GDT:Flatness
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Revision as of 20:59, 14 January 2026 by Admin (talk | contribs) (Created page with "{{Author Credit |author=Admin }} {{GDT Infobox |symbol=Flatness |type=Form |category=Form |standard=ASME Y14.5 |author=Admin |datums=None |modifiers=ST (statistical tolerance, where allowed) |tolerance_zone=Two parallel planes separated by the tolerance value |image=<!-- TODO: add diagram file name --> |mistakes=Adding datum references; applying flatness to a pattern; using prohibited modifiers |related=F4:Flatness Validation }} == Sum...")
| Symbol | Flatness |
|---|---|
| Type | Form |
| Category | Form |
| Standard | ASME Y14.5 |
| Author | Admin |
| Datums | None |
| Modifiers | ST (statistical tolerance, where allowed) |
| Tolerance zone | Two parallel planes separated by the tolerance value |
| Image | |
| Common mistakes | Adding datum references; applying flatness to a pattern; using prohibited modifiers |
| Related | F4:Flatness Validation |
Summary
Flatness controls the form of a planar surface or planar feature of size. It limits the entire surface to lie between two parallel planes.
Definition
The tolerance zone for flatness is two parallel planes separated by the tolerance value. The entire surface (or derived median plane for a feature of size) must fall within this zone.
When to use it
Use flatness to control overall surface form without referencing datums, such as ensuring a sealing face is flat or a mating surface is within a flatness requirement.
Examples
Example: Planar surface flatness
Flatness applied to a planar surface with no datum references.
Example: Planar feature of size flatness
Flatness applied to a planar feature of size where size is defined on the drawing.
Common mistakes
- Adding datum references to a flatness FCF.
- Applying flatness to a pattern.
- Using prohibited modifiers such as projected tolerance zone or tangent plane.