Can Basic Dimensions Be Omitted?

A question came up recently about how GD&T relates to CAD dimensions:  The rules of GD&T say that basic dimensions are required when using a position tolerance. But does that always mean that the print must show these “boxed” dimensions?

Answer:  No, it doesn’t. While classical GD&T seems to say that the boxed dimensions are required, there are two alternative ways to meet the rule.  First, if most or all of the dimensions on a print are to be basic dimensions, then a general note can be placed on the drawing that states “all dimensions shown are assumed to be basic” or something to that effect.  This means that the dimension is still shown, but without the box. Caution: if this is the case, make sure that the title block does not show any default plus/minus tolerances.

The other option is one that is becoming more common: Leave the dimensions off the print, and add a note saying something like “all undimensioned features are dimensioned in CAD, and are basic.”  To its extreme, this means that the entire print can leave off all dimensions, requiring the reader to have access to the CAD data if they really want to know the dimension.  While there are advantages to this (the designer doesn’t have to cram all the dimensions on the drawing), it also assumes that everyone downstream will have access to the CAD data.

Like anything else, moderation is the key.  I say that if you can show the dimensions, put them on there (with the box, for basic dimensions).  However, if it’s a complex shape, or if you’re certain that everyone can access CAD, then you could go dimensionless. As always, your comments are welcome!

5 Comments

  1. I agree with the explanation.

    As the drawing becomes the legal part of the document, It is always necessary to show it on the print irrespective of the CAD files.

    Further, to avoid all confusions it is always better to include ‘box’ for basic dimensions.

  2. Since you mention in your drawing on the general notes that the dimensions not shown are basic, go ahead

  3. As per my experience, it is better to indicate the basic dimensions with the “box”. In all the latest CAD packages, this facility is available. If we give the general notes in drawing, the chances of mistakes are more.

  4. Digging up an old thread here for sure. But curious to get further clarification on this. This is a two part question.

    First point/question: We have for some time had a note on our drawing that states “any dimension not shown should be derived from the 3D CAD”. I argued from day 1 that this was not nearly enough as it doesn’t indicate what tolerance on it should be. Is this correct? People attempt to use the statement that the “titleblock tolerances apply”, but at what accuracy? x.xx or x.xxx??

    The second: I have started to win the above argument or at least convince others that further clarification is needed. So we have now started to keep the note above AND add the following

    “any undimensioned features |Profile|X.XX|A|B|C|” (where A,B,C are valid datums for the part).

    This at least give more clarification on the size. HOWEVER….however my question becomes how do you measure such feature. Is it based on it was defined in the CAD? Meaning you would have to go to the way the feature was constructed to determine how wide something would be for instance, or is it measured from the datums only? If does that mean that a part which was intended to have a “thickness” now is measure not as a thickness but two separate dimensions from the datum to establish a thickness?

    We don’t go crazy with this. As you mention moderation. Typically we put this on our plastic parts that are fully of small features that are not consequential to form/fit/function. Or some of our casting where there are features that are important to function but dimensional stability is not as important as others. This is in an attempt to keep our drawings less cluttered.

    Thanks in advance for the input!!

    • I agree with your first couple of points… CAD data is considered basic (since it’s the perfect geometry), and you can’t appeal to a title block tolerance that has a number of decimal places mentioned. So yes, it’s quite common to have a general note giving a profile of a surface tolerance of |X.XX|A|B|C|.

      It’s mainly meant as a CYA thing, but if something needs to be measured, then it’s not a thickness to be measured. It’s the form/location of each surface back to A, B, C. So I would say that if there is a specific size dim that is really important then you’re better off dimensioning it (and tolerancing it) directly. But don’t forget to also control the feature’s location and orientation on the part. That’s the real value of profile: it covers so many bases with one callout.

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