Understanding Threads and Fastener Design

Threads can be found holding just about anything together- from bridges and airplanes to everyday appliances. Yet, despite their ubiquity, the science and design of threads often remain overlooked. Understanding the anatomy of threads, from their roots to their crests, can unlock insights into how they manage stress, enable precision, and ensure reliability in high-stakes applications.

This blog delves into the fascinating world of thread design, focusing on the differences between ISO and ASME standards, and exploring why details like rounded or flat roots matter. Whether you’re an engineer, machinist, or enthusiast working with fasteners, understanding these nuances will help you make informed decisions about thread selection and design, improving both performance and durability.

Before the world had standardized threads, engineers, blacksmiths, and machinists faced a chaotic reality—every bolt and screw was essentially custom-made, often incompatible with others. The introduction of standardized bolt threads and thread design was a transformative moment in industrial history, allowing for the mass production and interchangeability of fasteners. This seemingly small innovation became the backbone of modern manufacturing, construction, and engineering.

In the early days of threaded fasteners, there were no set standards. Each craftsman had their own way of cutting threads, meaning that a bolt made in one shop might not fit a nut made in another. This lack of uniformity made repairs and replacements difficult, especially as industrial machinery became more complex.

Ancient civilizations, such as the Greeks and Romans, used basic screw thread types for presses and lifting devices, but these were often handmade and lacked consistency. The first real push toward thread standardization came with the rise of the Industrial Revolution in the late 18th and early 19th centuries.

The Whitworth Thread featured:

• A 55-degree thread angle (see diagram below for more information)
• A fixed number of threads per inch, depending on the bolt diameter
• A rounded thread crest and root for improved strength (see diagram below for more information)

Whitworth’s system became the first national thread standard, adopted across Britain and its colonies. This development significantly improved the compatibility of bolts and nuts, making repairs and replacements much easier.

Across the Atlantic, the United States faced the same challenge: a lack of standardization in fasteners across industries, leading to inefficiencies and incompatibility. While inspired by Whitworth’s system, American engineer William Sellers recognized the difficulties in manufacturing Whitworth threads. In 1864, he introduced a new thread standard that was simpler to produce and more suitable for industry, laying the foundation for what would later become the Unified Thread Standard (UTS). The Sellers had key differences:

• A 60-degree thread angle (instead of Whitworth’s 55-degree angle) (see diagram below for more information)
• Flat crests and roots for easier machining (see diagram below for more information)
• Unified thread sizes across American industries

The Sellers Thread was widely adopted in the U.S. and later influenced the development of international thread standards.

Meanwhile, in Europe, engineers were working toward a standardized metric thread system to replace the fragmented national standards. This effort culminated in the ISO Metric Thread System, formally adopted by the International Organization for Standardization (ISO) in 1947.
The ISO Metric Screw Thread System established:

• A 60-degree thread angle (similar to Sellers’ and ASME UTS threads)
• Thread pitch measured in millimeters rather than threads per inch
• A globally consistent framework for metric fasteners

Today, the ISO Metric Thread System is the most widely used thread standard worldwide, while the ASME Unified Thread Standard (UTS) remains prevalent in the United States and Canada. Many industries rely on both systems, with dual-standard fasteners ensuring compatibility across international markets.

The standardization of bolt threads and thread design revolutionized manufacturing and engineering, enabling greater efficiency across industries. In the United States, the American National Standards Institute (ANSI) plays a crucial role in maintaining consistency in fastener standards, working alongside ASME to define specifications that ensure interchangeability and reliability. This enables:

• Mass production of fasteners – Eliminating the need for custom-made bolts for every machine.
• Interchangeability – A bolt from one factory would fit a nut from another, reducing inefficiencies.
• Stronger and more reliable connections – Properly designed thread tolerances improved load distribution and fastener durability.
• Global compatibility – A metric bolt made in Germany could now be used in machinery built in Japan, while ASME fasteners remained a staple in North America.

The Trapezoidal thread, often called Tr thread, is the metric equivalent of the Acme thread. It has:

• A 30-degree thread angle
• High efficiency in power transmission
• Common use in linear actuators, jackscrews, and machine tools

Both Acme and Trapezoidal threads continue to be widely used in machinery today due to their strength and durability.

Notice in the above diagram that as the pitch line moves downward, the thread changes to have wider roots and narrower crests, and the overall space between threads is increased. Having the correct pitch line location allows for the threads to fit together properly, with the correct spacing.

ISO threads, as defined in ISO 68-1, mandate a rounded root for external threads, with the option for either a fully rounded root (a single continuous arc tangent to the flanks) or a partially rounded root (two smaller arcs with a flat section between them). This mandatory rounding for external threads is intended to enhance fatigue resistance, particularly in high-stress applications. ISO internal threads, on the other hand, typically have flat roots, but they may also be rounded, depending on specific design or manufacturing requirements. The flexibility to incorporate a fully or partially rounded root in ISO internal threads allows for compatibility with external threads while ensuring performance under varying load conditions.

In contrast, ASME threads, as specified in standards like ASME B1.1 and ASME B1.13M, allow for both flat and rounded roots in external threads. Rounded roots are generally recommended for applications where fatigue resistance is critical, such as in rolled threads, as the smoother profile reduces stress risers. However, flat roots are commonly used in cut threads due to ease of manufacturing and sufficient strength for general-purpose applications. Internal threads in ASME standards typically feature flat roots, with rounding being less common and generally only employed when explicitly required for specific applications.