Ra is not the whole finish
Ra averages surface deviations and can miss isolated scratches, dents, folded burrs, or edge defects. Pair Ra with visual and functional inspection.
Quality control
Surface Roughness Measurement
Mass finishing can improve surface feel, but inspection must define exactly where and how roughness is measured.
Ra
Rz
Cutoff length
Sampling plan
Sampling location matters
Measure the same surface, direction, and distance from edges each time. Tumbling attacks exposed peaks differently than recessed features.
| Control item | Recommended practice | Why it matters |
|---|---|---|
| Parameter | Specify Ra, Rz, or both. | Different metrics describe different surface behaviors. |
| Cutoff length | Lock the cutoff and evaluation length in the inspection plan. | Changing cutoff can change reported values. |
| Direction | Measure in the agreed orientation relative to machining marks. | Directional surfaces can show different readings. |
| Sampling | Define parts per lot and features per part. | Mass finishing results can vary by load position and media contact. |
Link roughness to process variables
When roughness drifts, check media wear, compound concentration, water quality, loading ratio, and cycle time. The related controls are covered in Media Life Cycle Analysis and Abrasive Quality Control.