Flatness

Controls

Flatness requires all points on a surface to lie within two parallel planes separated by the tolerance value. It controls surface form only — there is no orientation or location component. A surface can be perfectly positioned but still fail flatness if it bows, warps or has high spots.

Flatness is tighter than the size tolerance by definition — applying a flatness tolerance looser than the size tolerance has no effect.

Example

A precision-ground machine vice jaw face must be flat within 0.005mm so it seats without rocking on the surface plate during a sensitive setup. A sealing face on a hydraulic manifold is another common application — bow in the face would allow fluid to bypass the gasket under pressure.

Inspection

• Surface plate + dial indicator swept across the full surface; record min and max
• CMM grid scan — fit minimum zone (two parallel planes enclosing all points)
• Feeler gauge under a precision straight edge at multiple orientations
• Optical flat + monochromatic light for lapped or ground surfaces

Datum & Modifiers