▭ Feature control frame
Feature control frame · Indicadores dimensionales · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
The feature control frame reads left to right: symbol → tolerance value → modifiers → datums. Each cell adds a requirement to the feature it points to on the drawing.
What controls
A rectangle divided into compartments that contains, in a fixed order, everything that defines a geometric tolerance: the characteristic symbol, the zone value with its shape (⌀, S⌀), the modifiers that apply to it (Ⓜ, Ⓛ, Ⓟ...) and, if the control needs them, the datums in their order of precedence. What the frame defines is the zone within which the feature must lie, not a measurement to be taken with an instrument. A frame without datums is a form tolerance; one with datums is orientation, location, runout or profile.
How to verify
Before measuring anything, break down the frame: which feature is indicated (the surface or its axis/median plane, depending on where the leader points), what shape the zone has, whether there is bonus tolerance from a material modifier, and which datums to align to and in what order. That breakdown is literally the inspection program. On a CMM, each compartment translates into a software option: with Ⓜ you have to enable evaluation with bonus, and with Ⓟ you have to project the zone above the face. A GO/NO-GO functional gauge is only valid for frames with Ⓜ.
Datums and notes
The datums occupy the last compartments and their order is order of precedence, not alphabetical. A frame without datums is pure form and is not measured against any external reference; if you see a datum next to a form symbol (flatness, circularity, cylindricity), the drawing has been misinterpreted or the symbol is not the right one.
Workshop guideline values
- General
- Values of 0.1–0.3 mm in position and orientation frames for ordinary machining.
- Accuracy
- 0.02–0.08 mm in frames for close-fitting parts or guided assemblies.
- High precision
- 0.005–0.02 mm in frames for spindles, tooling and precision parts.
Guidance only: the drawing's value always wins.
Common mistakes when interpreting it
- Reading the frame value as if it were a ± tolerance on each axis: with ⌀ it is the full diameter of the zone, not a margin per side.
- Ignoring where the leader points: on the surface it controls the face; on the dimension line it controls the axis or median plane of the feature of size.
- Dropping the modifiers when transferring the frame to the inspection sheet: the same control with and without Ⓜ accepts or rejects different parts.
How to get it in the workshop / if it comes out of tolerance
Read from left to right: 1) symbol, 2) value (with ⌀/S⌀/⌒ if applicable), 3) modifiers (ⓂⓁⓅ...), 4) datums |A|B|C|.
If a part is rejected on a frame, start by checking that what was measured is what the frame says: a leader on the surface controls the face, and one on the dimension line controls the axis or median plane; the result changes completely. Also check whether the value was applied as a diameter or as a radius and whether bonus was taken into account. Many parts reported as out of tolerance are really frames transcribed incorrectly onto the inspection sheet.
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