5.5 Free State, Restrained Condition, and Nonrigid Parts

Free State, Restrained Condition, and Nonrigid Parts

Quick Answer: Everything on a Y14.5 drawing applies in the free state unless a note says otherwise. A restraint note specifies which datum features restrain the part, with what force, in what direction. The free-state symbol releases one individual requirement from that restraint. Average diameter lets a nonrigid circular part satisfy size on an average of measurements while form is controlled separately.

A rigid machined block behaves the same on the surface plate as it does in the assembly. A stamped bracket, a thermoformed cover, a thin-wall aluminium casting, and a welded frame do not. They sag under gravity, spring after release, and change shape the moment a clamp touches them. Y14.5 addresses this with a default and three tools, and the Senior body of knowledge lists all of them.

The fundamental rule

One of Y14.5's fundamental rules governs the whole topic:

All dimensions and tolerances apply in a free state condition — that is, with no external forces applied other than the force of gravity — unless otherwise specified.

"Unless otherwise specified" is the entire practical content of this section. For a rigid part the free-state default is invisible, because a rigid part does not care. For a nonrigid part the default is often unusable: a sheet-metal panel measured unclamped may be 3 mm out of flat and perfectly acceptable once it is bolted into the car.

Restrained-condition notes

To measure a nonrigid part the way it is used, the drawing states a restrained condition. A complete restraint note answers four questions:

1. Which datum features restrain the part.
2. How they restrain it — bolts, clamps, vacuum, its own fasteners.
3. What force is applied, with units.
4. What else about the setup matters — orientation relative to gravity, temperature, sequence of clamping.

A typical general note reads along the lines of: unless otherwise specified, all dimensions and tolerances apply in a restrained condition, with the part restrained at datum features A, B, and C using the specified fasteners torqued to the stated value.

Two Senior-level points about restraint notes:

• A restraint note without a force value is incomplete. "Restrained at A, B, C" does not tell an inspector how hard to pull, and a nonrigid part measured at 5 N is a different part than the same one measured at 50 N.
• Restraint changes what the datum features do. Under restraint a warped planar datum feature is pulled flat against its simulator, so the established datum plane is the simulator plane rather than a three-point contact on high spots. That is usually the point.

The free-state symbol

The free-state symbol is placed in the tolerance compartment of an individual feature control frame, after the tolerance value and any material condition modifier. It means: this particular requirement applies in the free state, notwithstanding the drawing's general restraint note.

The logic is a two-layer system:

This is why the free-state symbol normally appears together with a restraint note rather than instead of one. On a drawing that never mentions restraint, adding the free-state symbol is redundant — everything was already free state.

The common application is a nonrigid ring or housing where location must be verified restrained (because that is how it assembles) but form must also be limited unrestrained (because a part that requires enormous force to pull into shape will fatigue or leak). The location controls carry no symbol; a separate circularity or profile requirement carries the free-state symbol.

Average diameter

For a nonrigid circular part — a thin-wall ring, a seal groove, a tube — the actual local size at any one cross-section may fall outside the size limits simply because the part has gone slightly oval in free state, even though the part is entirely serviceable once installed.

Average diameter notation addresses this. The diameter is qualified as an average, and conformance is judged on the average of several measurements taken around the feature, rather than on every actual local size. Form is then constrained separately, typically by a circularity tolerance carrying the free-state symbol, so that the part cannot be arbitrarily out of round just because its average is right.

Average diameter appears explicitly under Form on the Y14.5-2018 Senior body of knowledge and under the free-state provisions in 2009.

Mathematically defined surfaces and multiple frames

Two related body-of-knowledge items round out this territory.

Mathematically defined surface. Where a surface is too complex for drawing views — a turbine aerofoil, a body panel, a manifold transition — the true geometry is defined by the model, and the drawing carries the geometric tolerances that apply to it. The basic geometry lives in the dataset governed by ASME Y14.41, not in dimensioned views. A profile tolerance on such a surface refers to the model surface as its true profile. Senior candidates should recognize that the absence of dimensioned views does not make the requirement ambiguous when a model dataset is invoked.

Multiple datum reference frames. A nonrigid part often carries one frame for the restrained functional requirements and a different frame, or no datum references at all, for the free-state form requirements. That is legitimate and expected. As Section 5.4 established, features referencing genuinely different frames are never subject to simultaneous requirements.

Orientation alternative practice

One more small item lives nearby on the body of knowledge. Where an orientation tolerance must control individual line elements rather than the whole surface — a slightly bowed extrusion where each cross-section must be square even though the overall surface is not — the requirement can be expressed as an orientation control with a note such as EACH ELEMENT or EACH RADIAL ELEMENT beneath the frame. Y14.5-2018 expresses the radial case with profile of a line referenced to the datum axis instead. Either way the effect is the same: the zone applies to one line element at a time rather than to the entire surface simultaneously.

Traps

1. Assuming restraint is the default. Free state is the default; restraint requires a note.
2. Writing a restraint note with no force. Incomplete and unverifiable.
3. Using the free-state symbol on a drawing with no restraint note. Redundant.
4. Applying average diameter to a rigid part. The provision exists for nonrigid parts.
5. Assuming gravity counts as an external force. It does not; free state means no forces other than gravity.