GDT 8 1 2 Symbol form flatness

Flatness, including surface and DML flatness, will be explained with examples. One of the clear drawbacks of using straightness is that it only measures along a single line and can’t control for three-dimensional features very well. Thankfully GD&T also has a callout for flatness, which is like straightness with another dimensions. Instead of setting up a two-dimensional TZ in which a line must fit, flatness creates a three-dimensional TZ of two parallel planes in which a real or derived plane must fit. Similar to straightness, flatness can control for either a surface or derived median plane. Surface Flatness Surface flatness might be called out for a sealing feature which needs a precise fit. On a part like this, a laser scan of the surface might be taken to assure the needed flatness. No part of the surface could fall outside two parallel planes that are .005mm apart. That tight tolerance is needed for the surface to seal correctly. However, the location and even orientation can have a looser tol