Flatness (manufacturing)

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In manufacturing and mechanical engineering, flatness is an important geometric condition for workpieces and tools. Flatness is the condition of a surface or derived median plane having all elements in one plane.^[1]

Geometric dimensioning and tolerancing has provided geometrically defined, quantitative ways of defining flatness operationally. Flatness deviation may be defined in terms of least squares fit to a plane ("statistical flatness") or worst-case (the distance between the two closest parallel planes within). It can be specified for a given area and/or over an entire surface.

In the manufacture of precision parts and assemblies, especially where parts will be required to be connected across a surface area in an air-tight or liquid-tight manner, flatness is a critical quality of the manufactured surfaces. Such surfaces are usually machined or ground to achieve the required degree of flatness. Metrology of such a surface can confirm and ensure that the required degree of flatness has been achieved as a key step in a manufacturing processes.

Two parts that are flat to about 1 helium light band (HLB) can be "wrung" together, which means they will cling to each other when placed in contact. This phenomenon is commonly used with gauge blocks.