Overview of American Standard Fastener System and Thread Representation Methods

The American Standard fastener system is a complex and comprehensive set of standards covering all aspects from design, manufacturing to inspection. These standards are developed by different organizations, each with its specific responsibilities and focuses. Below is a brief explanation of the main American Standard fastener organizations and their meanings, as well as the characteristics of the American Standard fastener system and thread representation methods.

01 American Standard Fastener-Related Standards Organizations and Their Meanings

• ANSI (American National Standards Institute) - Founded in 1918, ANSI is a private non-profit organization in the United States responsible for coordinating various industrial standards. It does not develop standards itself but accredits standards from other organizations.
• AISI (American Iron and Steel Institute) - Representing the American steel industry, AISI develops standards related to steel products, including fastener materials and performance requirements.
• ASME (American Society of Mechanical Engineers) - A professional organization for mechanical engineers, ASME develops standards covering pressure vessels, boilers, and other mechanical systems, including fastener applications.
• ASTM (American Society for Testing and Materials) - ASTM standards are published by ASTM International, one of the world's oldest international standards organizations, predating similar organizations such as ISO (International Organization for Standardization) and IEC (International Electrotechnical Commission). ASTM standards cover a wide range of fields, including but not limited to materials, construction, chemistry, electrical, mechanical, and environmental.
• IFI (Industrial Fasteners Institute) - The American Standard IFI is a series of standards and specifications in the fastener field developed by the Industrial Fasteners Institute. These standards cover the manufacturing, use, and inspection of fasteners such as bolts and nuts, including testing and quality control requirements.
• SAE (Society of Automotive Engineers) - It has developed a series of standards related to automotive engineering, widely used in automotive design, manufacturing, testing, and maintenance, including but not limited to engines, drivetrains, materials, fuels, emission control, and vehicle safety. SAE standards aim to ensure the performance, reliability, and safety of automotive products, while promoting technological innovation and industry collaboration.

Characteristics of the American Standard Fastener System

The American Standard fastener system is characterized by diversity and meticulousness. It includes two independent standard systems: metric and inch, covering different precision grades from rough bolts to precision bolts. The American standards seemingly free and casual, are actually detailed and rigorous, focusing on practicality and operability.

02 American Standard Fastener-Related Standards

American Standard fasteners are widely used in various industrial fields, including automotive, aerospace, construction, and mechanical manufacturing.

Below are some American Standard fastener-related standards:

• ASME B1.1-2003(R2008) Unified Inch Thread (UN and UNR Thread Forms)
• ASME B1.2-1983(R2007) Gages and Measurements for Unified Inch Threads
• ASME B1.3-2007 Thread Measurement Methods for Inch and Metric Threads (UN, UNR, UNJ, M, and MJ) for Conformity
• IFI-166-2007 Acceptance Procedures for Damaged Threads
• IFI-301-2008 Calibration Procedures for Thread Gages

03 American Standard Fastener Thread Representation Methods

Unified Thread Form

The unified thread is the common thread for American inch-sized fasteners.

• a. Thread major diameter d is the nominal diameter;
• b. Thread minor diameter d1 = d - 2(5H/8);
• c. Thread pitch diameter d2 = d - 2(3H/8);
• d. H = 0.866P;

Where: P — pitch (P = 25.4/n mm or P = 1/n in; n — number of threads per inch)

Elements of American Standard Fastener Thread Representation

The representation method of American standard fastener threads usually includes the following elements:

• Thread Diameter: Represents the nominal diameter of the thread, in inches.
• Threads per Inch: Represents the number of threads per inch length, i.e., the pitch.
• Thread Type Code: Specifies the type of thread, such as UNC (Unified Coarse Thread), UNF (Unified Fine Thread), etc.
• Thread Accuracy Grade: Represented by a combination of numbers and letters, such as 2A or 3B, where numbers 1, 2, 3 represent the accuracy grade of external threads, and A represents the external thread symbol, B represents the internal thread symbol.

Unified Thread Specification Marking Method

The marking method for unified thread specifications: use a number (nominal diameter < 1/4") or an inch fraction of the nominal diameter (> 1/4"), threads per inch, and code (UNC - coarse thread, UNF - fine thread, etc.) to represent.

Below are some specific examples of American thread representations:

• Example: 1/2"-13UNC-2A: Indicates a nominal diameter of 1/2", 13 threads per inch coarse external thread, thread accuracy grade 2A;
• Example: 1/2"-20UNF-2B: Indicates a nominal diameter of 1/2", 20 threads per inch fine internal thread, thread accuracy grade 2B.

Specific Meanings of Thread Codes

• UNC/UNRC — Unified Coarse Thread/External Thread with Arc Bottom Coarse Thread;
• UNF/UNRF — Unified Fine Thread/External Thread with Arc Bottom Fine Thread;
• UNEF/UNREF — Unified Extra Fine Thread/External Thread with Arc Bottom Extra Fine Thread;
• UN/UNR — Unified Constant Pitch Thread/External Thread with Arc Bottom Constant Pitch Thread, such as 4UN, 6UN, 8UN, 12UN/4UNR, 6UNR, 8UNR, 12UNR, etc.;
• UNS — Special Series of Unified Threads.

Among them, commercial fasteners commonly use coarse and fine thread series (UNC/UNF, UNRC/UNRF) and constant pitch series threads such as 8UN and 12UN.

Fitting of American UN Series Threads

In practical applications, the thread representation method may vary according to specific industrial standards or manufacturer requirements. The detailed specifications and marking methods of threads should refer to the corresponding American standards, such as ANSI/ASME B1.1 or IFI standards.

Unified Thread Nominal Diameter and Threads per Inch Series (Partial)