GD&T concept · Rule

Free State (Ⓕ) — Non-Rigid Parts

The Ⓕ modifier: the tolerance applies to the part unrestrained — no clamping, no fixturing, natural shape.

Definition

The free-state modifier Ⓕ — the letter F in a circle, placed in the tolerance compartment after the tolerance value — states that the tolerance applies with the part completely unrestrained: no clamps, no fixtures, no fasteners, nothing but gravity. Free state means the condition where forces other than gravity are not applied; the part shows its natural shape. For rigid parts the distinction is academic — clamping does not meaningfully move a steel block. For non-rigid parts it is everything: thin sheet metal, gaskets, seals, and plastic parts deflect visibly under their own weight, and clamping them flat during inspection measures the fixture as much as the part.

The default, and why Ⓕ exists at all

Under ASME Y14.5, dimensions and tolerances are verified in the free state by default — no symbol is needed for that to be true. Ⓕ exists because of its interaction with the restrained-condition note: when a drawing requires a part to be restrained for inspection, Ⓕ attached to a specific tolerance opts that tolerance back out, requiring it in the free state while everything else is checked restrained. It is a per-tolerance override on a drawing-level requirement, and its most common home is exactly there — a restrained drawing with a few tolerances that only make sense unrestrained.

Left: the part in its free state — no forces applied beyond gravity, drawn in its natural deflected shape; a tolerance carrying Ⓕ is verified in this state. Right: the same part restrained to datum simulators per a restrained-condition note — forced toward the shape it assumes mounted in service. The note is text (there is no standardized restrained symbol); Ⓕ exists to opt individual tolerances back out of a drawing-wide restrained requirement. Schematic, exaggerated.

Average diameter: a special application

Circularity of a flexible cylinder — a rolled sheet-metal ring, a seal — carries a subtlety: the part can be out-of-round yet still function, because it will be forced round when installed. Y14.5 therefore allows circularity of a non-rigid part to be applied to the AVERAGE diameter: enough measurements are taken around the feature to establish an average, and circularity is evaluated about that. The Ⓕ symbol marks the tolerance as applying in this unrestrained, averaged condition — the part is judged on how round it naturally is, not on how round the fixture can make it.

Reading a feature control frame

⌓ 0.5 Ⓕ A reads as: profile of a surface, zone 0.5 wide, evaluated in the free state, referenced to datum A. The Ⓕ occupies the usual modifier slot after the tolerance value. Its presence on a drawing that carries a restrained-condition note is the signal: this particular tolerance is exempt from the restraint — measure the part loose.

Ⓕ after the tolerance value: this zone is verified with the part unrestrained — no clamping, no fixturing, natural shape under gravity alone.

When to use it

Non-rigid parts generally: sheet metal, gaskets, seals, springs, thin-walled plastic housings
Any tolerance on a drawing that carries a restrained-condition note, where this tolerance must still be verified unrestrained
Circularity of average diameter on flexible cylindrical parts
Never on a genuinely rigid part — the modifier adds nothing where clamping cannot move the feature