Feature Control Frame — Anatomy & Reading

The box that carries a geometric requirement: characteristic symbol, tolerance value, modifiers, and datum references.

Definition

The feature control frame is the box that carries a geometric requirement on a drawing. Every control in this library — position, flatness, profile, runout, all of them — is written inside one: the characteristic symbol, the tolerance, and the datums that locate the zone. The frame is a complete requirement in itself: nothing about the geometric control lives anywhere else on the drawing. The dimension lines give the geometry; the frame gives the geometric requirement on that geometry.

The compartments

A frame is read left to right, exactly as written. Compartment 1 holds the geometric characteristic symbol — which control applies. Compartment 2 holds the tolerance value: a Ø prefix means a diameter zone (a cylinder around an axis), no prefix means a width between two lines or planes; a material condition modifier (Ⓜ, Ⓛ, or Ⓘ) may follow the value in the same compartment. The remaining compartments hold datum references in precedence order — primary first, then secondary, then tertiary — and each datum letter may carry its own modifier.

How to read one

⌖ ⌀0.5 Ⓜ A B C reads: position control, 0.5 diameter zone at MMC, located from datum A first, then B, then C. Read the tolerance unit as one piece — Ø0.5 Ⓜ — then the datums as the reference frame. A frame with no datum compartments at all (e.g. ⏥ 0.2) is self-referencing: the zone is established from the feature itself, which is what makes a control a Form tolerance. The number of datum compartments is itself information: none = form only, one or more = the zone is anchored to the datum reference frame.

Common variations

• No datum compartment — a Form control, self-referencing (⏥ 0.2)
• One datum — surface-applied orientation controls like ⟂ 0.1 A
• Two or three datums — location controls and anchored profile, e.g. ⌖ ⌀0.5 Ⓜ A B C
• A Ø prefix — the zone is a cylinder, not a width: axis features and features of size
• Modifiers in the tolerance compartment — Ⓜ/Ⓛ for bonus tolerance, Ⓟ for a projected zone, Ⓘ for independency
• Modifiers after datum letters — a datum referenced at its MMC boundary, e.g. A Ⓜ
• Stacked frames — the same feature with several requirements, like composite position: each row is a separate requirement

Common mistakes

• Reading Ⓜ as if it modified the datums — it modifies the tolerance unless a modifier appears after a datum letter
• Assuming an absent modifier means MMC — it means RFS, the default
• Confusing Ø and no-Ø: Ø makes the zone a cylinder, never a width
• Reordering datums — A B C is a precedence order, not a list; swapping them changes the reference frame and the part's meaning
• Attaching the frame to the wrong element — a frame on a surface controls the surface; the same frame below a size dimension controls the axis