GD&T Flatness

A brief explainer on what is the meaning of Flatness in GD&T and how it is inspected

Flatness is defined as the allowable variation in the planar shape of the surface . It is used to define how much a surface can deviate from a perfectly flat plane.

Here is an example of two types of roads. The first is considered to be perfectly flat with hardly any undulations.

The second one has many undulations.

If we apply the same analogy to surfaces of machine components at the micron scale then that the degree of variations between the crests and troughs is flatness.

GD&T notation for Flatness is a Feature control frame attached to the surface or line (in orthographic view) indicating the surface. As shown

The meaning of the notation is that any point on the surface should lie within a rectangular tolerance zone of width 0.02 .

Inspection of flatness:

The part is placed on a guage block which has a holethrough which a dial gauge indicator is positioned
The part is moved to and from and side to side.
The total indicator reading on dial gauge is taken
Which gives the extent of surface variation
The value should be lesser than the value in feature control frame in Flatness control .

IF Flatness is being measured in a CMM (Co ordinate measuring machine ) then the probe shown with the red ball is moved all around the surface and readings are taken at many points .

The array of points taken from CMM all need to lie within a tolerance zone of 0.02 . That is maximum deviation between them from crest to trough should be within the value of 0.02 .