On the other hand, flatness does not need a datum. It needs another feature such as an axis or a surface to relate to. Thus, while straightness only makes sure that a single line on a surface has to be within the limits, GD&T flatness does the same for a collection of lines – a surface. While straightness has parallel lines representing its tolerance zone, the flatness tolerance zone is formed by two parallel planes. Flatness vs straightnessįlatness is the 3D equivalent of the surface straightness control. So let’s do some 1:1 comparisons in order to make sure the difference is clear to everybody reading this article. It may seem like flatness is very similar to other geometric as well as regular tolerances in terms of the final result. All the points on the specified surface must lie between these two planes for part approval.Īs flatness refines a surface, we can also use it in a tolerance stack without interfering with other requirements. It defines two parallel planes on either side of the flat surface as the tolerance zone for the surface. The flatness callout controls the uniformity of a surface or a median plane as needed. No surface is perfectly flat but using GD&T, we can develop parts with a surface that is flat enough for our application. Many applications need parts with a flat surface. VI Benefits of Using Flatness Tolerance What is Flatness?
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