Datum Reference Frame Definition Template
A datum reference frame is the coordinate system every position, profile and runout tolerance on the drawing is measured from. When the shop, the CMM and the design disagree about how to hold the part, the same part can measure good and bad. This template records the scheme once.
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Datum Reference Frame Definition Template
Datum Reference Frame Definition
Company: Acme Precision Ltd Part: PN-48213 Rev C Prepared by: M. Iyer, Manufacturing Engineering on 2026-10-20
Datum features and simulators
| Datum | Feature on part | Physical simulator | Degrees of freedom constrained |
|---|---|---|---|
| A (primary) | Mounting face, planar surface (3 points of contact) | Ground fixture plate, flatness 0.005 | 1 translation, 2 rotations |
| B (secondary) | Bore B1 diameter 25 H7, axis (4 DOF with A) | Expanding mandrel, 25.000 mm nominal | 2 translations |
| C (tertiary) | Side slot, width 8 mm (rotation about B) | 1 rotation |
Material condition modifier on B
RFS (regardless of feature size)
Fixture and setup notes
Clamp on rib R2 only, torque 3 Nm, to avoid distorting the bore.
Approvals
The datum precedence A, B, C and the six degrees of freedom model follow common GD&T practice (ASME Y14.5 and ISO 5459 describe the rules). Verify against the standard your drawing cites.
Link setup documents to ERP routings
Optional review of how fixture and datum documentation could attach to operations.
Optional. The tool and every export work without this. No spam.
How to use it
Name the real feature that establishes each datum, the physical simulator you will use to contact it (plate, mandrel, collet, soft jaws) and which degrees of freedom each datum removes. A part has six: three translations and three rotations. The primary datum usually removes three, the secondary two and the tertiary one. Share the sheet with tooling and the CMM programmer.
Reading the datum precedence
Order matters. A, B, C is not the same as B, A, C because the primary datum gets first contact. The material condition modifier on a datum feature of size changes whether the simulator is fixed at size or can float, which affects shift and therefore measured position.
- Primary: most stable, usually a large planar face
- Secondary: constrains location perpendicular to the primary
- Tertiary: stops the last rotation or translation
- RFS simulators expand or contract to the actual size of the feature
Common mistakes
Drawings sometimes name a datum that cannot physically be contacted, or the shop fixture locates on features different from the drawing datums. Both create arguments at final inspection. Also check that clamping does not distort thin features before you measure.
Frequently Asked Questions
What is a datum reference frame?
It is the set of up to three mutually perpendicular planes established by datum features in the order given in a feature control frame. All the related tolerances are measured relative to it.
What are the six degrees of freedom?
Translation along and rotation about each of the three axes. Datums A, B and C together remove all six, so the part location is fully defined.
Do I always need three datums?
No. A feature may need only one or two datums depending on which orientation or location it controls. Use as many as the function requires.
What is a datum feature simulator?
It is the real equipment surface, such as a gauge plate or mandrel, that stands in for the theoretically perfect datum during measurement.
If your ERP routings do not carry the fixture and setup information, we can discuss attaching setup documents to operations.
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