Ⓕ Free state
Free state · Modificadores de zona · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
On the drawing, “⌓ 0.5 Ⓕ |A|” states a free state requirement with a tolerance of 0.5 mm, measured from datum A.
What controls
Ⓕ means that this particular tolerance must be met with the part unrestrained, with no clamping force straightening it out: just resting under its own weight. It is a modifier intended for non-rigid parts (thin sheet metal, long extrusions, rubber gaskets, plastics, welded assemblies) that change shape depending on how they are held. It matters because these drawings usually carry a note requiring all other dimensions to be measured with the part restrained in a fixture that brings it to its assembled condition; Ⓕ is the exception to that note: this characteristic is verified unrestrained. What it does NOT mean is that the part may be measured any old way: the drawing usually specifies how it must be supported and in what orientation relative to gravity, and that is part of the requirement.
How to verify
Let the part rest on neutral supports in the orientation called out on the drawing and measure it touching it no more than absolutely necessary. CMM probing force alone deflects thin sheet and elastomers, so turn the force down to the minimum or measure non-contact with a laser scanner, structured light or an optical comparator; on very soft parts, optical measurement is the only reliable method. Record the support position on the inspection sheet: the same part in the free state lying flat or on edge gives different values, and without that information the result is not repeatable. Also let the part stabilize thermally, especially plastic parts.
Common mistakes when interpreting it
- Measuring the part clamped in a vise or in the assembly fixture: with Ⓕ that invalidates the result, because the requirement is precisely no external force.
- Reading Ⓕ as a looser tolerance: the zone is the same; what changes is the condition in which it is checked.
- Ignoring the support orientation specified on the drawing: on a flexible part, measuring it flat or on edge changes the result without the part changing.
How to get it in the workshop / if it comes out of tolerance
On the shop floor, a flexible part will give you whatever reading you want depending on how hard you squeeze it, so the criterion has to be settled before you make anything. If the characteristic carries Ⓕ, set up minimal, stable support (three points, a cradle, a clean table) and do not use a vise, clamps or a magnet. If the part is measured in the free state but assembled clamped, don't be surprised if it looks off at the end: the assembly deflection is taken up in the assembly and is not a machining defect.
If readings don't repeat between measurements, it is almost always the setup: reduce the number of support points, check that the part isn't rocking and always repeat the same orientation. If the part is out of tolerance in the free state but fits perfectly in the assembly, talk to design: the characteristic may need to be dimensioned in the restrained condition rather than the free state. And if the deviation shows up after machining, suspect released residual stresses: in sheet metal, from prior laser cutting or bending; in plastic, from molding. A stress-relief anneal or a change in the sequence of operations usually fixes it better than clamping harder.
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