The Language of Roughness
In CNC machining, surface finish is critical for sealing surfaces, bearing fits, and aesthetics. The two most common metrics are Ra and Rz.
Ra (Average Roughness)
The arithmetic average of the absolute values of the profile height deviations from the mean line.
Best for: General indication of surface texture. Most common in US/ISO.
Weakness: Hides singular defects (scratches).
Rz (Mean Roughness Depth)
The average of the maximum peak-to-valley heights of 5 consecutive sampling lengths.
Best for: Sealing surfaces and sliding fits. Common in DIN (German) standards.
Strength: More sensitive to extreme peaks/valleys.
Conversion Chart (Approximate)
While there is no exact mathematical conversion (because they measure different things), this chart provides the industry-accepted equivalence.
| Grade | Ra (µm) | Ra (µin) | Rz (µm) | Process |
|---|---|---|---|---|
| N12 | 50 | 2000 | 200 | Flame Cut / Saw |
| N9 | 6.3 | 250 | 25 | Rough Milling/Turning |
| N7 | 1.6 | 63 | 6.3 | Finish Milling |
| N6 | 0.8 | 32 | 3.2 | Grinding / Fine Turn |
| N4 | 0.2 | 8 | 0.8 | Lapping / Polishing |
How to Calculate Theoretical Finish
In turning, theoretical surface finish is determined by Feed Rate and Tool Nose Radius.
Where:
f = Feed per revolution (mm/rev or ipr)
r = Tool nose radius (mm or inch)
Takeaway: To improve finish, decrease feed rate or increase nose radius.