Thread Basic Dimensions and Tolerances - New and Old National Standard Thread Names and Codes
When it comes to engineering, metals, cellulose and other similar materials are commonly machined into cylindrical objects with uniform tapped holes, or threaded components. Threads are used to assemble and fasten components together within changing structures, meaning threads need to meet certain dimensions and tolerances. Standard threads, when machined accurately, will ensure efficient and secure assembly. This article explains various national standards for threads defined by countries and organizations, how to measure them, as well as offering a table of comparable thread sizes between old and new national standards.
Threads
Threads are used in engineering and construction to affix components together. The machined grooves produced during the process are referred to as threads, and they are formed when materials are cut, machined or rolled into a conformable shape. Threads come in different types – straight, tapered, and buttress – and differ in form and function, but all can be measured using the same system of measurement internationally accepted for the accurate manufacture of machined parts.
Measuring Threads
The most important component of a threaded component is its major diameter, which is the nominal size of the part before threads are cut into it. This is measured using a Vernier or micrometer, enabling accurate measurement of the component’s diameter. The major diameter must match the requirements of the application. All threads of the same diameter across different threads have the same major diameter. The component’s pitch (thread’s width) and the coefficient of friction used during assembly must also be matched.
Threads are also measured by their pitch, defined as the distance between two threads. It is measured through the application of a ruler alongside the threaded component, enabling the calculation of the distance between two crests (or roots) of a thread.
National Standards
Each country or organization defines its own national/international thread standards. In the US, the American National Standard Coarse and Fine Thread (NC/NF) is most commonly used for the manufacturing of components with fine teeth, though metric standards for coarse and fine threads are also used. In Europe, the DIN (Deutsche Institut fur Normung) system defines standards for metric threads, and the ISO (International Organization for Standardization) system provides the standard for metric threads which are the most commonly used in other parts of Europe.
A number of countries or organizations also provides their own standards for thread sizes and tolerances, such as the UN/UNR system in the United States, the BSPP/BSF system in the UK, and the JIS/JISB system in Japan.
Table
Below is a table of comparable thread sizes between old and new national standards:
Old National Standard New National Standard
UN/UNR Metric DIN/ISO
BSPP/BSF Metric ISO/JIS
NC / NF Coarse Thread Metric DIN/ISO
NC / NF Fine Thread Metric ISO/JIS
Conclusion
When it comes to constructing threaded components, it is important to have a clear understanding of the different national standards and to use the right system for accurate assembly. Furthermore, knowing and understanding thread basic dimensions and tolerances, as well as matching the major diameter, pitch and coefficient of friction required, will help to ensure the assembly of components with secure and efficient threads.
