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Flatness is one of the easier GD&T controls to recognize, but it is also commonly misunderstood. Flatness controls the shape of one surface without referencing a datum.
What Flatness Controls
A flatness tolerance creates a zone between two parallel planes. The controlled surface must fit inside that zone.
No Datum Required
Unlike orientation controls such as parallelism, flatness does not require a datum reference. It controls the surface itself.
Flatness Is Not Size
A surface can have the correct overall dimensions and still be too wavy. Dimensional tolerance and geometric tolerance control different characteristics.
When Is Flatness Useful?
Use it when surface form matters to assembly, sealing, sliding or another functional requirement. Do not add tight flatness simply because the surface looks important.
Inspection
Inspection method depends on the tolerance and part size. The inspection setup should be capable of demonstrating whether the surface remains within the specified zone.
Keep learning: Explore more practical engineering articles in the GetCADFile Design Guide.