GD&T REFERENCE

SYMBOL REFERENCE

All 14 GD&T symbols · ASME Y14.5-2018 · Definitions, mistakes & checklists

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FLATNESSFORMNO DATUM

ASME Y14.5-2018 §12.4 · Controls how flat a surface is

WHAT IT CONTROLS

Flatness controls how much a surface can deviate from a perfect plane. The tolerance zone is defined by two parallel planes within which all points of the surface must lie. It controls form only — not orientation or location.

WHEN TO USE

Use flatness when a mating surface must seal or mate precisely, such as gasket surfaces, bearing seats, or any interface where surface contact quality matters. It is also used to refine the form requirement beyond what Rule #1 (the envelope principle) already imposes.

COMMON MISTAKES

Adding a datum reference — flatness is a form control and never requires a datum

Using the diameter symbol (⌀) — flatness applies to a planar zone, never a cylindrical zone

Applying a flatness tolerance looser than the size tolerance — this creates a conflict with Rule #1

Confusing flatness with parallelism — flatness does not reference any datum, parallelism does

IS YOUR CALLOUT CORRECT?

Is the FCF free of any datum reference letters?

Flatness never references a datum. If you see |A| or |B| in the FCF, remove it.

Is there no diameter symbol (⌀) before the tolerance value?

Flatness defines a zone between two parallel planes, not a cylindrical zone.

Is the flatness tolerance smaller than the size tolerance on the same feature?

Per Rule #1, the flatness tolerance cannot exceed the size tolerance or it becomes meaningless.

REDPRINT CHECKS THIS

Redprint flags flatness FCFs that include datum references, use the diameter symbol incorrectly, or conflict with Rule #1 size tolerances.

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