Flatness
Does flatness need a datum?
No. Flatness is an independent form control applied to the surface itself.
What does the tolerance zone look like?
Two parallel planes separated by the flatness value.
Can a surface pass size and still fail flatness?
Yes. Size tolerance and flatness are separate controls.
Flatness is a GD&T form tolerance that limits how far a surface may deviate from a perfect plane; the entire surface must lie between two parallel planes separated by the specified tolerance. It is evaluated independently of any datums, controlling only form. Flatness ensures a surface is planar across its entire area, critical for seating, sealing, and flush assembly in precision machining and millwork.
In CNC machining and millwork, flatness is relied upon when a part must seat, seal, or register flush without rocking, leaking, or inducing assembly stress. It is especially critical on mating faces, fixture pads, gasket lands, drawer bottoms, panel reference surfaces, and machine-mount interfaces. On the shop floor, flatness errors frequently arise from internal stress release, clamping distortion, uneven stock removal, thermal growth, or post-release warpage. A surface may appear acceptable while still clamped but shift out of tolerance after unclamping. Flatness is verified by collecting points across the surface using CMMs, height gauges, feeler gauges, or 3D scanners, then finding the smallest parallel-plane zone that contains all points. Typical flatness expectations vary by process: rough general machining targets 0.05–0.10 mm, precision machining 0.01–0.03 mm, and high-precision work at 0.005 mm or less. Crucially, a part can be dimensionally correct yet fail assembly if flatness is poor, because size controls how big the part is, while flatness controls how uniformly planar the contact face is across its full area.
Clamp-induced warp: Thin plates, large panels, and plastics forced flat during machining spring back after unclamping, creating a surface that passes inspection while clamped but fails final inspection after release.
Uneven stock removal: Aggressive roughing, poor toolpath strategy, or cutting only one side of a plate leaves residual stress imbalance, causing banana-shaped or dish-shaped surfaces that miss the flatness tolerance zone.
Confusing flatness with parallelism or levelness: Flatness does not require a datum and does not control orientation. A surface can be perfectly flat yet tilted, or level-ish yet still not flat enough for sealing or precision assembly.