GlossaryKind of term: Manufacturing termFlatness
What is flatness?
Flatness is a form tolerance that limits how far a surface may depart from an ideal plane, as a zone between two parallel planes, without a datum.
The symbol is ⏥, and the distance between the two parallel bounding planes equals the tolerance value: every point of the toleranced surface must lie between them. Those two planes may tilt and shift freely; they are tied to no datum. Flatness therefore says nothing about the orientation or height of the surface, only about its form. A flatness of 0.05 mm allows a concave, convex or twisted surface, as long as it fits between two planes 0.05 mm apart.
What does flatness depend on?
How flat a surface comes out depends on the stiffness of part and workholding, residual stress in the material, clamping force, heat from cutting, machine geometry and strategy. A thin or asymmetric part can spring back after unclamping or distort as stress is released. Roughing first, then releasing or reclamping with low force before finishing, can help; stress relieving in between is another option. Face milling, grinding, lapping and flat honing each reach a different level, depending on size and material.
What does the standard say about flatness?
ISO 1101 sets the symbol and the tolerance zone, ISO 12781 the terms and measurement conditions for flatness. The default reference for evaluation is the minimum zone: the two parallel planes closest together that enclose all points. Another reference, such as a least-squares plane, gives a different result and is then indicated as a modifier in the frame. A surface without a frame of its own falls under the general geometrical tolerance, where the drawing gives one.
How does flatness work in practice?
Flatness is measured on a coordinate measuring machine, with a dial indicator over a surface levelled on adjustable supports, or optically for very small values. Laying the part flat on the surface plate and running a dial indicator over the top face measures parallelism to the underside, not flatness. Whether a tight flatness calls for an extra operation or a subcontracted step depends on size, wall thickness, material and machine.
What is flatness often confused with?
Flatness is not parallelism: parallelism refers to a datum and limits the orientation of the surface, and with it the flatness. Nor is flatness roughness: flatness concerns the form of the whole surface, roughness the fine texture within a short evaluation length. A mirror-smooth surface can still be concave.
How do you see flatness in Blake?
In Blake the flatness requirement sits with the part as a proposal from the drawing, alongside the other tolerances. If you subcontract a grinding or lapping step, it is its own line from your own subcontract library; if its price is missing, the line stays open. More on estimating.
Frequently asked questions
Can I hold a tight flatness by milling?
On a stiff part, with low clamping force and a light last cut, often yes. On a thin or asymmetric part that springs back after unclamping, an intermediate step, grinding or lapping is sometimes needed. What is reachable depends on size, material and machine.
Does flatness need a datum?
No. Flatness compares the surface only with an ideal plane. If a flatness frame carries a datum letter anyway, the indication is wrong; ask whether parallelism or perpendicularity was meant.