⏥ Flatness
Flatness · Forma · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
The whole indicated surface must lie between two parallel planes 0.08 mm apart, with no requirement on its orientation to any other face of the part.
What controls
Limits the variation of an actual surface relative to two theoretical parallel planes separated by the tolerance value: the entire surface must fit between the two planes, regardless of its orientation to other faces of the part (which is why it takes no datum). Passing flatness does not guarantee that the face is square or parallel to another one — that is controlled separately by perpendicularity and parallelism.
How to verify
The simplest method in the shop is bluing (Prussian blue) on a granite surface plate: the high spots are marked and guide scraping or rework. With a dial indicator on a surface plate, sweep the whole surface in a grid and record the difference between the highest and lowest point (not the absolute reading at a single point). For tight tolerances, optical interferometry against optical flats is used, or a CMM with a dense point map fitted to the best plane by least squares.
tolerance zone
Two parallel planes separated by the tolerance value; the entire actual surface must fit between them.
Workshop guideline values
- General
- 0.05–0.2 mm on seating faces and flanges from conventional machining.
- Accuracy
- 0.01–0.03 mm on sealing surfaces or fitted machine beds.
- High precision
- 0.002–0.008 mm on inspection surface plates or precision guideways.
Guidance only: the drawing's value always wins.
Common mistakes when interpreting it
- Thinking a flat surface is correctly oriented: flatness does not control the angle to other faces, only the form itself — a face can be perfectly flat and still be tilted.
- Checking only a couple of diagonals with an indicator instead of sweeping the whole surface: a localized high spot in a corner will not show up if you only measure through the center.
How to get it in the workshop / if it comes out of tolerance
Needs no datum: it is a pure form tolerance. Useful on mounting bases, flange faces and sealing surfaces. A concave/convex face can be within flatness but poorly oriented.
Suspect the workholding first: too much clamping force in a vise or on a magnetic chuck distorts the part, which springs back ("bows") when released. Reduce clamping to the minimum needed and take a finishing pass without forcing. If the part is thin or still hot from roughing, let it cool before the final pass and before measuring: thermal warping disappears once it stabilizes. In milling, check the overlap between passes (stepover) so no step is left between them.
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