BSW thread
What is a BSW thread?
The BSW thread, or British Standard Whitworth thread, is one of the oldest industrial thread forms, developed by Joseph Whitworth in 1841. It is an imperial (inch-based) thread profile with a 55-degree flank angle and rounded thread crests and roots. BSW threads were long widely used, especially in British mechanical engineering and workshop industries and machine construction, but today they have been partly replaced by metric threads and other international standards.
BSW threads in mechanical engineering
In machine shops and machining workshops, BSW threads are still used mainly for the maintenance, repair, and spare-part manufacture of older British machines and equipment. Because the thread is inch-based, its use requires separate taps and dies conforming to the Whitworth standard thread taps. This means that metric tools are not directly suitable for producing or repairing BSW threads.
Dimensions and characteristics of BSW threads
BSW threads are dimensioned in inches, and thread pitch is expressed as a TPI value (threads per inch). For example, a 1/2" BSW thread has 12 threads per inch. The 55-degree thread angle and rounded crests and roots reduce stress concentrations, which makes the thread form more durable compared to sharp-angled profiles.
Manufacturing BSW threads in machining
BSW threads can be produced by single-point threading on a lathe, or by using taps and dies. In machining, it is important to use the correct pitch and thread profile, as an incorrect tool geometry can lead to incompatibility. The tools and taps used for producing the threads follow Whitworth standard dimensions and profiles, and they differ clearly from metric tools.
Summary
The BSW thread is a historically significant inch-based thread form that is used today mainly in maintenance and repair work, particularly on British-made machines and equipment. It is identified by its 55-degree angle and rounded thread crests and roots. In machine shops, correct production of BSW threads requires tools that comply with the Whitworth standard and precise dimensioning.
