GD&T symbol · Form tolerance

Flatness — Form Tolerance

Limits the deviation of a surface from a perfect plane.

Definition

Flatness controls how far a surface may depart from a perfect plane. It is a Form control — self-referencing, with no datum reference frame — and the entire surface must satisfy the requirement at once, not line by line. Because the zone is established from the surface itself, flatness says nothing about where the surface sits: a surface can be perfectly flat and still be tilted, offset, or poorly oriented relative to the rest of the part. Flatness controls form only, which is exactly what a gasket face or mounting pad needs.

The zone: two parallel planes

The flatness tolerance zone is the space between two parallel planes separated by the tolerance t. Every point of the actual surface must fall between the two planes in a single simultaneous evaluation. The planes are not anchored to the nominal surface direction — any pair of parallel planes t apart that contains the whole surface satisfies the requirement, so a uniformly tilted or slightly shifted surface can still pass. A surface that bows, dishes, or warps within the band passes; anything that needs more than t of separation anywhere fails.

Cross-section through a plate: the zone is two parallel planes 0.2 apart, and the entire surface — every point, over the full area — must lie between them in one simultaneous evaluation. Unlike surface straightness, there is no per-section pass or fail. Schematic, exaggerated.

Reading a feature control frame

A flatness frame has no datum references and no diameter symbol — the zone is a width between planes, not a cylinder. ⏥ 0.2 reads as a flatness tolerance of 0.2 width. Compare the position frame ⌖ ⌀0.5 Ⓜ A B C: position locates the feature relative to datums and may take material modifiers, while flatness references only the surface itself and never carries a material condition. The frame says nothing about where the surface is — only how flat it must be.

The feature control frame, compartment by compartment. There is no datum reference and no diameter symbol — the zone is a width between planes — and, like all Form controls, no material modifier is possible.

Measurement

Surface plate + height gauge / dial indicator — the classic method: sweep the indicator over the surface and read the total range of travel; it must stay within t. Fast and adequate for most plates, pads, and seal faces.

CMM point sampling — probe a grid of points over the surface and fit the tightest pair of parallel planes that bounds them. Used for larger or flexible parts, and whenever measured flatness feeds a statistical process.

When to use it

Gasket and seal faces, where a full-surface contact band is what prevents leaks
Base plates and mounting pads, where the part must sit without rocking on its mating surface
Datum features that other controls will reference — the datum feature's own form must be good enough to establish a stable reference plane
Any surface that must contact another over its full area rather than at a few high spots