Machining
Surface roughness handbook
Indicative Ra values by IT grade and machining method — as in CNC Mate: match roughness to tolerance and process.
Surface roughness reference
Ra (µm) reference by IT grades and CNC machining methods. Indicative Ra values by IT grade and machining process.
Recommendation for IT7
Ra 0.8 – 1.6 µm
Class N6 – N7
Fine turning, finish grinding, reaming, broaching
| IT grade | Ra, µm | Processes / note |
|---|---|---|
| IT5+N1 – N3 | ≤ 0.1 | Fine honing, final polishing, mirror lapping with pastes |
| IT6N4 – N5 | 0.2 – 0.4 | Fine grinding, honing, lapping |
| IT6 – IT7N5 – N6 | 0.4 – 0.8 | Grinding, fine turning, fine milling, finish reaming |
| IT7N6 – N7 | 0.8 – 1.6 | Fine turning, finish grinding, reaming, broaching |
| IT8 – IT9N7 – N8 | 1.6 – 3.2 | Fine milling, finish turning, reaming, fine grinding |
| IT10 – IT11N8 – N9 | 3.2 – 6.3 | Finish turning and milling, counterboring, rough grinding |
| IT12 – IT13N9 – N10 | 6.3 – 12.5 | Semi-rough turning/milling, drilling, abrasive snagging |
| IT14 – IT17N10 – N12 | 12.5 – 100 | Rough turning and milling, casting, forging, blanking, thermal cutting |
Frequently asked questions
How do IT grades relate to Ra?
Tighter IT grades usually need finer surface finish. The handbook lists indicative Ra ranges and typical processes for each grade band.