2026/06/05
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4 minutes read
In a previous blog post, we introduced Geometric Dimensioning and Tolerancing (GD&T), the symbolic language used by engineers working under the American Society of Mechanical Engineers (ASME) standards to optimally control and communicate allowable variations. In this blog, we’re diving deeper into a core GD&T concept: datums.
But before we get there, we need to understand the feature control frame.
A feature control frame is a standardized way to call out geometric dimensions and tolerances.
Depending on the type of characteristic, it may reference no datums for form tolerances, or up to three datums.
A datum is a theoretical reference point, line, or surface on a part that serves as the origin for measurement and manufacturing. It acts as a frame of reference against which other features are defined and inspected, ensuring consistency in how parts fit and function together.
Datums are usually selected from important functional surfaces or features, such as a flat face, a hole, or an axis, and are labeled with capital letters.
When you place datum symbols on a drawing, you’re instructing the manufacturer on how to align the part within this framework. This makes sure that both the person making the part and the person measuring it are holding it the same way.
Datums are usually labeled A, B, C if there are only three, but you can assign more datums to a part. Plus, different control frames might reference different datums depending on what’s needed.
In mechanical engineering, degrees of freedom refers to the number of independent parameters needed to fully define the position or motion of a mechanical system, encompassing both translational and rotational movements.
Datums are applied in a specific order – primary, secondary and tertiary – to progressively constrain a part in space. Together, they remove the six degrees of freedom that any rigid body possesses. The sequence of datums in a datum reference frame is intentional and meaningful.
It’s important to remember that a manufactured part will never be perfect. When inspecting a manufactured part, the inspector must compare the imperfect part with the theoretically perfect version shown on the drawing. This can only be done by creating a “perfect grid” to measure the manufactured part against. This “perfect grid” is the datum reference frame.
A datum reference frame is the coordinate system created by the datums specified in the part drawing.
The order of the datums referenced in a feature control frame is important for the measurement of the part. The primary datum, which is the first one in the control frame, will be the first to be constrained. It requires at least three points of contact, and controls three degrees of freedom.
The secondary datum, which is the second datum listed in the control frame, will always be perfectly perpendicular to the primary datum within the datum reference frame. The secondary datum requires two points of contact and is constraining an additional two degrees of freedom.
The tertiary datum, which is the third datum listed in the control frame, will always be at 90 degrees to the primary and secondary datums in the datum reference frame. It is the last datum to be fixed during measurement. The tertiary datum requires one point of contact and is constraining the last degree of freedom.
Remember, a datum callout order is a big deal!
A common mistake in reading GD&T drawings is assuming that the alphabetical order of datums (A, B, C) is arbitrary. It is not. The order in which datums appear in the feature control frame dictates the order of precedence.
The order matters because parts are imperfect.
If Datum A is your primary datum, the part is clamped for Datum A first. The part aligns primarily to A. If the angle between Datum Feature A and Datum Feature B is not a perfect 90 degrees, the gap will appear at Datum B.
If you were to switch them, making Datum B the primary, the part would align to B first, and any angular error would be forced onto A.
Changing the order changes the alignment, which changes the measurement results. A part might pass inspection with an A-B-C order but fail with a B-A-C order. This hierarchy ensures we inspect the part based on how it functions in the final assembly.
When it comes to GD&T, not all datums work the same way. Some datums, called designated datums, don’t need any specific geometric tolerance on their features before being used. These are the actual physical surfaces, holes, or edges on a part. They’re directly tied to the datum feature and can be used right away to establish the datum reference frame.
On the other hand, we have qualifying datums. These are a bit different because they do require the geometric tolerance on the datum feature to meet certain criteria first. Basically, you’ve got to ensure the geometric tolerance is met before you can use the datum for referencing.
So, to sum it up: designated datums are ready to go as they are, while qualifying datums need to meet their geometric tolerance requirements before they can be used. It’s a key distinction to keep in mind when working with GD&T!
When selecting datums, choose them based on function, that is how the part sits, locates and clocks within the assembly. The datum order defines the setup order, so it’s best to prefer features that are stable, accessible and repeatable.
You should also use datum modifiers when you want the inspection to reflect the actual clearance in the real-world assembly.
Our High-Velocity Mass Customization (HVMC) model includes inspection as one of our services. To get to know more about our inspection services, including how we use GD&T as an inspection language, email our team at info@protospacemfg.com.
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