10.2 Patterns, Simultaneous Requirements, and Multiple Segments

10.2 Patterns, Simultaneous Requirements, and Multiple Segments

Quick Answer: When two or more features are located by the same datum reference frame (same datums, same order, same modifiers), Y14.5-2009 treats them as one simultaneous requirement — they are inspected together in a single setup. When the datum references differ, they become separate requirements (SEPREQ) and are inspected independently. The default is determined by the datums, not by a note.

The simultaneous requirements rule

If a drawing has multiple features or patterns, each located by a position (or profile) tolerance referencing the same datum reference frame — same datum features, same order, same material condition modifiers — the standard considers them a single simultaneous requirement. In inspection, the part is set up once on the datums and every feature is verified against its true position in that one setup. The features must pass together: if one fails, the requirement fails, even if every other feature is perfect.

The reasoning: the datum reference frame is the common origin. If two patterns are both located to A|B|C with identical modifiers, they share the same origin and orientation, so a single setup measures them to the same frame. Inspecting them separately would allow the datum simulators to shift between setups, hiding the true relationship between the patterns.

When features become SEPREQ

Features are separate requirements when any of the following differ between their datum references:

• Different datum features (one references A|B|C, another A|D|C).
• Different order of the same datums (A|B|C vs. A|C|B).
• Different modifiers on a datum (B at MMC vs. B at RFS, or B at LMC).
• Different degrees of freedom constrained by the datums (e.g., one uses a tertiary datum the other omits).

When any of these differ, the features are inspected independently — each is verified against its own datum reference frame in its own setup. A failure of one does not fail the other.

Composite vs. multiple single-segment frames

A composite frame (one position symbol, multiple segments) always treats the pattern as one simultaneous requirement within each segment: in the PLTZF segment the whole pattern is one simultaneous requirement to A|B|C; in the FRTZF segment it is one simultaneous requirement internally to the repeated datums.

Multiple single-segment frames — each with its own position symbol — are independent controls by default. Two separate position callouts that happen to reference A|B|C are not automatically simultaneous; they are separate requirements unless explicitly noted SIM REOQ (simultaneous requirement). Conversely, features that share datums but should be inspected separately can be noted SEPREQ.

Worked example — two bolt circles on one flange

A flange has two concentric bolt circles: an outer 8-hole circle of Ø10 holes and an inner 4-hole circle of Ø6 holes. Both are located to A|B|C (A flange face, B central bore, C clocking notch), all at RFS.

• Because both patterns share the same datum reference frame (A|B|C, same order, same modifiers — RFS throughout), the standard treats them as one simultaneous requirement. The part is set up on A, B, and C once; the CMM verifies all 12 holes (8 + 4) against their true positions in that single setup. If the outer pattern is in tolerance but the inner pattern is out, the simultaneous requirement fails — both must pass together.
• If the outer pattern were referenced to A|B|C and the inner to A|B only (dropping C), the datum references would differ, and the two patterns would be separate requirements, each inspected in its own setup.
• If the outer pattern were referenced to A|B|C with B at MMC and the inner to A|B|C with B at RFS (no modifier shown), the modifier difference would make them separate requirements, even though the datum names are identical.

Decision table — simultaneous or separate?

Why the rule matters in inspection

Consider two bolt circles that share datums but are inspected separately. The first setup measures the outer pattern and passes; the part is unclamped and re-fixtured for the inner pattern. The second setup shifts slightly relative to the first, and the inner pattern now passes too — but the angular relationship between the two patterns is uncontrolled, because the two setups did not share a common frame. If the two patterns must align rotationally (e.g., to engage two mating covers bolted together), the inspection must be simultaneous to catch misalignment. The simultaneous requirements rule ensures the inspection reflects the functional assembly.

Senior-level traps

1. Assuming features are simultaneous because they share datum names. The modifier on each datum matters. A|B|C with B at MMC is not the same datum reference frame as A|B|C with B at RFS — they are separate requirements.
2. Treating multiple single-segment frames as composite. Two separate position symbols are independent by default; only a shared position symbol across segments makes a composite (and therefore simultaneous) control.
3. Adding SIM REOQ to features with genuinely different datums. Forcing simultaneous inspection when the datums differ can reject good parts or hide real misalignment; the note is meant to override the default for same-datum features, not to merge different datum schemes.
4. Forgetting that a composite frame is simultaneous within each segment. A composite frame's PLTZF is one simultaneous requirement for the whole pattern to the datums, and its FRTZF is one simultaneous requirement for the holes to each other.
5. Ignoring the order of datums when comparing frames. A|B|C and A|C|B are different reference frames because the secondary and tertiary roles differ; they produce separate requirements.