Properly defining, qualifying, and referencing datum features is essential to ensure that parts are properly manufactured and inspected.  To begin exploring proper use of datum features, it is helpful if we first review definitions and rules for selecting, identifying, and qualifying these features.

A Datum Feature is a feature on a physical part that is used to reference other features.  The ASME Y14.5 2009 Standard tells us, “A datum feature is selected on the basis of its functional relationship to the tolerance feature and the requirements of the design.  To ensure proper assembly, corresponding interfacing features of mating parts should be selected as datum features.  However, a datum feature should be accessible on the part and of sufficient size to permit its use.  Datum features must be readily discernible on the part.  Therefore, in the case of symmetrical parts or parts with identical features, physical identification of the datum feature on the part may be necessary. “

Note that a datum feature is different from a datum, which refers to an intangible, theoretical feature with perfect form.  When referring to a datum feature on a part or drawing, “datum feature” is the proper term.  A datum simulator is what we call the theoretical surface that we use to approximate the datum feature for measurement purposes.  In the example, Fig. 1 below, a part with a planar datum feature ‘A’ can be placed on a granite block to locate adjacent features.  The datum feature is the actual part surface, and the surface of the granite block that we are measuring from is the datum simulator. 

Fig. 1

Datum features can be permanent or temporary.  Temporary datum feature may be used for in-process parts such as castings, forgings, machinings or fabrications.  Temporary datum features may or may not be later modified or removed by subsequent manufacturing processes.  Permanent datum features should be selected so that they are not appreciably changed by subsequent processing. 

One common mistake when selecting datum features is to use an axis, centerline, or center plane.  According to the ASME standard, these items cannot be datum features.  This is because multiple features could share the same centerline, center plane, or axis, making it difficult to determine which feature should be prioritized as a datum feature for inspection or manufacturing purposes. 

Figure below illustrates the proper form and proportion of Datum Feature Symbols

Image Source: ASME Y14.5 – 2009 Standard – Appendix C, page 194

According to the 2009 ASME Y14.5 standard, datum features must be controlled (also known as datum qualification) by applying appropriate geometric tolerances, or through Rule #1 with size for features of size.  The 1994 version of the same standard is less strict and says only to consider applying tolerances to datum features.  Typical GD&T constraints used to qualify primary datums are size, circularity, cylindricity, linearity, flatness, profile, or location.  Secondary and Tertiary datum features further define orientation and/or location, referencing back to higher precedence datums.  Typical constraints for Secondary and Tertiary datums can be divided into two different categories, depending on the manufacturing process.  For milled parts, perpendicularity, angularity, parallelism, location, and profile are common, while perpendicularity, runout, total runout, and profile are typical.  This is what we call Datum feature qualifying.

In the example, Fig 3 drawing below, the primary datum feature A is the part surface highlighted in red and is qualified with flatness.  Datum A is a plane.  The secondary datum feature B is highlighted in blue and is qualified with perpendicularity in reference to datum A.  Datum B is also a plane.  Finally, datum feature C is highlighted in green, and it is qualified with perpendicularity in reference to both datum plane A and datum plane B.  Datum C is also a plane, and reference to two higher-order datums makes it a tertiary datum.

Fig 3.

In practice, qualification of datum features with their own tolerance is important so that inspection considers the possibility for error in the datum features.  If no tolerance is shown, errors in datum features may magnify errors in other areas, leading to unnecessarily rejected parts.

Check out the video overview of relating secondary datum features to primary datums below: