Flatness tolerance in GD&T controls the flatness of a surface regardless of any datum feature. It makes sure required surface is flat enough.
Value of Flatness tolerance is always less than the dimensional tolerance associated with a part feature. LMC and MMC conditions can be used with flatness tolerance.
Flatness Tolerance Zone
Reference surface should lie in between two parallel planes
Application Of Flatness
As shown in left image width of the block is controlled with in 0.2 mm tolerance zone. For the requirement of the flatness within 0.1 mm limit, flatness tolerance is used.
Without Flatness tolerance, to achieve flatness within 0.1 mm limits, tolerance value is reduced to 0.1 mm
As a result of reducing the tolerance limit increases the chances of rejection and increases part cost.
Measurement Of Flatness
Dial Gauge is used to measure the value of flatness tolerance of a block.
To measure the Flatness, part is moved in a 2-dimensions over the surface plate and variation in the reading of dial gauge is observed.