BA-thread
What is a BA Thread?
The BA thread, or British Association thread, is a series of small screw threads, with the largest size being 0BA, which has a diameter of 6 mm. These threads are still widely used in miniature devices and modeling, even though their usage has declined over time. BA threads were originally designed "scientifically" so that the largest thread would have a diameter of 6.0 mm and a pitch of 1.0 mm, with each subsequent larger number meaning 0.9 times the pitch of the previous thread pitch. The thread measurements were rounded to two significant decimal places in both metric and inch measurements, anticipating the global adoption of the metric system nearly a hundred years in advance.
History and Development of BA Threads
The British Association for the Advancement of Science first proposed the design of the BA threads in 1884, and they were standardized and published in 1903. The design was based on the Swiss Thury thread, but the BA thread simplified the defined thread profile to a 47.5° angle and made it symmetrical, with thread depth set at 3/5 of the thread pitch (p).
In the Thury thread, the peaks of the thread profile were rounded to 1/6p and the roots to 1/5p, which made the thread angle close to 47.5 degrees but not exact. In BA threads, the thread profile angle was defined exactly as 47.5 degrees, which simplified the standard.
Characteristics of BA Threads
BA threads are defined in a range of sizes, where higher numbers indicate smaller screws. For example, a 0BA thread has a pitch of 1 mm and a diameter of 6 mm. The pitch of each higher-numbered thread is calculated by multiplying the previous number's pitch by 0.9 and rounding the result to two significant digits. This approach makes BA threads distinctive compared to other thread systems.
The thread angle is 47.5°, and the thread depth is 0.6 times the pitch, resulting in rounded crests and roots. This makes BA threads particularly suitable for precision instruments and other small-scale applications.
Uses and Importance of BA Threads
BA threads are still used in some precision instruments, such as optics, moving-coil ammeters, and relays. For instance, a 2BA thread attaches a metal part to a dart shaft, and 4BA threads are still used in wiring boxes and switch boxes that follow old British engineering standards.
Especially in modeling and small-scale engineering, BA threads are popular because their small hexagonal heads make it easier to create scale-appropriate fasteners. This has made BA threads a popular choice, especially in the United Kingdom.
BA Thread Standards and Compatibility
BA threads are defined in British Standards, such as BS 93:1951, which covers sizes from 0BA to 16BA. Although the 0BA thread diameter and pitch are the same as the M6 thread, their thread profiles are not compatible, which prevents reliable operation of loaded threaded joints.
Summary
BA threads are specialized, small-sized screw threads originally designed in 1884 and standardized in 1903. These threads are scientifically dimensioned and have specific thread profiles, making them ideal for precision instruments and modeling. Although usage of BA threads has declined, they remain significant in certain industries and precision instrumentation.
