5.3 Functional Datum Feature Selection, Multiple Datum Reference Frames & Simultaneous vs. Separate Requirements

5.3 Functional Datum Feature Selection, Multiple Datum Reference Frames & Simultaneous vs. Separate Requirements

Quick Answer: Three closely related paragraphs govern how many datum reference frames a part has and which features build them. Para. 4.15 requires that only the required datum features be referenced in a feature control frame, with the functional requirements of the design as the basis for selection. Para. 4.14 permits more than one datum reference frame on a part, and requires that where the relationship between reference frames must be determined, that relationship shall be specified. Para. 4.19 defines a simultaneous requirement: two or more geometric tolerances that apply as a single pattern or part requirement. It applies to position and profile tolerances located by basic dimensions and related to common datum features referenced in the same order of precedence at the same boundary conditions — and in a simultaneous requirement there is no translation or rotation between the datum reference frames of the included tolerances. SEP REQT overrides it.

---

Functional Datum Feature Selection (Paras. 4.8, 4.15, 4.18)

Para. 4.8 states the selection criterion directly: a datum feature is selected on the basis of its functional relationship to the toleranced feature and the requirements of the design. The standard adds three practical qualifiers:

• To ensure proper assembly, corresponding interfacing features of mating parts should be selected as datum features.
• 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; on symmetrical parts or parts with identical features, physical identification of the datum feature on the part may be necessary.

Para. 4.15, Functional Datum Features, adds the discipline that separates a competent drawing from a cluttered one: only the required datum features should be referenced in feature control frames. Understanding the geometric control a given tolerance provides is necessary to determine how many datum feature references a given application actually needs.

How Many Datum References Does a Tolerance Need?

The over-referencing trap. A stem shows ⟂ | 0.05 | A | B | C on a face perpendicular to A. Adding B and C constrains degrees of freedom that an orientation tolerance cannot use, and none of them changes the tolerance zone. Para. 4.15 is the authority for calling that specification excessive.

Precedence Follows Function, Not the Alphabet

Datum precedence should mirror the order in which the part actually seats in its assembly: the feature carrying the most contact and the largest seating load is normally primary, the feature providing the next constraint is secondary, and the clocking feature is tertiary. The standard's own worked example follows exactly this logic — a shoulder that carries the bolt load into contact becomes primary, a pilot diameter becomes secondary, and a clocking feature becomes tertiary.

---

Multiple Datum Reference Frames (Para. 4.14)

More than one datum reference frame may be necessary for certain parts, depending upon functional requirements. A gearbox housing may locate its bearing bores from one frame and its mounting-foot holes from a different frame, because the two feature sets mate with different parts.

The requirement that follows is the tested one: where more than one datum reference frame is used and it is necessary to determine the relationships and calculate boundaries between the reference frames, the relationship between the datum reference frames shall be specified. In other words, two independent frames on one part are legal — but the moment a stack-up must cross from one frame to the other, the drawing owes the reader a defined relationship, typically expressed with basic dimensions and a geometric tolerance tying one frame's datum features to the other's.

        ONE PART, TWO DATUM REFERENCE FRAMES (para. 4.14)

   ┌──────────────────────────────────────────────────────────┐
   │  Datum features A + B  ────►  DRF #1  ──► bearing bores   │
   │                                                           │
   │  Datum features C + D  ────►  DRF #2  ──► mounting holes   │
   └──────────────────────────────────────────────────────────┘
     Legal. But if a stack-up must cross from DRF #1 to DRF #2,
     the RELATIONSHIP between the two frames must be specified.

Related tools: para. 4.12.5 permits only part of a surface to serve as a datum feature, indicated by a chain line drawn parallel to the surface profile and dimensioned for length and location, by a note, or by a datum target. Para. 4.21 allows the axes or center planes of an established datum reference frame to be labeled on the drawing where illustrating them aids clarity.

---

Simultaneous and Separate Requirements (Para. 4.19)

A simultaneous requirement is where two or more geometric tolerances apply as a single pattern or part requirement. The standard states its scope precisely, and every element of that scope is a possible exam discriminator:

1. It applies to position and profile tolerances. Orientation and form tolerances are not simultaneous requirements.
2. The features must be located by basic dimensions.
3. They must be related to common datum features.
4. Those datum features must be referenced in the same order of precedence.
5. They must be referenced at the same boundary conditions — an A | B Ⓜ | C frame and an A | B | C frame are not a simultaneous requirement, because B is at MMB in one and RMB in the other.

When all five conditions hold, there is no translation or rotation between the datum reference frames of the included geometric tolerances — the separate patterns are locked to one another as rigidly as if they had been called out in a single frame, and a functional gage must check them together on one fixture.

Overriding the Default

Where the design does not require the patterns to be related to each other, the note SEP REQT (separate requirement) is placed beneath each applicable feature control frame. Each pattern is then verified independently, its own datum reference frame free to translate and rotate relative to the other pattern's.

Cost consequence. Simultaneous requirements are stricter and are what a single functional gage enforces. A supplier who discovers two patterns cannot both be held on one fixture is usually asking for SEP REQT — which is a design decision about whether the two patterns must line up in the assembly, not an inspection convenience.