Why is Flatness used?
Surface flatness helps a complete face seat evenly. Median-plane flatness limits bow of the derived median plane of a feature of size.
FORM CONTROL / GD&T SYMBOL GUIDE
FLATNESS
Flatness can control an entire surface or, when applied to a feature of size, its derived median plane. The examples compare both applications with and without an explicit flatness requirement.
INTERACTIVE APPLICATION
Surface and median-plane flatness
NO SURFACE FLATNESS CONTROL
The whole face may warp
- Size limits alone do not specify a separate flatness zone for the complete face.
- A warped mounting face can rock or leave contact gaps.
NO MEDIAN-PLANE FLATNESS CONTROL
The derived median plane may bow
Limitations
- The part is a feature of size, but no separate flatness requirement controls its derived median plane.
- Thickness variation can make the mid-plane uneven.
SURFACE FLATNESS
Every point on the face is controlled
- The entire surface fits between two parallel planes 0.05 apart.
- Surface flatness needs no datum.
FEATURE-OF-SIZE FLATNESS
The derived median plane is controlled
Advantages
- A flatness frame applied to the size dimension controls the derived median plane of that feature of size.
- Its median-plane zone is between two parallel planes 0.05 apart.
Where is Flatness used?
Use surface flatness on mounting or sealing faces. Apply median-plane flatness to a plate, tab or width feature when its mid-plane form matters.
How is Flatness used?
For surface flatness, attach the frame to the face. For feature-of-size flatness, apply it to the size dimension so the derived median plane is controlled. Neither example uses a datum.
The whole face fits between parallel planes 0.05 apart. No datum is required.
The derived median plane of the feature of size fits between parallel planes 0.05 apart.
Flatness vs Straightness: Which one to use?
Straightness controls individual line elements or a derived median line. Surface flatness controls the whole face; flatness applied to a feature of size controls its derived median plane. Use straightness for a line or axis and flatness for a complete face or median plane. See the straightness example →
Commonly defined variable values
The tolerance value is the separation of the two planes in either example. The starting values below describe surface applications; choose a median-plane value from the feature’s function and inspection method.
Scroll the table sideways to see every column.
| Application | Illustrative starting value | Example feature | Notes / caution |
|---|---|---|---|
| Machined mounting face | 0.05–0.20 mm | 200 × 200 mm face | Check clamping distortion. |
| Ground sealing face | 0.01–0.05 mm | 100 × 100 mm face | Verify gasket or seal requirements. |
| Fabricated plate | 0.3–1.0 mm | 500 × 500 mm panel | Agree free-state support during inspection. |
These are planning examples, not universal rules or guaranteed process capabilities. Set the actual tolerance from function, size, length, process capability, inspection method and the standard specified on the drawing.
How is flatness measured in real life?
For surface flatness, sample the complete face. For median-plane flatness, measure the opposing surfaces and evaluate their derived median plane. Choose the method shown or agreed for the drawing.
- Surface plate + indicator: Support and level the face independently, then scan a grid of points. Do not treat the opposite face as a datum for this form check.
- CMM: Sample across both length and width, then evaluate the full surface against the specified two-plane zone.
- Optical or interferometric methods: For fine finishes, capture an areal height map and evaluate it with the agreed filtering and sampling method.
- Median plane: On a CMM, sample both opposing faces of the feature of size and evaluate the derived median points across its extent.
ASME Y14.5-2018 (R2024)
ASME Y14.5 surface flatness uses a datum-free two-plane zone. Size limits may also impose a form boundary for a regular feature of size under Rule #1.
ASME Y14.5 edition reference. Examples are learning schematics; use the complete requirements on the drawing.
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