Metric Thread

Metric threads are one of the most widely used types of threads globally. They are standardized by the International Organization for Standardization (ISO)¹ and are commonly used in many industries, from automotive to electronics. Metric threads are measured in millimeters, with specifications defined by the diameter and pitch (the distance between threads).

Key Types of Metric Threads:

1. ISO Metric Thread: The standard used internationally for screws, bolts, and other fasteners. It comes in both coarse and fine variations.
   • Coarse Thread: For example, M6×1, where M6 means a nominal diameter of 6 mm and 1 mm is the pitch (distance between threads).
   • Fine Thread: For example, M6×0.75, where M6 means the diameter is 6 mm, and the pitch is finer at 0.75 mm.
2. JIS Metric Thread²: The Japanese Industrial Standard (JIS) is used in Japan and regions influenced by it. Common specifications include M4, M6, M8, M10, etc., which are frequently used for industrial applications in Japan.

Examples:

• M8×1.25: This means the screw has a nominal diameter of 8 mm, and the thread pitch is 1.25 mm.

Imperial Thread

Imperial threads, commonly used in the United States³ and some Commonwealth countries⁴, are based on inches and threads per inch (TPI). There are several standards for imperial threads, including the Unified Thread Standard (UTS)⁵ and British Standards⁶.

Key Types of Imperial Threads:

1. Unified Thread Standard (UTS): This standard includes:
   • UNC (Unified Coarse Thread): Used for general-purpose applications. For example, 1/4-20 UNC means a nominal diameter of 1/4 inch and 20 threads per inch.
   • UNF (Unified Fine Thread): A finer thread used for more precision. For example, 1/4-28 UNF means a nominal diameter of 1/4 inch and 28 threads per inch.
2. British Standard Whitworth (BSW)⁶: This standard was developed by the British Standards Association and is mainly used for older mechanical equipment. For example, 1/4 BSW means a nominal diameter of 1/4 inch.
3. British Standard Fine (BSF)⁶: A finer thread compared to BSW, used for precision machinery.

Examples:

• 1/4-20 UNC: The nominal diameter is 1/4 inch with 20 threads per inch.
• 3/8-16: The nominal diameter is 3/8 inch, and it has 16 threads per inch.

Summary: Choosing the Right Thread for Your Project

Whether you are working on an international project requiring ISO standards or dealing with older mechanical systems that use British or American thread standards, understanding these key thread specifications is essential to choosing the correct screw or fastener.

• Metric threads are measured in millimeters and are used worldwide, with ISO and JIS standards being the most common.
• Imperial threads are based on inches and are used primarily in the United States and some Commonwealth countries, with standards such as UNC, UNF, BSW, and BSF.

By knowing the diameter and pitch of your required threads, you can confidently select the correct fasteners for any application, ensuring both precision and reliability.

Footnote

1. International Organization for Standardization (ISO): The ISO is an international body that defines worldwide standards for various industries, including thread specifications.
2. Japanese Industrial Standard (JIS): The JIS is the standard used in Japan for a variety of manufacturing processes, including threading specifications.
3. United States: The U.S. uses its own set of standards for screws and fasteners, most notably the Unified Thread Standard (UTS), which is based on inches.
4. Commonwealth countries: Commonwealth countries like Canada, Australia, and India, while largely using metric threads today, still rely on some imperial thread specifications for certain industries.
5. Unified Thread Standard (UTS): The UTS is a system of thread standards used primarily in the U.S., which includes both coarse and fine thread variations (UNC, UNF).
6. British Standards: These include older thread standards like BSW and BSF, often used in mechanical equipment, particularly in the UK and countries influenced by British engineering traditions.