Don't Add Tolerance by Rounding
Off.
A common
question relates to rounding off measurements or numbers when
converting between metric and inch dimensions and tolerances.
This Tip will address measurements. A subsequent Tip will take
on conversions and rounding.
Y14.5 tells us in
section 2.4 that all numbers are absolute. In other words, 1
means 1.000000…….forever. Y14.5 also refers us to ANSI/IEEE
268 standard on rounding off. In this standard we are told
that you cannot increase the limits of a dimension as a result
of rounding "where the original limits may not be violated."
Let's say the part thickness shown measures .236. Back in
school most of us were told that we could round this to .24.
That might be fine in sixth grade, but when measuring parts,
it won't pass. Bottom line, the .236 measurement is out of
tolerance. Another consideration is the uncertainty of the
measurement. Due to uncertainty of the measuring process, the
.236 measurement may actually be less than the true value. Of
course, the best approach is to stay away from the limits if
your process is capable.
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