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!
November 24th, 2009 at 7:05 am
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.
March 3rd, 2010 at 6:41 am
Since you mention in your drawing on the general notes that the dimensions not shown are basic, go ahead
July 5th, 2010 at 1:14 am
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.