threading tools and taps used. It's the inverse of pitch: TPI = 1/pitch(inches). This system evolved from historical manufacturing practices in inch-based countries.
When should I use fine threads versus coarse threads?
Use coarse threads (UNC) for general applications, quick assembly, and when thread strength is not critical. Use fine threads (UNF/UNEF) for precision applications, better holding power, thinner wall sections, and where vibration resistance is important.
Are unified threads compatible with metric threads?
No, unified threads are not compatible with metric threads. They have different thread angles (both 60°), different pitch systems (TPI vs mm), and different diameter standards. Always verify thread type before assembly to prevent damage.