Estimate flatness tolerance based on part size per ISO 2768 and common machining standards. Essential for surface plate inspection, CMM measurement planning, and blueprint tolerance specification.
Flatness is the condition of a surface where all elements lie in one plane. It is one of the four form tolerances in GD&T (along with straightness, circularity, and cylindricity). This calculator provides approximate flatness tolerances based on part size.
Flatness controls a single surface relative to itself -- all points must lie between two parallel planes. Parallelism controls one surface relative to a datum surface. A surface can be perfectly flat but not parallel to the base, or parallel but not flat (wavy).
Place the part on a surface plate with three equal-height gauge blocks. Sweep a dial indicator across the top surface. The total indicator reading (TIR) is the flatness error. For CMM inspection, probe a grid of points and compute the minimum zone of two parallel planes containing all points.
Lapping and precision grinding can achieve flatness of 0.001 mm per 100 mm (1 micron/100mm). Optical flats achieve sub-micron flatness measured in wavelengths of light (helium light band = 0.0003 mm). These are used for gauge blocks, bearing surfaces, and sealing faces.
Enter the length of the surface. The calculator estimates the flatness tolerance based on common ISO 2768 and machining standards. For critical surfaces, always reference the engineering drawing tolerance rather than rule-of-thumb estimates.
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SPECIFYING FLATNESS? Flatness on a carbide blank is set at the sintering stage, not at final grinding. See what we check on a supplier's process.
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Cutting formulas with the units worked through · Speed and feed starting points for all six ISO material groups · How to read a carbide grade past its ISO code. 12 pages.
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