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( Read more about measuring straightness.) Flatness vs.

Axis straightness controls how much bow or curve is allowed in the axis of a part. There is also axis straightness, a characteristic we deal with often here at Metal Cutting Corporation. Its tolerance zone exists between two parallel lines and is an indicator of flatness along one line on a part’s surface. Surface straightness, on the other hand, is two-dimensional (2D). straightnessįlatness, then, is a characteristic of a three-dimensional (3D) object as it exists between the two parallel planes that delineate its tolerance zone. Looking at it another way, the tolerance zone is the highest and lowest (±) acceptable points across the plane of the surface that is being measured. The flatness tolerance zone is the area between two parallel planes, into which the surface in question would need to fit in order to qualify as “flat.” The flatness tolerance would be the allowable distance between those parallel planes. It is also useful where two surfaces must meet flush and form a tight seal, as with two surfaces bolted together with a gasket in between. The characteristic of flatness is useful in helping to ensure even wear across a surface.

Symbolized in GD&T by a parallelogram, flatness allows you to limit waviness or variation in a surface without tightening its dimensional tolerance. What is surface flatness?įlatness is a measure of a surface’s form compared against itself, indicating that all the points along the surface lie in the same plane. It can also be an important consideration when optimizing for manufacturability.Īlong with positioning, cylindricity, and other geometric tolerancing characteristics, flatness tolerance may be indicated on an engineering drawing to help ensure the metal parts that are produced will have both the correct form and the desired functionality.

Surface flatness is one of the Geometric Dimensioning and Tolerancing (GD&T) standards used to communicate how a part should be manufactured. Does flatness matter in small parts production?