Surface finish affects everything from part appearance to function—friction, wear, sealing, fatigue life, and coating adhesion all depend on it. This guide explains surface roughness parameters, measurement methods, and how to specify the right finish.
Understanding Ra (Roughness Average)
Ra is the most commonly specified surface roughness parameter. It represents the arithmetic average of the absolute deviations from the mean line, measured over a sampling length.
Ra is expressed in:
- Microinches (μin): Common in the US
- Micrometers (μm): International standard
1 μm ≈ 40 μin (more precisely, 39.37)
Ra to N-Grade Conversion
ISO N-grades provide a standardized roughness classification:
| N Grade | Ra (μin) | Ra (μm) | Typical Process |
|---|---|---|---|
| N1 | 1 | 0.025 | Superfinishing |
| N2 | 2 | 0.05 | Lapping |
| N3 | 4 | 0.1 | Honing |
| N4 | 8 | 0.2 | Fine grinding |
| N5 | 16 | 0.4 | Grinding |
| N6 | 32 | 0.8 | Fine turning |
| N7 | 63 | 1.6 | Turning/Milling |
| N8 | 125 | 3.2 | Machining |
| N9 | 250 | 6.3 | Rough machining |
| N10 | 500 | 12.5 | Sawing |
Convert Surface Finish Values
Convert between Ra, Rz, RMS, and N-grades. See typical machining methods for any finish.
Other Roughness Parameters
Rz (Mean Roughness Depth)
Average of the five highest peaks to five lowest valleys over the sampling length. More sensitive to occasional deep scratches than Ra. Typically 4-7× the Ra value.
RMS (Root Mean Square)
Similar to Ra but squares values before averaging, giving more weight to extremes. RMS ≈ 1.11 × Ra for typical surfaces.
Rt (Total Height)
Maximum peak to valley height over the evaluation length. Single extreme value, useful for clearance fits.
Specifying Surface Finish on Drawings
The ISO surface finish symbol includes:
- Basic symbol (check mark shape)
- Roughness value and parameter (e.g., Ra 1.6)
- Optional: process, lay direction, sampling length
For critical surfaces, specify the parameter (Ra, Rz), the value, and any additional requirements like lay direction or maximum waviness.
Process Capabilities
What Each Process Can Achieve
- Superfinishing/Lapping: 1-4 μin (0.025-0.1 μm)
- Honing: 4-16 μin (0.1-0.4 μm)
- Grinding: 8-32 μin (0.2-0.8 μm)
- Fine turning/boring: 16-63 μin (0.4-1.6 μm)
- Standard machining: 63-250 μin (1.6-6.3 μm)
- Rough machining: 125-500 μin (3.2-12.5 μm)
Application Guidelines
- Bearing surfaces: 8-32 μin (0.2-0.8 μm)
- Sealing surfaces: 32-63 μin (0.8-1.6 μm)
- Sliding fits: 32-125 μin (0.8-3.2 μm)
- General machined: 125-250 μin (3.2-6.3 μm)
- Non-critical: 250+ μin (6.3+ μm)
Cost Implications
Finer finishes cost more due to:
- Slower feeds and speeds
- Additional operations (grinding after turning)
- Sharper/fresher tooling requirements
- More precise machine requirements
Each halving of Ra roughly doubles the cost. Don't over-specify—match the finish to actual functional requirements.
Measurement Basics
- Stylus profilometers are most common
- Cutoff (sampling length) affects readings—use appropriate settings
- Take multiple measurements across the surface
- Direction matters—measure perpendicular to lay when possible
Surface finish specification requires balancing function, cost, and achievability. When uncertain, consult with your machinist about what's practically achievable and whether the specified finish is actually necessary for the application.
