Subject: Re: How do I quantify accuracy?

From: [email-masked] (Tim Kirby)
Date: 10 May 94 19:40:19 CDT

David Jenkins ([email-masked]) wrote:
:In article <[email-masked]>, shampain@hightop (Shampain) writes:
:> If a piece is accurate to x over a span of y inches, how is this expressed?
:> ie .0015 over 10 inches...

: In my experience, this dimension and tolerance would be described as 

: 10" + .0015"
:     -
...

The description read fine to me, Dave... I have seen it printed on devices
incapable of the plus-over-minus sign as 10" +/- 0.0015 or, where there are
different values for the tolerances, 10" +0.000/-0.003 (for example).
On a drawing, particularly where the values are different, it would probably
be written as
                    +0.000
                 10
                    -0.003

With the tolerances in a smaller print size and closer together.

The terminology of the original question bothered me a little, though. "If a
piece is accurate to ..." sounds like the results of measuring something that
is already made; such a measurement might be described as you had it based on
the accuracy of the tool used to measure the piece... but it is not often
recorded as such - in my experience measured dimensions are often recorded
(rightly on wrongly) as absolute values with the accuracy being implied by the
accuracy of the recorded measurement - e.g. how many decimal places it is
written down to.

The drawings for a part would be the more common place to find the degree of
tolerance as you described it. The design calls for a part to be made within
the specified tolerance, 10" + 0.0015 giving a possible deviation of 0.003"
                             -
between any two parts made to that dimension.

Hmmm. I probably just confused everyone, didn't I?... *sigh*.

Tim
--
Tim Kirby --------- Cray Research Inc., Eagan, MN, USA ---------- [email-masked]
Disclaimer: Cray didn't say this... I did...
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