ISO 1101 flatness tolerance on a surface plate

The drawing of an engine cylinder head specifies a flatness tolerance of 0.02 mm on the gasket face. The metrologist has to check that the whole surface lies between two parallel planes 0.02 mm apart, with no outside datum. A flatness error lets combustion gases leak.

Definition and measurement of an ISO 1101 flatness tolerance

Measuring method	Instrument used	Reference surface	Estimated uncertainty
Three-point method	Granite surface plate + dial indicator	Plane set on 3 jacks	±0.003 mm
CMM probing	Automatic 3D CMM	Least-squares plane	±0.001 mm
Precision straightedge	Knife-edge straightedge + feeler gauges	Linear bearing line	±0.010 mm

All values are given for the reference environmental conditions of 20 °C.

Levelling on three adjustable jacks on a surface plate

Measuring on a surface plate means setting the part on three adjustable threaded jacks to cancel the overall tilt of the plane. The dial indicator sweeps the surface along a grid. The difference between the highest and lowest points gives the flatness. To explore other form tolerances, see our guide to ISO 1101 geometric symbols.

Reading the flatness tolerance zone

Flatness is a pure form tolerance. It needs no datum (no letter A or B). The zone is bounded by two parallel planes set apart by the specified value.

Worked example: inspecting a base plate

A 200x200 mm aluminium plate with a 0.02 mm flatness tolerance is measured on a CMM. 1. Probe 49 points. 2. Highest point: +0.014 mm, lowest point: -0.011 mm. 3. Total deviation: 0.025 mm. Verdict: the value (0.025 mm) exceeds 0.020 mm. The part is declared non-conforming.

Shop-floor pitfalls when inspecting flatness

The first pitfall is not cancelling the part’s tilt before taking readings. The second is a surface plate that has lost its own flatness. The third comes from the part springing once unclamped after machining. To record your readings without errors, use our dimensional quality inspection modules.