2.3 Feature Control Frame Structure and Reading

FCF Anatomy — Left to Right

A feature control frame is a rectangular box divided into compartments. Reading left to right:

┌──────┬──────────────┬───────┬───────┬───────┐
│  ⌖   │  Ø0.2 Ⓜ    │  A  │  B  │  C  │
└──────┴──────────────┴───────┴───────┴───────┘
  (1)      (2)            (3)   (4)   (5)

1. Geometric characteristic symbol — the type of control (position, profile, flatness, etc.).
2. Tolerance zone — zone shape prefix (Ø for cylindrical, Ⓢ for spherical, no prefix for a width zone), tolerance value, and any material condition modifier (Ⓜ MMC, Ⓛ LMC; RFS is implied by default in 2009 and shown by no symbol).
3. Primary datum reference — the first datum in the DRF, the most constraining.
4. Secondary datum reference — constrains remaining degrees after the primary.
5. Tertiary datum reference — constrains the final degrees (typically location in one direction or clocking).

Each compartment has a fixed meaning; the order is not interchangeable. Swapping the primary and secondary datum changes the DRF and the inspection setup. A control written A|B|C is not equivalent to B|A|C.

Characteristic Symbol Catalog

Form characteristics (straightness, flatness, circularity, cylindricity) never reference datums — a form FCF with a datum reference is malformed. Profile of a line/surface may reference datums optionally: without datums, profile controls only form (the shape of the true profile); with datums, profile controls orientation and/or location to the DRF. This optional-datum rule is a frequent Senior question.

Reading an FCF Aloud

A useful Senior technique is to verbalize the FCF in plain language:

• ⌖ | Ø0.2 Ⓜ | A | B | C reads as: The axis of the feature must lie within a Ø0.2 cylindrical tolerance zone at maximum material condition, the zone oriented and located relative to primary datum A, secondary datum B, and tertiary datum C.
• ⌢ | 0.3 | A reads as: The surface must lie within a 0.3-wide zone bounded by two parallel surfaces 0.3 apart, centered on the true profile, oriented to datum A.
• // | 0.05 | A reads as: The surface must lie between two parallel planes 0.05 apart, parallel to primary datum A.

Single-Segment vs Multi-Segment vs Composite

A single-segment FCF has one row of compartments and controls one relationship to one DRF. A multi-segment FCF stacks two or more independent single-segment rows (each with its own symbol) when two different controls apply to the same feature — for example, a position control and a perpendicularity control on the same hole.

A composite FCF shares a single characteristic symbol across multiple segments (rows). Composite position is the common case:

┌──────┬──────────┬─────┬─────┬─────┐
│  ⌖   │  Ø0.4 Ⓜ│  A  │  B  │  C  │
├──────┼──────────┼─────┼─────┼─────┤
│  ⌖   │  Ø0.1 Ⓜ│  A  │     │     │
└──────┴──────────┴─────┴─────┴─────┘

• The upper segment (Pattern-Refining Feature Control, PLTZF) locates the pattern to the DRF: Ø0.4 at MMC to A|B|C.
• The lower segment (Feature-Relating, FRTZF) refines the pattern internally: Ø0.1 at MMC, oriented to A only. The lower segment controls hole-to-hole location within the pattern, not pattern-to-datum location.

Composite profile works similarly: the upper segment controls profile to the full DRF, the lower segment refines the profile's form and orientation to a subset of datums. The Senior exam tests the rule that composite segments share the symbol and the lower segment cannot have a larger tolerance than the upper. A lower segment datum reference must be a subset (in order) of the upper segment datums.

Reading Order and What Each Compartment Decides

• The symbol decides the characteristic and whether datums are allowed.
• The tolerance compartment decides zone shape (prefix), size (value), and material condition (modifier).
• The datum compartments decide which surfaces simulate the DRF and in what precedence.
• The modifiers on datums (Ⓜ/Ⓛ or implied RFS) decide how each datum feature is established (at its MMC boundary, LMC boundary, or actual mating envelope).

Missing compartments carry meaning. An FCF with no tertiary datum leaves one rotational degree of freedom unconstrained; an FCF with no datum compartments at all (for profile of a surface) controls only the form of the true profile, not its location.

Common FCF Reading Errors at the Senior Level

The Senior exam rewards candidates who read the FCF literally rather than assuming design intent. Three reading errors appear repeatedly. First, confusing the tolerance compartment's prefix with the zone shape: a position control written without the diameter symbol is a width zone (two parallel planes) applied to a center plane, not a cylindrical zone. A position callout of 0.2 (no Ø) on a slot is a width zone on the slot's center plane; reading it as a cylinder reverses the geometry and the acceptance boundary. Second, misreading a datum modifier as a tolerance modifier: the symbol Ⓜ in the tolerance compartment applies bonus tolerance to the controlled feature, while the same symbol Ⓜ in a datum compartment permits datum shift between the datum feature and its simulator. These are two different effects that share one glyph, and a candidate who conflates them will miscalculate both the acceptance boundary and the functional gage. Third, assuming a multi-segment frame is always composite: two stacked single-segment frames, each carrying its own characteristic symbol, are independent controls that both fully apply; a composite frame shares one symbol across its segments, and the lower segment only refines orientation or pattern location without re-locating to the datums. Reading the symbol placement and the segment sharing correctly is what distinguishes these cases.

Datum Modifiers in the Datum Compartments

A datum reference may carry its own material condition modifier (Ⓜ or Ⓛ) in Y14.5-2009, applied to the datum feature simulator rather than to the controlled feature. A datum referenced at MMC, written for example as AⓂ, allows the simulator to shift as the datum feature departs from MMC — this is datum shift, a freedom of the datum reference frame itself, not bonus tolerance on the controlled feature. Datum shift is especially common in composite position frames and on patterns located to a datum feature of size. The Senior distinction is precise: bonus tolerance grows the controlled feature's tolerance zone as the feature departs from its material condition; datum shift moves the entire DRF relative to the part as the datum feature departs. They are independent mechanisms, and a candidate who treats datum shift as bonus will mis-state the gage design and the worst-case acceptance boundary. When no modifier is shown on a datum reference in Y14.5-2009, the datum is applied regardless of material boundary (the RFS default), and the datum feature simulator contacts the datum feature at its actual mating envelope with no shift available.