British Standard Whitworth
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British Standard Whitworth (BSW) is one of a number of imperial unit based screw thread standards which use the same bolt heads and nut hexagonal sizes, some others being British Standard Fine thread (BSF) and British Standard Cycle. These three are collectively called Whitworth threads.
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[edit] History
The original Whitworth thread form was proposed by Joseph Whitworth in 1841 to replace various proprietary fasteners. This standard specifies a 55° thread angle and a thread depth of 0.640327p and a radius of 0.137329p, where p is the pitch. The thread pitch is specified by a chart. The Whitworth thread system was later to be adopted as a British Standard to become British Standard Whitworth. Contrast this with the American Unified Coarse another standard based on imperial fractions used in the United States. These are similar except that the Unified thread angle is 60° and has flattened crests (Whitworth crests are rounded). There is one important exception in this comparison; the thread pitch for the 0.5 in bolt is 12 threads per inch (tpi) in BSW vs 13 tpi in the AUC.
With the adoption of BSW by all the British railway lines, many of which had previously used their own standard both for threads and for bolt head and nut profiles, it came to dominate mechanised British manufacturing.
In the USA, BSW mostly died out as steel bolts replaced iron, but was still used for aluminium parts into the 1960s and 1970s when metric based standards replaced the Imperial ones. In some other countries, such as Australia, BSW is still used. Though metric standards are more commonly used.
[edit] Comparison of standards
The British Standard Fine (BSF) standard has the same thread angle as the BSW, but has a finer thread pitch and smaller thread depth. This is more like the modern "mechanical" screw and was used for fine machinery and for steel bolts.
The British Standard Cycle (BSC) standard which replaced the Cycle Engineers' Institute (CEI) standard was used on British bicycles and motorcycles. It uses a thread angle of 60° compared to the Whitworth 55° and very fine thread pitches.
(To simplify matters the term hexagon will be used in this paragraph to denote either bolt head or nut). Whitworth spanner (wrench) markings refer to the bolt diameter rather than the distance across the flats of the hexagon (A/F) as in other standards. Confusion also arises because BSF hexagon sizes can be one size smaller than the corresponding Whitworth hexagon. This leads to instances where a spanner marked 7/16BSF is the same size as one marked 3/8W. In both cases the spanner jaw width of 0.710 in, the width across the hexagon flat, is the same. However, in World War II the size of the Whitworth hexagon was reduced to the same size as the equivalent BSF hexagon purely to save metal during the war, and they never went back to the old sizes afterwards. Thus it is today uncommon to encounter a Whitworth hexagon which takes the nominally correct spanner. Spanners in this case may be marked 7/16BS to indicate that they have a jaw size of 0.710 in and are designed to take either the (later) 7/16 BSW or 7/16 BSF hexagon. The table here[1] illustrates the differences between the old and new hexagon standards.
The British Association screw thread (BA) standard is sometimes classed with the Whitworth standard fasteners because it is often found in the same machinery as the Whitworth standard. However it is actually a metric based standard that uses a 47.5° thread angle and has its own set of head sizes. BA threads have diameters of 6 mm (0BA) and smaller, and were and still are particularly used in precision machinery.
The Whitworth 55° angle remains commonly used today worldwide in form of the 15 British standard pipe threads defined in ISO 7, which are commonly used in water supply, cooling, pneumatics, and hydraulic systems. These threads are designated by a number between 1/16 and 6 that originates from the nominal inner inch-diameter of a steel pipe for which these threads were designed. These pipe thread designations do not refer to any thread diameter.
Other threads that used the Whitworth 55° angle include Brass Threads, British Standard Conduit (BSCon), Model Engineers (ME), and British Standard Copper (BSCopper).
