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Old March 3, 2013, 06:34 AM   #20
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Join Date: November 13, 2006
Posts: 5,249

From ASME Y14.5 2009: "Geometric Dimensioning and Tolerancing"

Datum — A theoretically exact point, axis, or plane derived by contact a datum feature; it then becomes the origin from which geometric characteristics are measured.

You cannot buy a datum.Datums are theoretical geometric features,such as planes ,axis,etc.

We can use tools to represent datums ,for example,a granite surface plate may be used to represent a datum plane.Or,as in the Hornady case length caliper gage,the ring privided will rest on the case shoulder to give a useful approximation of the datum .

ASME Y14.5 is THE standard for what terms and definitions apply to engineering drawings.

I am quite comfortable working with it.

Last edited by HiBC; March 3, 2013 at 06:44 AM.
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