4.1 Multiple Datum Features and Datum Reference Frames
Key Takeaways
- A datum reference frame (DRF) is the complete three-dimensional coordinate system of three mutually perpendicular datum planes (primary, secondary, tertiary) that together arrest the six degrees of freedom of a part; every toleranced feature is measured from this frame.
- Y14.5-2009 uses the term multiple datum features when two or more datum features act together to establish one datum; the simulators contact all identified features simultaneously to derive a single plane, axis, or center plane.
- A single drawing may carry multiple DRFs when different features are located by different functional references; the Senior-level judgment is to use the minimum set that preserves functional intent.
- Once the DRF is established, features are located by basic dimensions from the DRF origin; the datums themselves carry no tolerance — only the relationship between the DRF and each controlled feature does.
- When two or more feature control frames share the same datums in the same order with the same modifiers, ASME Y14.5-2009 treats them as a simultaneous requirement by default — one setup, all zones must fit at once.
The Datum Reference Frame Concept
A datum reference frame (DRF) is the complete three-dimensional coordinate system from which datum-referenced geometric tolerances are evaluated. A rigid part has six degrees of freedom: three translations and three rotations. The ordered datum references in a feature control frame constrain the applicable degrees of freedom and establish the origin and orientation for the tolerance zone.
A typical rectangular part uses three mutually perpendicular planar datum features:
- Primary datum A constrains one translation normal to the plane and two rotations: 3 DOF.
- Secondary datum B constrains one additional translation and one rotation: 2 DOF.
- Tertiary datum C constrains the final translation: 1 DOF.
The order matters. A|B|C and A|C|B are different datum reference frames because B and C perform different secondary and tertiary roles.
Multiple Datum Features in Y14.5-2009
Y14.5-2009 uses multiple datum features for two or more physical datum features that act together to establish one datum. Their datum feature simulators contact the identified features simultaneously, and the combined contact establishes one perfect datum plane, axis, or center plane. This is not one feature serving as two datums, and it is not an ordered primary-secondary reference.
Worked Example: Two Journals Establish One Datum Axis
Suppose a shaft is supported in service by two separated, nominally coaxial bearing journals. If both journals are identified as the multiple datum features that establish datum A, the inspection setup simulates both journals at the same time and derives one datum axis from their combined contact. A runout or position control referenced to datum A is evaluated from that single jointly established axis.
Do not rewrite that function as A|B: separate compartments specify datum precedence, with A established before B. The revision-specific vocabulary matters: the 2009 body of knowledge calls this multiple datum features, while the 2018 body of knowledge uses common datum features.
A Cylindrical Primary Does Not Fully Lock the Part
A cylindrical datum feature used as the primary datum establishes an axis and constrains four DOF: two translations perpendicular to the axis and two rotations about perpendicular axes. It still permits translation along the datum axis and rotation about the datum axis. A planar end face may be referenced second to constrain axial translation, and a keyway, slot, or other clocking feature may be referenced third to constrain rotation. The resulting sequence is written in separate datum compartments, such as A|B|C.
Multiple DRFs in One Drawing
A drawing may use multiple DRFs when different controlled features have different functional interfaces. Each feature control frame carries the datum reference sequence appropriate to that requirement; there is no single mandatory global DRF for the whole drawing. Use the minimum number of DRFs that preserves function, because every additional setup adds inspection effort and another relationship to manage.
Once a DRF is established, basic dimensions locate the true position or true profile relative to it. The datum features establish the reference; the controlled feature's geometric tolerance defines the permissible zone.
Simultaneous Requirements (Preview)
Separate position or profile requirements that locate features or patterns from the same datum reference frame are simultaneous by default under Y14.5-2009: one valid DRF setup must satisfy all of the applicable tolerance zones together. The note SEP REQT overrides that default when the requirements are intended to be evaluated separately. Requirements using different datum references are separate by their construction. Chapter 10 develops the rule and its exceptions.
How many degrees of freedom does a primary planar datum feature arrest?
A cylindrical bore is used as the primary datum feature. Before any secondary or tertiary datum is referenced, which degrees of freedom remain unconstrained?
Which is a valid reason to use multiple DRFs on one drawing under ASME Y14.5-2009?