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.