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.

8 Comments

  1. 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?

  2. 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.

  3. Please explain the situation,
    On the part drawing I specified a flat surface as damum A.
    The actual part has this surface bowed.
    The toolmaker asked me to put a flatness requirement on this datum.
    Is it legal?
    Thank you

  4. Yes, that is legal. Just because a surface is labeled as a datum doesn’t actually impose any tolerance on it. So it’s very common to have a surface be labeled as a datum and then also apply a flatness tolerance to it.

    That is why, to be more precise, we say that the actual surface of the part isn’t really the datum — it is called the “datum feature.” The true datum would be a theoretically perfect plane that contacts the actual surface at the high points.

  5. I am Working In quality Department last time am attend one interview they ask what is mean by Datum Target And how can explain that.

    • A datum target is a way to specify on a product drawing the exact location and size of gage elements that are to be used to establish a datum. That theoretical datum will then be the origin for some sort of geometric tolerance on the product.

      For instance, suppose I have a plastic part that has no flat surfaces — it may be curved on all sides. Well, I can’t just set it down on a flat table to measure it. So I might create a special fixture with three vertical pins; the curved bottom of the part could rest on those three pins. All subsequent measurements could be taken from the imaginary plane created by those three pins.
      Thus, the pins establish the “datum,” but that datum was taken by having the fixture pins be located at the “datum targets.”

      • Its really good explanation.
        Thank you very much John.
        Thanks a lot.

  6. In case the tolerance derived from datum A is relatively high, the actual part has this A surface bowedabove 40% of the tolerance we need to improve the flatness or to employ target points with basic dimensioned coordinates: A1; A2; A3
    Michael, Senior Metrologist

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