Dimensions with Datum Targets
Another question came in recently, having to do with basic dimensions and their use with datum targets. If you are up to speed on GD&T, you should know that a basic dimension is any number enclosed in a box:
The purpose of this is to show a theoretical dimension, without any tolerance. (Even a general title block tolerance does not apply!) Instead, the feature that is being dimensioned will have some GD&T that provides the actual tolerance for manufacturing.
Now here’s the question: datum targets are usually located using basic dimensions, but there is no GD&T to provide a tolerance:
Is this legal? What governs the accuracy of where those datum targets are?
Yes, it is legal, and here is the key: Geometric tolerances are applied to features of a part. A datum target is an imaginary point, line, or area that is simply used for fixturing or gage setup. It’s not the responsibility of a product drawing to worry about the tolerancing of a gage or fixture!
The ASME Y14.5 standard says it this way in paragraph 4.6.2: “The location and size, where applicable, of datum targets are defined with either basic or toleranced dimensions. If defined with basic dimensions, established tooling or gaging tolerances apply.”
Thus, most GD&T people use basic dimensions to locate datum targets. Of course, there is the question of where these “established tooling or gaging tolerances” come from, but that is something for a design group to discuss, and perhaps even reference in a note on the drawing.
May 1st, 2008 at 4:42 pm
I work in Quality Control and sometimes we are required to prepare dimensional reports that have Basic Dimensions. How can that be done if they have no tolerances?
May 27th, 2008 at 5:34 pm
Sorry for the delay in replying, Mario. Basic dimensions themselves have no tolerance, but they point to a feature that usually does have a tolerance. That tolerance is shown in the form of a GD&T “feature control frame.” So for quality control tracking, any statistical data are not really looking at the basic dimension, but at the tolerance given in the feature control frame. It’s a minor point, but remember that the purpose of the basic dimension is merely to avoid having us mistakenly apply title block tolerances to that number. My blog entry about basic dimensions and datum targets emphasized that there is no direct tolerance, but the datum targets would never be measured or tracked by quality control.