RE: Tolerance in text

[eric -dot- dunn -at- ca -dot- transport -dot- bombardier -dot- com]

"Jonathan West"  wrote on 02/21/2006 09:46:33 AM:

> I disagree with this concept of implied tolerance.

Couldn't agree more. There is NO rounding in tolerances. The fact that the 
design requires and sets tolerances rules out the possibility that 
rounding exists.

25 ± 0.1 means 24.9 to 25.1 is acceptable. From everything I was taught, 
the measuring tool used should be accurate to one more decimal place than 
the tolerance. So, a micrometer that measures in ten thousandths is good 
for measuring any part requiring tolerances in the thousandths. I'd have 
to delve back into other text books for the exact allowances.

So, in the preceding example 25.11 is too large and 24.89 is too small.

Which brings up an important point. NEVER convert measurements without 
providing the measurement in the original units. And if you're converting 
measurements with tolerances, tread VERY carefully. Lot's of mathematical 
rules depending on number of significant digits and whether the conversion 
factor is absolute or approximate (rounded). The rounding error introduced 
by the conversion MUST be smaller than the required tolerance. Or, in 
other words, the tolerances of the converted measurement must be tighter, 
and not looser, than the tolerances of the original measurement.

Eric L. Dunn
Senior Technical Writer

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