How to Set Realistic Flatness Tolerances on CNC Machined Aluminum Plates

Flatness is a form control of a surface itself, not a position relative to another datum. When a flatness 0.10 mm callout appears, the entire surface must lie between two parallel planes 0.10 mm apart. If the part also needs alignment to another feature, add a datum-dependent profile or orientation tolerance instead of substituting a plain flatness value.

For a conventional 6061-T6 aluminum plate, a realistic overall flatness range is 0.05–0.10 mm across a 150 mm x 100 mm machined face, assuming the material is not highly stressed. To specify 0.02 mm or tighter, plan on stress-relieved tooling plate, balanced machining from both sides, and a final skim cut after roughing. These steps increase cycle time and cost, so reserve sub-0.05 mm flats for gasket areas, optical mounts, or other functional sealing surfaces.

Steels behave differently. 304 and 316 stainless work-harden and distort more than 303 stainless or low-carbon steel, so keep flatness values generous unless the blank is stabilized before finishing. Plastics such as POM/Delrin are thermally sensitive and can bow in the inspection room; establish a controlled temperature condition when measuring tight flatness on plastic.

Do not confuse a Ra 0.8 surface finish with flatness. They are independent characteristics and require separate callouts. Use ISO 2768-2 general geometrical tolerances only for nonfunctional areas, and add a specific flatness tolerance when function demands it. For tolerances below 0.05 mm, verify with a CMM using enough points across the surface rather than assuming the machining process will hold the value indefinitely.

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For more guidance on related work, see: Press-Fit CNC Machining Tolerances: How to Specify Holes That Assemble Right.