Hi All
I was talking with Roger Moment this evening discussing the thread (no pun
intended) on Whitworth Tread Forms and I thought that I would try and clarify
things a bit.
<< In a message dated 04/02/2001 12:18:50 AM Mountain Daylight Time,
rader@interworld.net writes:
<< Gary:
> Can you please give me a definition of the term "whitworth"
> I am using it as a rally question and I want to have it correct. >>
Robert Larson responded:
>>For the definition of the term Whitworth it might be interesting to add
that it is a proper name. It came from Sir Joseph Whitworth (1803-1887) who
wrote papers on thread form that were published by the Institution of Civil
Engineers in England. He has also been credited with the invention of the
micrometer, an application of precision threads.<<
O.K., so far correct but let me elaborate further. There are four (4)
distinct Whitworth Thread Forms, British Standard Whitworth (BSW) which was
the original coarse thread developed by Sir Joseph Whitworth in 1841 and used
on things like locomotives (then) and for threading into Aluminum where a
coarse thread is less prone to stripping than a fine one (now). Second in
the series was British Standard Fine (BSF) which was developed in 1908 for
other engineering applications. The next two (2) Whitworth Thread Forms are
British Standard Pipe (BSP) and British Standard Pipe Tapered (BSPT). What
makes these all Whitworth Thread Forms is the fact that they all have a 55
degree thread angle as well as rounded crests and roots. American Threads
all have 60 degree thread angles and flat crests and roots.
British Association Thread Form (BA) is a metric based form (47 1/2 degree
thread angle) which was originally developed by the Swiss for very small
watch and clock screws, and you will sometimes here the type referred to as
the "Swiss Small Screw Standard". The British adopted this thread form on
1903 and call it the British Association Thread. BA sizes are designated by
the numbers 0BA through 16BA, similar to our American machine screws, but in
the BA system, the larger numbers designate the smaller screws. You will
normally find BA fasteners where bolts smaller than 1/4" are required e.g.,
in instruments and electrical components.
British Standard Cycle (BSC), also known as Cycle Engineers Institute (CEI).
Just as the name implies, this thread form is found on bicycles and
motorcycles where the fine threads are less prone to vibrating loose. This
thread form has a 60 degree thread angle as with the American thread forms
but with rounded crests and roots as the rest of the British systems. The
majority of the cycle fasteners are all 26 threads per inch (TPI) throughout
all bolt diameters e.g., 1/4 X 26 TPI, 5/16 X 26 TPI, 3/8 X 26 TPI, 7/16 X 26
TPI, etc. To confuse matters even more, some of the older cycle threads had
either 20 or 24 TPI.
By now anyone who has read this far and better yet who understands all of
this has no life!
Cheers
Curt Arndt
Carlsbad, CA
'55 BN1, '60 AN5
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/// (If they are dupes, this trailer may also catch them.)
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