## Re: Tolerance in text

Subject: Re: Tolerance in text
From: eric -dot- dunn -at- ca -dot- transport -dot- bombardier -dot- com
Date: Tue, 21 Feb 2006 09:57:09 -0500

A chance for my Bachelor's in Mechanical Engineering to be put to use! :p

"Links, Diederik" wrote on 02/21/2006 08:35:57 AM:
> --------
> Point 1:
> --------
> I normally write:
> "Drill a hole with a diameter of 25 +/-0.2 mm."
> My colleque writes:
> "Drill a hole with a diameter of 25 ±0.2 mm."

Either is correct, both mean exactly the same thing. however, +/- should

> --------
> Point 2:
> --------
> What notation is correct or more, preferrable:
> A. "Drill a hole with a diameter of 25 +/-0.2 mm."
> "Drill a hole with a diameter of 25 mm +/-0.2 mm."

The preferable notation is 25 ±0.2 mm. No reason what so ever to rewrite
the units. No self respecting engineer or machinist would pull units out
of thin air. It is also the method shown in my university text book
(Engineering Drawing and Design, Third Edition SI Metric, Cecil Jensen).

The only time that units are required in both the nominal measurement and
the tolerance is in the very unusual case that the tolerance is being
expressed in different units. In fact, I can't find a single instance of
different units in tolerances in the text book. And, the mere possibility
just seems absolutely wrong to me. EVERY tolerance table of standard
tolerance expresses nominal value and tolerance in the same units of
measurement (except sometimes with the notation that the tolerances are
expressed in thousandths to avoid all the extra zeros).

> B. "Drill a hole with a diameter of 25 +0.2/0.5 mm."
> "Drill a hole with a diameter of 25 +0.2 mm/0.5 mm."

Without resorting to equation editors or typography, the correct notation
would be: 25 +0.2/-0.5 mm. I'm assuming that the .5 is the negative
tolerance, it doesn't make sense otherwise.

Ideally, the + value would be superscript and the - value would be
subscript. But, it isn't possible to vertically align the two in most
publishing software. Someone may be able to prove me wrong on that point.

Note that in the SI or metric system, the basic (nominal) measurement need
not be expressed to the same number of decimals as the tolerance (25 ±0.2
mm NOT 25.0 ±0.2 mm). In the inch system, it's the opposite. The basic
(nominal) measurement is expressed to the same number of decimals as the
tolerance (.500 ±0.002 in NOT .50 ±0.002 in).

Whether or not to put a space after the ± seems more open to debate and
simple style choice.

Eric L. Dunn
Senior Technical Writer

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References:
Tolerance in text: From: Links, Diederik

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