| Whitworth size | Core diameter | Threads per inch | Pitch | Tapping drill size |
|---|---|---|---|---|
| 1/16" | 0.0411 in | 60 | 0.0167 in | Number Drill 56 (1.2 mm) |
| 3/32" | 0.0672 in | 48 | 0.0208 in | Number Drill 49 (1.85 mm) |
| 1/8" | 0.0930 in | 40 | 0.025 in | Number Drill 39 (2.55 mm) |
| 5/32" | 0.1162 in | 32 | 0.0313 in | Number Drill 30 (3.2 mm) |
| 3/16" | 0.1341 in | 24 | 0.0417 in | Number Drill 26 (3.7 mm) |
| 7/32" | 0.1654 in | 24 | 0.0417 in | Number Drill 16 (4.5 mm) |
| 1/4" | 0.1860 in | 20 | 0.05 in | Number Drill 9 (5.1 mm) |
| 5/16" | 0.2414 in | 18 | 0.0556 in | Letter Drill F (6.5 mm) |
| 3/8" | 0.2950 in | 16 | 0.0625 in | 5/16 inch (7.94 mm) |
| 7/16" | 0.3460 in | 14 | 0.0714 in | Letter Drill U (9.3 mm) |
| 1/2" | 0.3933 in | 12 | 0.0833 in | Letter Drill Z (10.5 mm) |
| 9/16" | 0.4558 in | 12 | 0.0833 in | 12.1 mm (0.4764 in) |
| 5/8" | 0.5086 in | 11 | 0.0909 in | 13.5 mm (0.5315 in) |
| 11/16" | 0.5711 in | 11 | 0.0909 in | 15 mm (0.5906 in) |
| 3/4" | 0.6219 in | 10 | 0.1 in | 16.27 mm (0.6406 inch) |
| 13/16" | 0.6845 in | 10 | 0.1 in | 18 mm (0.7087 inch) |
| 7/8 " | 0.7327 in | 9 | 0.1111 in | 19.25 mm (0.7579 in) |
| 15/16" | 0.7953 in | 9 | 0.1111 in | 20.75 mm (0.8169 in) |
| 1" | 0.8399 in | 8 | 0.125 in | 22 mm (0.8661 in) |
| 1 1/8" | 0.9420 in | 7 | 0.1429 in | |
| 1 1/4" | 1.0670 in | 7 | 0.1429 in | |
| 1 1/2" | 1.2866 in | 6 | 0.1667 in | |
| 1 3/4" | 1.4939 in | 5 | 0.2 in | |
| 2" | 1.7154 in | 4.5 | 0.2222 in | |
| 2 1/2" |
[edit] Current usage
Within the United States, the Whitworth thread that most people encounter is the quarter-inch thread on the bottom of most cameras for mounting on a tripod. The Leica Thread-Mount used on rangefinder camera and on many enlarging lenses is 39mm by 26 turns-per-inch Whitworth, an artifact of its having been developed by a German company specializing in microscopes and thus equipped with tooling capable of handling threads in inches and in Whitworth.
[edit] Historical misuse
British Morris and MG engines from 1919 to 1955 were built in a factory that used metric threads but with bolts and nuts for Whitworth spanners (wrenches) and sockets.[2]
[edit] See also
Other thread standards:
- British standard fine thread (BSF)
- Unified Thread Standard (UTS, including UNC, UNF and UNEF)
- British standard pipe thread (BSP)
- British standard brass thread
- British Association screw threads (BA)
- ISO metric screw thread
[edit] References
- ^ British Standard Whitworth thread profile - British Tools and Fasteners, New York
- ^ Wood, 1977
[edit] Bibliography
- British Standards Institution (1956) Parallel screw threads of Whitworth form, British standard 84, 2nd Rev., London : British Standards Institution, 66 p.
- Oberg, E., Jones, F.D., Hussain, M., McCauley, C.J., Ryffel, H.H. and Heald, R.M. (2008) Machinery's handbook : a reference book for the mechanical engineer, designer, manufacturing engineer, draftsman, toolmaker, and machinist, 28th Ed., New York : Industrial Press, ISBN 978-0-83112-800-5, p. 1858–1860
- Wood, J. (1977) The restoration and preservation of vintage & classic cars, Yeovil : Haynes, ISBN 0-85429-186-5
[edit] External links
- Spanner Jaw Sizes Additional information and spanner jaw size table.
- BSW Tapping drill for 65% and 100% and clearance drill sizes.
- [2] Discusses Whitworth thread and pitch sizes and spanner sizes.
- Conversion chart Whitworth/BSF/AF and metric spanner sizes at foot of page
