You should know these imperial British thread types
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Getting your bearings in a different system of units is a bit like learning a foreign language. At first you only understand the first thing and have to laboriously translate everything word for word, but the longer you practise, the more intuitive it becomes. For craftsmen and DIY enthusiasts alike, it is useful and sometimes even unavoidable to know at least the basics of customs units as well as the metric system. The Anglo-American measurement system plays an important role not only in the USA. In Great Britain, too, nothing works without inches, as the unit is called in English. And some of the British inch thread types have now also become widespread in this country. Above all, the Whitworth pipe thread, which has become an integral part of the sanitary industry in Germany. So that you know exactly what you are dealing with in future when you come across an imperial British thread type, we have taken a closer look at the subject.
Joseph Whitworth and the origin of imperial British thread types
British thread types are primarily associated with one name: Joseph Whitworth. If you often work with threads, then you have probably heard the term Whitworth thread. The British engineer has obviously left a more than lasting impression, but why? Joseph Whitworth is nothing less than the father of thread standards. Can you imagine a world without standardized threads? No interchangeability, no reproducibility and no well-stocked screw shelves in the DIY store. That was the world in which Joseph Whitworth lived. Until he decided to change that and presented the world's first national thread standard in 1841. The example set a precedent and soon thread standards were springing up all over the world. The name Whitworth became immortal and is still used every day by countless craftsmen - even if perhaps not everyone knows exactly what it actually stands for.
The special feature of the imperial British thread types
A special feature that still distinguishes British thread types from many metric and imperial American threads dates back to the early days of thread standards. Whitworth threads have a flank angle of 55°, in contrast to 60° for the other widely used screw threads.
The pitch of the inch British Whitworth threads differs from the metric threads. The pitch is not the distance between two thread tips, but the number of threads per inch. In this context, you will often see the abbreviation TPI, which stands for threads per inch. Fortunately, however, this specification can easily be converted into the pitch we are familiar with by dividing 2.54 cm (i.e. 1 inch) by the number of threads.
Imperial British Whitworth screw thread
The British standard thread, which corresponds to our metric standard thread in terms of application and function, is the British Standard Whitworth (BSW). This is actually still the original thread standard, which goes back to the designs of Joseph Whitworth himself. The BSW thread has long since become part of the British Standards, the standards collection of the British Standards Institution (BSI). Many other British thread types were developed on the basis of the BSW profile. This is why many British threads are colloquially referred to as "Whitworth threads", even if they did not originate directly from the pen of the resourceful engineer.
One of these is the British Standard Fine (BSF). The British fine thread is the counterpart to the metric fine thread with a smaller pitch and shallower thread depth. It is used for thin-walled components and in precision mechanics.
Another British thread type that is often grouped with the Whitworth threads, although strictly speaking it does not belong to them, is the British Association thread (BA). Although the BA thread is often found side by side with BSW and BSF in components, it has a completely different geometry. The flank angle here is 47.5° and the BA thread is not an imperial thread at all, but the nominal diameter and pitch are specified in mm, as with metric threads.
Imperial British Whitworth pipe threads
If you were familiar with the name Whitworth before this article, it was probably in the context of pipe thread types. This is the area in which the sphere of influence of imperial British threads finally extends beyond the borders of Great Britain. The pipe threads are grouped together under the name British Standard Pipe (BSP) - probably better known to you as G pipe threads.
As with most types of pipe thread, there are also conical and cylindrical variants of BSP threads. British Standard Pipe Parallel thread (BSPP) stands for the cylindrical thread with a constant diameter, while the tapered version is known as British Standard Pipe Taper thread (BSPT).
With the G pipe thread, which is widely used in Germany, both the internal and external threads are cylindrical and a connection is created that does not seal in the thread. The G thread is standardized according to ISO 228. Less commonly used in Germany are the sealing connections consisting of a conical external thread R and either the cylindrical Rp internal thread or the likewise conical Rc internal thread.
It is also important to note a special feature of the dimensions of Whitworth pipe thread types. The inch specifications appearing in the thread designation do not correspond to the actual outer diameter of the thread. This is due to the fact that the wall thickness of modern pipes has been significantly reduced compared to that of old cast iron pipes. This has been compensated for by increasing the internal diameter, but without adjusting the size specifications accordingly. As a result, a G 1" thread actually has an outer diameter of 1.309 inches or 33.250 mm.
Are you looking for taps and dies for inch British Whitworth threads? The BAER online store has everything you need. And if you have any questions, our friendly customer service team is always on hand to help.