[Bell Historians] Fractions

Bickerton, Roderic K (SELEX) (UK) roderic.bickerton at pQYRvBXfc3SC0c8VnruGK2wQYK2kp-4dYwdoN7vIBp5F-JHcjIMuw4v13P85292Yp5dyM3v3LoD0cEGNfmbW0kyDJj8QOXE9.yahoo.invalid
Thu May 25 13:30:42 BST 2006

There is good reason to keep fractions when describing pre metrication
engineering, be it drawing or specification.
Fractional inches were used for hand made parts. Decimal inches were
used for  precision parts.
Early drawings often had a mix with different tolerances being indicated
by the way the dimensions were given so 1 1/2" could refer to a 1/64"
tolerance but 1.5" could be a 0.002" tolerance. Where a variety of
tolerances were needed any dimensions outside of the "general"
tolerances were specified beside the dimension.
Unthinking decimalisation of information reproduced from earlier
drawings and specifications can remove tolerance information. 
This would reduce the value of the information considerably if it were
to be used to remake a old item.
Unfortunately very few engineering drawings still exist for early parts
like self aligning plain bearings.


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