## Re: Tolerance in text

Subject: Re: Tolerance in text
From: Geoff Lane <geoff -at- gjctech -dot- co -dot- uk>
To: TECHWR-L <techwr-l -at- lists -dot- techwr-l -dot- com>
Date: Tue, 21 Feb 2006 14:27:38 +0000

On Tuesday, February 21, 2006, Links, Diederik wrote;

> I _obviously_ prefer the first one.
> Because the '±'-sign means 'approximately' (the hole can be 24 mm or
> 26 mm) and not 'plus or minus' (the hole must be between 24.8 mm and
> 25.2 mm).
---

In my book, "±" specifies a tolerance. The notation that I've used
both as a marine engineer and as a tech writer is to specify the
nominal dimension, then the maximum oversize, then the maximum
undersize. So, you could write, "25 mm + 0.2 mm / - 0.2 mm".

- Where the oversize and undersize tolerances are the same, you can
combine them using "±", so you could write "25 mm ± 0.2 mm".

- Using the first notation, where either the undersize or oversize
tolerance is omitted, it is assumed to be zero.
So, "25 mm - 0.2 mm", "25 mm / - 0.2 mm", and "25 mm + / - 0.2 mm"
all mean "25 mm + 0.0 mm / - 0.2 mm".

That said, the most common form specifies both tolerances explicitly
where they are unequal and most people now take "25 +/- 0.2" to mean
"25 plus or minus 0.2" even if it isn't strictly correct. Thus, using
"+/-" is ambiguous and probably should be avoided.

Don't omit the units from the nominal dimension because that too can
be ambiguous. For example, does "25 ± 0.2 mm" have a nominal dimension
of 25 mm, 25 cm, or something else?

Perhaps as important is implied tolerance. Any dimension has an
implicit rounding tolerance. So, "25 mm" actually means
"25 mm + 0.49 mm / - 0.50 mm" because any dimension in that range
rounds to 25 mm to the nearest millimetre, which is the accuracy
implied by omitting the first decimal place.

So, IMO, "25.0 mm ± 0.20 mm" is the best way of writing the tolerances
you need without introducing ambiguity.

HTH,

--
Geoff

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References:
Tolerance in text: From: Links, Diederik

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