How to choose datums in GD&T: a practical guide
Choose datums from the features that locate the part in its assembly: the primary datum is usually the largest stable mating face, the secondary is the feature that sets orientation or position next, such as a locating bore or edge, and the tertiary stops the last rotation. Datums chosen this way let the shop fixture and the inspector measure the part exactly as it works.
Guilherme Rodrigues ItinoseUpdated 3 min read
Key takeaways
- Datums describe how the part mates, not where the designer started sketching.
- Primary datum: the largest stable contact face; it removes three degrees of freedom.
- Secondary and tertiary datums stop the remaining translations and rotation.
- A datum feature must be real, accessible and measurable on the finished part.
- Dimension functional features from the datum reference frame, not from each other.
Datums are where GD&T starts and where most drawing problems begin. A position tolerance of 0.1 mm means nothing if the inspector cannot reproduce the datum reference frame the designer had in mind.
Think about the assembly first
Ask how the part sits in the machine or product:
- Which face rests on the mating part? That large, stable contact surface is usually the primary datum (A). It removes three degrees of freedom.
- What locates it next? A dowel bore, a shaft or a shoulder that sets position becomes the secondary datum (B).
- What stops the last rotation? A second pin, a slot or an edge becomes the tertiary datum (C).
If the drawing follows the assembly, the shop can build a fixture that mimics it and the inspector can measure on a CMM or surface plate the same way.
Rules for good datum features
| Rule | Why |
|---|---|
| Real and accessible | The inspector must touch it on the finished part |
| Stable and large enough | Small or curved faces rock on the plate |
| Functional | It should matter in the assembly |
| Machined early | Later operations can locate from it |
Common mistakes
- Using the CAD sketch origin or a center plane nobody can touch.
- Choosing a cast or rough surface as primary datum for precision features.
- Dimensioning features chain-to-chain instead of from the datum frame, which stacks tolerances.
- Changing datum order between related parts in the same assembly.
ASME or ISO?
The logic of choosing datums is the same in ASME Y14.5 and ISO GPS; the symbols and some default rules differ. State the standard on the drawing so nobody guesses.
We review and detail drawings with datums that the shop can fixture and inspect. See CAD engineering and manufacturing documentation.
When to outsource this
- Your drawings use datums that the inspector cannot fixture
- You are moving from ± tolerances to GD&T on critical parts
When not to
- Simple parts where general tolerances already guarantee function
FAQ
Do I always need three datums?
No. Use as many as needed to control the toleranced feature. A cylindrical part may only need a primary face and a secondary axis.
Can a datum be an axis?
Yes. A cylindrical feature such as a bore or shaft can be a datum feature; its axis becomes the datum.
What is the most common datum mistake?
Choosing a feature for convenience in CAD, like the edge where the sketch started, instead of the surface that actually mates in the assembly.
Is the datum order important?
Yes. The order of datums in the feature control frame defines which contacts first, and that changes how the part is fixtured and measured.
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