Finish reference
Surface Roughness Conversion Chart
Convert common Ra, Rz, RMS, and N-grade roughness language into a planning reference before inspection review.
Direct answer: surface roughness conversion is only a planning approximation between Ra, Rz, RMS, and N-grade values. Final acceptance must follow the drawing, standard, and approved measurement method.
Finish reference
Surface Roughness Conversion Chart
Planning conversion chart for Ra, Rz, RMS, and N-grade surface roughness values before final drawing or inspection review.
| Finish callout | Approx Ra | Approx Rz / RMS | Planning note |
|---|---|---|---|
| Rough machined face | 3.2 - 6.3 um / 125 - 250 uin | Rz often 4x - 7x Ra | Use for clearance faces and non-sealing surfaces. |
| General machined finish | 1.6 - 3.2 um / 63 - 125 uin | RMS slightly above Ra | Common for routine milling and turning features. |
| Fine machined finish | 0.8 - 1.6 um / 32 - 63 uin | Confirm metric or imperial callout | May require sharp tooling, stable feed, and finish pass. |
| Precision sliding or sealing face | 0.4 - 0.8 um / 16 - 32 uin | Rz must be explicitly checked | Review process capability and measurement method. |
| Ground / lapped reference face | 0.1 - 0.4 um / 4 - 16 uin | Conversion uncertainty is high | Use the actual standard and instrument parameter. |
| N-grade planning band | N5 - N10 typical machining range | Maps only approximately to Ra | Do not convert acceptance criteria without customer approval. |
Standards handoff
Use the standards guide before treating a converted finish as an acceptance limit.
Troubleshooting
Diagnose chatter, feed marks, tool wear, and measurement issues when finish misses target.
Calculator handoff
Estimate theoretical finish from feed and nose radius before changing the process.
Download chart
Save the conversion table for print review and setup planning.