Flatness vs Surface Finishįlatness and Surface Finish are typically on quite different scales, with Surface Finish being a much finer scale. The Flatness feature control frame may also rest on an extension line for a dimension of the surface. Simply callout the Flatness allowance in a feature control frame with an arrow leader designating the surface being controlled. Use of Flatness in a drawing is straightforward. The range of indicator travel must be less than the specified flatness tolerance. Place the surface to be measured face down on the surface plate and sweep it over the indicator tip. One way to achieve this measurement is to place a dial indicator such that its tip comes up through a hole in a surface plate. The sweep must be in a plane parallel to the desired flatness measurement plane. Measurement of Flatnessįlatness is typically measured by sweeping an indicator, probe, or other sensor over the surface. While Flatness is measured over a plane, Straightness is measured over a line. Think of Straightness as the 2D equivalent of Flatness. A surface may be within flatness tolerance but not parallel to any other surface or datum. Parallelism uses a datum to control a surface but there are no datums associated with Flatness. These two are often confused, but in fact they’re quite different. Since Flatness refines a surface, it may be used in a Tolerance Stack.

The distance between these two planes is the Flatness of the surface. The surface controlled must lie entirely within the space between the two planes. The other lies at the deepest trough of any irregularity of the surface. One lies at the peak of the tallest irregularity in the surface. The Tolerance Zone for a Flatness Tolerance is based on two parallel planes. It controls how much a surface on a part may deviate from an ideal plane.

Flatness is a form control, because it is used to control the form or shape of a feature.

It is not dependent on any other datums or features. The GD&T Flatness symbol references how flat a surface is. GD&T Flatness Symbol Definition of Flatness

Surface flatness symbol