How do you apply the concept of datum reference frame in GD&T?

1 What is DRF?

DRF is a coordinate system that defines the orientation and location of a part or a feature relative to a set of reference planes, called datums. Datums are imaginary, ideal, and infinitely large planes that are derived from real features of the part, such as surfaces, holes, or axes. By using datums, you can establish a common reference for all the geometric tolerances applied to the part or the feature.

2 How to select datums?

When selecting datums, one should consider the design intent, function, and manufacturing process of the part or feature. Datums should be stable, accessible, and repeatable during inspection, and correspond to the primary, secondary, and tertiary planes of the DRF in that order. Additionally, datums should be related to the mating or assembly conditions of the part or feature to minimize the accumulation of tolerances and variations.

3 How to identify datums?

To identify datums on a drawing, you need to use datum feature symbols and datum identifiers. A datum feature symbol is a rectangular box with an extension line that points to the feature that serves as the datum. A datum identifier is a letter (A, B, C, etc.) that labels the datum and indicates its precedence in the DRF. For example, datum A is the primary datum, datum B is the secondary datum, and datum C is the tertiary datum. You can also use modifiers, such as M (maximum material condition) or L (least material condition), to specify how the datum feature should be contacted during inspection.

4 How to apply geometric tolerances to datums?

To apply geometric tolerances to datums, you need to use feature control frames and datum references. A feature control frame is a rectangular box that contains the geometric symbol, the tolerance value, the material condition modifier (if any), and the datum references. A datum reference is a letter that refers to the datum identifier and indicates how the feature is constrained by the datum. For example, A|B|C means that the feature is located and oriented by datum A, then by datum B, and then by datum C. You can also use modifiers, such as M or L, to specify how the feature should be related to the datum at different material conditions.

5 How to interpret geometric tolerances with datums?

To interpret geometric tolerances with datums, you need to understand how the DRF establishes the tolerance zone for the feature. The tolerance zone is a three-dimensional space within which the feature must lie to meet the geometric requirement. The shape, size, and position of the tolerance zone depend on the type of geometric tolerance, the tolerance value, and the datum references. For example, a position tolerance with a value of 0.1 and datum references A|B|C means that the feature must be within a cylindrical tolerance zone with a diameter of 0.1 that is aligned with datum A, perpendicular to datum B, and parallel to datum C.

6 What are the benefits of using DRF in GD&T?

Using DRF in GD&T offers several advantages for designers and manufacturers alike, such as providing a clear and consistent way of defining and communicating the geometry of parts and features. This reduces ambiguity and errors in interpretation and measurement, while allowing for flexibility and optimization in design and manufacturing. Additionally, it ensures the functionality and interchangeability of parts and assemblies, as well as improves quality while reducing costs.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?