10.2 Patterns, Simultaneous Requirements, and Multiple Segments
Key Takeaways
- The simultaneous requirements rule states that two or more features located by the SAME datum reference frame (same datums, same order, same modifiers) are inspected as one simultaneous requirement — a single setup verifies all of them together.
- If datum references differ in feature, precedence, or material-boundary modifier, the requirements are separate by construction; the notation SEP REQT is used to override simultaneity that would otherwise arise from a common datum reference frame.
- Separate single-segment position or profile callouts that locate features from the same datum reference frame are simultaneous by default; having separate geometric symbols does not by itself make them separate.
- A common trap is assuming features are simultaneous when they repeat datum letters but differ in datum boundary (for example, B at MMB in one frame and B at RMB in another); the boundary difference creates different datum reference frames.
- Use the exact notes SIM REQT and SEP REQT: SIM REQT explicitly groups otherwise separate but compatible requirements, while SEP REQT separates requirements that would otherwise be simultaneous.
10.2 Patterns, Simultaneous Requirements, and Multiple Segments
Quick Answer: Applicable position and profile requirements that locate features or patterns from the same datum reference frame are simultaneous by default in Y14.5-2009. They must all be satisfied from one permissible datum setup. Different datum references create separate requirements. Use SEP REQT to override default simultaneity and SIM REQT to explicitly group otherwise separate, compatible requirements.
What simultaneous means
Two patterns controlled by separate position frames can still be one simultaneous requirement. If both reference A|B|C in the same order and at the same datum material boundaries, the applicable zones must be satisfied together from one realization of that DRF. This matters whenever datum mobility is available: the part cannot use one allowed shift to pass the first pattern and a different shift to pass the second.
The word simultaneous describes the geometric requirement, not a mandate that every point be probed at the same instant. A CMM may measure features sequentially while holding one datum alignment. What is prohibited is independently optimizing or re-establishing the DRF for each member of the simultaneous group.
Default separate cases
Requirements are separate when their datum reference frames differ, including:
- different datum features, such as
A|B|CversusA|D|C; - different precedence, such as
A|B|CversusA|C|B; - different material-boundary modifiers on a datum reference; or
- a different set of referenced datums, such as
A|B|CversusA|B.
Those frames permit different setup logic, so simultaneity cannot simply be inferred from repeated datum letters.
The two exact notes
- SEP REQT means separate requirement. It is placed with requirements that would otherwise be simultaneous so each may use its own permissible realization of the datum reference frame.
- SIM REQT means simultaneous requirement. It explicitly groups requirements that would otherwise be separate, but only where the drawing defines a coherent common setup. The note is not a license to erase contradictory datum precedence or incompatible simulators.
Use the spaces and spelling exactly as shown: SEP REQT and SIM REQT.
Composite and multiple single-segment controls
A composite feature control frame is one structured control with an upper pattern-locating segment and a lower feature-relating segment. Its segments have defined hierarchical roles: the upper segment locates the pattern to its referenced DRF, while the lower segment refines feature-to-feature relationship and any repeated datum-controlled orientation.
Multiple single-segment feature control frames are not composite merely because they are stacked or applied to the same feature. Each frame states a complete control. If separate single-segment requirements locate different features or patterns from the same DRF, however, the simultaneous-requirement rule still applies by default. Separate symbols do not create an automatic exception. Nor does SEP REQT transform the segments of a composite frame into independent controls; a designer who needs different controls must specify the appropriate separate frames.
Worked example — two bolt circles
A flange has an outer 8-hole pattern and an inner 4-hole pattern. Each has its own position feature control frame, and both reference A|B|C with the same datum boundaries. The two patterns are simultaneous by default. One datum setup must allow all 12 feature axes to satisfy their respective zones.
If the designer adds SEP REQT to make the two pattern requirements separate, each pattern may be evaluated with its own permissible DRF realization. If one pattern instead references B at MMB and the other references B at RMB, they are already separate because the datum reference frames differ; a SEP REQT note is unnecessary.
Decision table
| Specification | Default | Correct treatment |
|---|---|---|
| Separate position/profile FCFs; same datums, order, and boundaries | Simultaneous | One permissible DRF realization satisfies all; use SEP REQT only to separate |
| Different datum feature, order, set, or boundary | Separate | Evaluate each requirement from its stated DRF |
| Multiple single-segment FCFs locating features from the same DRF | Simultaneous where the rule applies | Separate symbols alone do not cancel simultaneity |
| Composite FCF | Structured composite control | Interpret upper and lower segments by their defined roles; notes do not convert its structure |
| Otherwise separate but compatible requirements explicitly grouped | Simultaneous by note | Use the exact notation SIM REQT and ensure a coherent common setup |
Senior-level checks
- Compare the full datum reference: letters, order, omitted datums, and modifiers.
- Do not confuse a common DRF with separate inspection optimization.
- Spell the notes exactly:
SEP REQTandSIM REQT. - Do not assume multiple single-segment frames are separate by default.
- Do not use a note to paper over incompatible datum schemes.
Two bolt patterns on a part are both located to A|B|C with all datum references at the same boundaries. What is the default requirement?
Two patterns reference the same datum letters, but the first references datum B at MMB and the second references datum B at RMB. Are they simultaneous or separate requirements?
A drawing shows two position callouts, each with its own position symbol, both referencing the same A|B|C datum reference frame. What is the default, and how can the designer make them separate?
Within a composite position frame, how is the simultaneous requirement applied?