10.1 Composite Position Tolerancing

10.1 Composite Position Tolerancing

Quick Answer: A composite position control is one position symbol with two or more segments. The upper segment is the Pattern Locating Tolerance Zone Framework (PLTZF) — it locates the whole pattern to the datums. The lower segment is the Feature Relating Tolerance Zone Framework (FRTZF) — it refines feature-to-feature within the pattern. The lower segment may drop a locating datum; it never re-locates the pattern to the part.

Why two levels are needed

A pattern of holes has two independent geometric concerns: (1) where the pattern as a group sits on the part (location to the datums), and (2) how accurately the holes are spaced relative to each other within the pattern. Functionally these are different — a bolt circle may need to be located to the part within ±0.25, while the holes within the circle must be spaced to each other within ±0.05 so all four bolts engage a mating cover. A single position tolerance cannot separate these; either it is loose enough for location (and the holes can drift within the pattern) or tight enough for spacing (and the pattern is over-controlled to the part). Composite position separates them into two segments of one control.

Anatomy of the feature control frame

| ⊕ | Ø0.5  | A | B | C |
|    | Ø0.1  | A | B |   |

One position symbol (⊕) governs both rows — this is what makes it composite. The upper row is the PLTZF: Ø0.5 to A|B|C, locating the pattern. The lower row is the FRTZF: Ø0.1 to A|B, refining the pattern internally. The lower segment carries a smaller tolerance, and it may reference fewer datums.

Rules for which datums carry into each segment

1. The upper (PLTZF) segment must reference all datums needed to locate the pattern — typically the primary, secondary, and tertiary datums (A|B|C).
2. The lower (FRTZF) segment may drop one or more datums, but the datums it does reference must appear in the same order as in the upper segment. You cannot reorder or introduce new datums in the lower segment.
3. A datum dropped from the lower segment means the pattern is free to translate (and in some cases rotate, if a datum constraining rotation is dropped) as a group relative to that datum — but the feature-to-feature relationship within the pattern stays at the tighter FRTZF value.
4. If a datum is repeated in the lower segment, it constrains only what that datum controls at that level — typically orientation, not location. For example, repeating A (a planar primary datum) in the FRTZF keeps the pattern perpendicular to A but does not re-locate the pattern along B or C.

Worked example — 4-hole bolt circle

A flange has a 4-hole bolt circle of Ø80 with four Ø10 holes. The drawing specifies:

| ⊕ | Ø0.5 | A | B | C |
|    | Ø0.1 | A | B |   |

Where A is the flange face, B is the Ø40 central bore, and C is a radial datum notch.

• PLTZF (upper, Ø0.5 to A|B|C): the four true positions of the holes form a perfect Ø80 bolt circle located to A, B, and C. Each hole's axis must lie within a Ø0.5 cylinder centered on its true position. This controls where the bolt circle sits on the flange — its height relative to A, concentricity to B, and angular position relative to C.
• FRTZF (lower, Ø0.1 to A|B): the four holes must also lie within Ø0.1 of their true positions relative to each other, with the pattern oriented to A (perpendicularity) and B (clocking). Because C is dropped, the pattern may rotate as a group around B by up to the PLTZF allowance, but the four holes stay spaced to Ø0.1 internally.

Verification: the CMM first checks the pattern's location to A|B|C against Ø0.5 (PLTZF). Then it checks each hole against its true position within the pattern at Ø0.1 (FRTZF), using only A and B for orientation. The lower check is not a stricter location of the pattern to C; it is a refinement of the holes to each other.

Reading-decision table — which datum controls what

Reading a segment from the frame

To read any composite position frame:

1. Confirm one position symbol governs both segments. If each segment has its own position symbol, it is not composite — it is two single-segment controls (see §10.2).
2. Read the upper segment as the pattern's location tolerance to the full datum reference frame.
3. Read the lower segment as a refinement of the pattern internally, with only the datums shown (same order as upper, possibly fewer).
4. Note any datum modifiers (MMC, LMC, RFS implied) — they must be considered per segment. A modifier on a datum in the upper segment does not automatically apply to the lower; the lower segment's datum references are read independently for modifiers shown there.

Senior-level traps

1. Treating the FRTZF as a stricter location to all datums. The lower segment refines the pattern internally; if it drops C, the pattern is free to rotate as a group relative to C. A candidate who applies Ø0.1 to all three datums is over-constraining the part and rejecting good hardware.
2. Reordering datums in the lower segment. The FRTZF may only drop datums from the end of the upper sequence; it cannot reorder them. Upper A|B|C allows lower A|B or A, but not B|A or A|C.
3. Adding a new datum in the lower segment. The FRTZF may only use a subset of the upper segment's datums. Introducing a new datum is not permitted in a composite frame.
4. Forgetting that the lower segment's tolerance is a refinement, not a replacement. A feature must satisfy both segments simultaneously — the lower segment adds a tighter internal constraint, it does not relax the upper one.
5. Misreading multiple single-segment frames as composite. Two separate single-segment position frames (each with its own symbol) are independent controls, not a PLTZF/FRTZF pair. The composite frame is identified by the single position symbol shared across segments.