Surface Finish Inspection: Callouts That Prevent Disputes

Surface finish inspection should be defined on the drawing before quoting, because an ambiguous Ra callout often causes rejected lots, extra measurement time, and avoidable cost. A simple note like “Ra 0.8” leaves open where, how, and how many parts to check.

State the parameter, value, and measurement convention. Ra is common, but Rz or Rmax may suit sealing faces and fatigue-critical radii. Add the cutoff length (often 0.8 mm for machined surfaces), evaluation length, and direction relative to dominant tool marks. For a 6061-T6 housing, an Ra 1.6 finish on sealing faces may be sufficient; 316 stainless or 7075-T6 may need tighter control after finishing. POM/Delrin can tear, so specify whether visual defects or Ra governs.

Define inspection method and sampling. A portable profilometer is typical; optical or 2D vision inspection can verify edge burrs and shallow marks. For ISO 2768-m general tolerances, finish is usually not covered, so add explicit notes. Say whether 100% inspection or AQL sampling applies, and identify datum/surfaces that are not finish-critical.

Finally, tie acceptance to function. Over-specifying Ra 0.4 across a whole bracket increases polishing and lead time, while under-specifying a seal face risks leaks. Put finish requirements and inspection method in the RFQ so the shop can quote the right process and measurement.

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For more guidance on related work, see: CNC Machining Industry Insights for Better Part Sourcing.