Complete surface finish comparison chart with Ra vs RMS values and machining process correlations. Essential reference for specifying surface quality in CNC machining applications.
Surface finish is a critical specification in CNC machining that affects part function, appearance, and performance. The most common measurement is Ra (arithmetic average roughness), which represents the average deviation of surface peaks and valleys from the mean line.
RMS (Root Mean Square) roughness is approximately 11% higher than Ra values and provides a different statistical representation of surface texture. Understanding these measurements helps specify appropriate surface finishes for different applications.
| Ra (μm) | Ra (μin) | RMS | Typical Process | Description |
|---|---|---|---|---|
| 50 | 2000 | 55 | Rough Turning, Drilling | Very rough finish, heavy tool marks visible |
| 25 | 1000 | 28 | Rough Turning, Boring | Rough finish, tool marks clearly visible |
| 12.5 | 500 | 14 | Medium Turning, Milling | Medium finish, tool marks visible |
| 6.3 | 250 | 7 | Fine Turning, Milling | Good finish, light tool marks |
| 3.2 | 125 | 3.5 | Fine Turning, Reaming | Very good finish, faint tool marks |
| 1.6 | 63 | 1.8 | Grinding, Fine Boring | Excellent finish, barely visible marks |
| 0.8 | 32 | 0.9 | Fine Grinding, Honing | Superior finish, mirror-like |
| 0.4 | 16 | 0.44 | Precision Grinding | Ultra-fine finish, highly reflective |
| 0.2 | 8 | 0.22 | Superfinishing, Lapping | Ultra-precision finish |
| 0.1 | 4 | 0.11 | Polishing, Lapping | Mirror finish, optical quality |
| 0.05 | 2 | 0.055 | Diamond Polishing | Ultra-precision optical finish |
| 0.025 | 1 | 0.028 | Diamond Turning | Ultra-precision mirror finish |
| Application | Ra Range (μm) | Description |
|---|---|---|
| General Machining | 6.3 - 25 | Standard production parts, non-critical surfaces |
| Precision Machining | 1.6 - 6.3 | Close tolerance parts, bearing surfaces |
| Optical Components | 0.025 - 0.4 | Mirrors, lenses, precision optics |
| Sealing Surfaces | 0.8 - 3.2 | O-ring grooves, gasket surfaces |
| Bearing Races | 0.2 - 0.8 | Ball bearing races, precision shafts |
| Gauge Blocks | 0.025 - 0.1 | Precision measurement standards |
Ra: 12.5-50 μm
Ra: 3.2-12.5 μm
Ra: 0.4-3.2 μm
Ra: 0.025-0.4 μm
Most common measurement, represents average deviation from mean line.
Statistical measurement emphasizing larger deviations from mean.
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