Flatness cannot be measured by simply placing the part on a granite slab and running a height gauge or microheight over it. This is a 3D measurement so points must be measured across the length and width of the part to ensure the entire surface is in tolerance. Modern CMM’s are best for measuring the part as they can create virtual planes that the true surface profile can be compared to. You are trying making sure that any point along the surface does not go above or below the tolerance zone. Two Sets of Parallel Planes where the entire referenced surface must lie.įlatness is can be measured using a height gauge run across the surface of the part if only the reference feature is held parallel. Flatness tolerance is always less than the dimensional tolerance associated with it. The flatness tolerance references two parallel planes (parallel to the surface that it is called out on) that define a zone where the entire reference surface must lie. It comes in useful if a feature is to be defined on a drawing that needs to be uniformly flat without tightening any other dimensions on the drawing. It is a common symbol that references how flat a surface is regardless of any other datums or features. Symbol: Relative to Datum: No MMC or LMC applicable: Yes – New in 2009 Drawing Callout: Description:

Flat symbols