Question about Basic DimensionsIs it necessary that a basic dimension should always be from a datum feature in a case where three datums are formed by a surface of a cube and there are two blind holes in this surface? There are two other holes on the other surface perpendicular to the datum surface.

I indicate the first surface as datum A and the two blind holes on that surface as B and C, respectively. When dimensioning the other two holes that are on the face perpendicular to datum A (other side of cube), I gave position tolerance w.r.t. A B C. I indicate the location of the hole from the edges as basic.

Since those edges are not datums, only the one edge that is common between the datum surface and the said surface on which these holes are, is a datum surface edge. The other edge (which is a perpendicular edge) is not a datum, but to show a theoretically correct dimension, I have shown it as basic, then I have a feature control frame associated with this basic dimension. Is that correct?

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Reply:

Thanks for the example and graphic. It really helps to have an application to discuss.

The ASME standard sets the datum as the origin of all basic dimensions. ETI’s Fundamentals of Geometric Dimensioning and Tolerancing textbook illustrates this especially well in Figure 5-10 on pg. 124. A basic dimension may be shown between two non-datum features if there are additional basic dimensions that relate it to the datum reference frame (chained basic dimensions).

What you want to avoid is using a basic dimension that has no relationship to a datum reference frame. This is often termed a “floating basic.” Without the ability to relate to a datum reference frame, these floating basics are immeasurable and cannot be accurately analyzed in a tolerance stack-up.

In your example, Datum A is the primary datum and, as used in the position tolerance, establishes the orientation of the tolerance zone (parallel to datum A). The basic dimension of 10 from datum A established the location of the tolerance zone from datum plane A.

So what else is required? We need to know the horizontal location (left and right variations) of the holes and some additional orientation requirements, because the holes could theoretically spin on the plane of A. Datum references B and C could define that for us, but we don’t know where the holes are from B. The basic dimension of 10 is related to a non-datum feature (this makes it a floating basic).

This drawing will require a section view to show and dimension the relationship between the two pairs of holes. This dimension must be a basic dimension and will allow us to remove the horizontal basic 10.

Michael Adcock
ASME GDTP Senior Level
Dimensional Engineering Mentor