Be aware also of the tolerancing classes for straightness and flatness specified by ISO 2768. The images below show this with the two lighter blue planes (actual part surfaces) forming the derived median plane (the ‘virtual’ plane shown in darker blue):įor more details, see ISO 12781, and ASME 14.5-2018, Section 8. For instance, where two flat surfaces are parallel but offset, the derived median plane between the two faces can be controlled with a flatness callout. It can also apply over several discontinuous faces, treating them as a single continuous plane (a ‘common zone’ tolerance) The images below show examples of surfaces to which a flatness control might apply.įlatness can also be applied to certain specific ‘virtual’ features. A flatness requirement can apply to an entire surface, or just to a small sub-section of a face (this latter case can create some challenges for measurement with some traditional flatness gauging tools). The below shows a 3D view of a surface that cannot be bounded between the two tolerance planes this is a fail.įlatness can apply to any surface (or any area of a surface) that is nominally flat, whatever the orientation of that plane.

Note that just because the surface may have a nominal angle on your CAD or drawings, the two planes need not be oriented at that exact angle, as long as they are parallel to one another. between these two parallel planes) the part is a ‘pass’. If all measured points on the surface lie within the tolerance band (i.e. The flatness tolerance zone is visualised as two parallel planes, spaced apart by the number indicated in the flatness control. a zonal flatness of ‘x’ microns per millimetre). Flatness may be expressed as a local rate over a larger area (e.g. As well as on 2D prints, you may also encounter flatness callouts directly in a 3D CAD model.Ī flatness callout with no other information should be interpreted to mean that the flatness control applies to the entire surface. Keep in mind that a flatness measurement is almost always a 2D measurement in 3D space (it is sometimes measured using a 1D curvilinear scanning path, although this gives limited coverage). Register and download Now Drawing calloutĪ typical flatness callout on a drawing print (2D views) is shown below: Download Keep our complete GD&T eBook as a reference.

Flatness gd&t symbol