GD&T concept · Rule

Continuous Feature (CF)

The CF symbol that makes separated, interrupted features evaluate as one single continuous feature.

Definition

The continuous feature symbol — the letters CF in an irregular hexagon, or the note CONTINUOUS FEATURE — declares that two or more physically separate features are to be evaluated as one single continuous feature. The separation can be grooves turning one cylinder into three segments, or slots splitting one face into two. Without CF, each segment is its own feature: its own size limits, its own Rule #1 envelope, its own geometric zone. With CF, one dimension and one set of controls governs them all, exactly as if the interruption were not there.

The interrupted feature of size: one `Ø30 CF` dimension spans all three segments of the boss, so Rule #1 and any geometric controls treat them as a single continuous feature — the grooves do not reset form or location between segments. The symbol is the letters CF in an irregular hexagon; the note CONTINUOUS FEATURE is the text equivalent. Without CF, each segment would be its own feature of size, free to tip and bow independently within its limits. Schematic, exaggerated.

What changes when CF is applied

On a feature of size, CF unifies the size and the form boundary: a boss interrupted by O-ring grooves becomes one Ø30 feature of size, so Rule #1 applies a single perfect-form-at-MMC envelope over all segments, and any geometric control — perpendicularity, position — applies to the unified feature rather than per segment. That is usually the point: the mating O-ring or seal rides across the interruption and only a single continuous envelope describes what it actually feels.

The 2018 extension: surfaces and datums

ASME Y14.5-2009 introduced CF for features of size only. The 2018 edition extends it: the symbol may now follow a feature control frame controlling surfaces — so a flatness callout with CF over two faces separated by a slot evaluates both faces as one continuous surface — and may accompany a datum feature symbol, treating separated datum features as one datum. An optional nX (e.g. 2X) beside the symbol states how many features are being united; it is clarification, not requirement.

The 2018 extension to surfaces: flatness frames over two faces separated by a slot, each carrying the CF symbol, so both faces are evaluated as one continuous surface — the flatness zone must hold across the gap as if the slot were not there. Before Y14.5-2018 this needed a profile-with-nX workaround. Schematic, exaggerated.

CF vs. nX

The distinction matters and the two are often confused. 4X on a callout means four INDEPENDENT features, each held separately. CF means the features are NOT independent — they are one. A position callout ⌖ Ø0.5 Ⓜ A B C with 4X gives each hole its own zone; adding CF would fuse them into one requirement evaluated across all four, which is rarely intended. CF exists for the rarer case where separation is physical (grooves, slots, clearance reliefs) but function is continuous.

When to use it

Sealing and bearing surfaces interrupted by grooves — the seal sees one surface, so specify one
Mating ways and slides split by relief gaps, where the mating part bridges the gap
Clamping or bolted faces separated by slots that must stay coplanar as one surface
Interrupted threads or splines where the envelope must span the interruptions