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- Flatness is a measure of a surface’s form compared against itself, indicating that all the points along the surface lie in the same plane. Symbolized in GD&T by a parallelogram, flatness allows you to limit waviness or variation in a surface without tightening its dimensional tolerance.
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Mar 30, 2021 · The flatness callout controls the uniformity of a surface or a median plane as needed. It defines two parallel planes on either side of the flat surface as the tolerance zone for the surface. All the points on the specified surface must lie between these two planes for part approval.
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Two things to note about flatness for a feature of size (Flatness DMP) compared to surface flatness: Flatness, as applied to a feature of size, automatically overrides Rule #1 of Geometric Dimensioning and Tolerancing. (Rule #1 is still in effect for surface flatness.)
Feb 17, 2024 · In this article, we will discuss what is flatness in GD&T, the flatness callout, explain how to measure flatness, and present a short comparison of straightness vs flatness.
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Mar 3, 2022 · Applying Flatness or Straightness to a Feature of Size. Reason for Use: Since Rule #1 is the default condition, it is used always and ignored only when specified. Rule #1 helps fully define features of a part and ensure the form of them is being controlled.
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Flatness is a GD&T form tolerance that is conceptually simple. According to the ASME Y14.5 standard, it “specifies a tolerance zone defined by two parallel planes within which the surface must lie.” Figure 1. ASME GD&T Flatness Example. Figure 1 illustrates this concept very well.
GD&T Flatness is a common symbol that references how flat a surface is regardless of any other datum’s or features. 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.
With flatness, you can allow the table to be flat without constraining the tabletop thickness very tight. (You would be rejecting tables that were good thicknesses and normally in spec if using Geometric Dimensioning and Tolerancing.)
