Surface Finish Inspection: Define Cutoff and Sampling

Surface finish inspection starts with the drawing, not the profilometer. A callout like Ra 0.8 without cutoff length, evaluation length, or sampling frequency forces the shop and inspector to guess. ISO 4288 and ASME B46.1 give default cutoffs—0.8 mm for Ra 0.8, 2.5 mm for Ra 3.2—but functional surfaces often need a custom plan.

Cutoff length controls which wavelengths the instrument averages. Using a 2.5 mm cutoff on a Ra 0.8 sealing face can hide short-wavelength chatter that causes leaks. For 6061-T6 manifolds or 316 stainless valve bodies, specify the cutoff, evaluation length, and whether Ra, Rz, or Rk is reported. A comparator plate is fine for cosmetic POM/Delrin covers, but a stylus profilometer is required when the finish affects fit or sealing.

Sampling frequency drives cost and lead time. Inspecting every part with a profilometer adds minutes per piece; for a 500-piece run, that can shift the quote. Instead, define functional zones on the drawing and set an AQL sample for non-critical surfaces. For tight-tolerance hydraulic parts, 100% finish inspection may be justified, but only on the sealing diameter.

Write an inspection note that says: “Measure per ISO 4288, cutoff 0.8 mm, evaluation length 4 mm, report Ra and Rz.” This closes the gap between machining and quality. If parts are anodized or passivated, inspect after coating because the finish changes. Match the surface finish inspection method to the function, and you avoid disputes without overpaying for unnecessary measurements.

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For more guidance on related work, see: Total Runout Tolerance: Specify It Right on CNC Shafts.