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What is a standardized thread form?

Did you know there are various standardized thread forms which offer different allowances, tolerances and designations for use with bolts, nuts and fasteners?

The oldest thread form is the British Standard Whitworth (BSW), developed by Sir Joseph Whitworth in 1841. This thread form is characterized by a 55° profile angle with a rounding of the crest and a root at 1/6 height of the fundamental triangle.

British Standard Fine (BSF) is a thread used in conjunction with BSW when finer pitches are required. The British Standard Pipe (BSP) has a Whitworth profile with two different types, Parallel (BSPP) and Tapered (BSPT). These thread forms are often used for fluid applications, such as water pumps and radiator valves.

Despite being established in the 19th Century, the thread forms are still widely used today. With the development of modern technology, their development has grown from taps and dies through to being produced by indexable inserts.

Willem Wittebol

Dormer Pramet, for example, has recently launched a new T8010 grade for thread turning applications and our inserts can be used in the production of BSW, BSF and BSPP thread forms. This assortment of inserts has either a partial or full profile.

A partial profile finishes only the root of the thread and does not touch the crest. The same partial profile insert can be used to cut threads with any pitch and common thread angle (i.e. 55°). It has a small root-corner radius suited to the smallest pitch.

An additional operation is needed to complete the external/internal diameter. The main advantage is cost-reduction as it eliminates the need for a different insert, however, it is not recommended for mass production.

A full profile insert finishes the complete thread – the root and the crest. Each thread pitch requires a different insert, offering better dimensional accuracy and a well finished thread without burrs.

In addition, Dormer Pramet offers a range of inserts for UN and NPT threads. These thread forms were standardized in 1945 in the USA, Canada and UK. The basic diameters are largely the same as BSW, but the 60° profile angle with different rounded peaks and valleys originates from the Seller thread, named after its creator William Sellers in 1868.

Willem Wittebol

Technical education manager

Dormer Pramet

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