Surface Roughness Reference Chart
Complete reference for surface roughness values, Ra/Rz/RMS conversions, finish methods by process, and cost impact analysis for CNC machined parts.
Surface Roughness Ra Values
| Ra Value | Description | Process |
|---|---|---|
| 0.1 μm | Mirror finish | Lapping, polishing |
| 0.2 μm | Super fine finish | Fine grinding, honing |
| 0.4 μm | Very fine finish | Precision grinding, fine milling |
| 0.8 μm | Fine finish | Finish turning, finish milling |
| 1.6 μm | Good machine finish | Careful turning/milling |
| 3.2 μm | Standard machine finish | Standard CNC milling/turning |
| 6.3 μm | Rough machine finish | Rough milling, turning |
| 12.5 μm | Very rough finish | Heavy roughing, sawing |
Surface Roughness Conversion Table
Convert between Ra (\u03BCm), Ra (\u03BCin), Rz, RMS, and N-grade standards. Rz values are approximate (typically 4–5× Ra).
| Ra (\u03BCm) | Ra (\u03BCin) | Rz (\u03BCm) | RMS (\u03BCm) | N Grade | ISO Grade |
|---|---|---|---|---|---|
| 0.1 | 4 | 0.5 | 0.11 | N1 | N1 |
| 0.2 | 8 | 1.0 | 0.22 | N2 | N2 |
| 0.4 | 16 | 1.8 | 0.44 | N3 | N3 |
| 0.8 | 32 | 3.6 | 0.88 | N4 | N4 |
| 1.6 | 63 | 7.2 | 1.76 | N5 | N5 |
| 3.2 | 125 | 14.4 | 3.52 | N6 | N6 |
| 6.3 | 250 | 28.8 | 6.93 | N7 | N7 |
| 12.5 | 500 | 57.6 | 13.75 | N8 | N8 |
* RMS ≈ Ra × 1.1. Rz is approximate and varies by measurement standard (ISO vs DIN).
Surface Finish by Process Method
| Finish Type | Ra Range | Method | Cost Impact |
|---|---|---|---|
| As-machined | Ra 1.6–3.2 μm | Standard CNC with sharp tooling | Baseline (1x) |
| Bead blasted | Ra 1.0–3.0 μm | Glass bead or aluminum oxide media | Low (+10–15%) |
| Brushed | Ra 0.4–1.6 μm | Abrasive belt or scotch-brite wheel | Low (+15–20%) |
| Tumble polished | Ra 0.2–0.8 μm | Vibratory tumbling with media | Medium (+20–30%) |
| Hand polished | Ra 0.05–0.2 μm | Progressive sanding + buffing | High (+50–100%) |
| Mirror polished | Ra < 0.05 μm | Diamond paste lapping | Very high (+100–200%) |
Frequently Asked Questions
What is Ra in surface finish measurement?
Ra (Roughness Average) is the arithmetic average of the absolute values of the profile heights measured from the mean line. It is the most widely used surface roughness parameter. A lower Ra number means a smoother surface.
What is the difference between Ra and Rz?
Ra is the arithmetic average roughness across the entire measurement length. Rz is the average of the five highest peaks and five deepest valleys within the sampling length. Rz is typically 4–5 times the Ra value and better captures occasional deep scratches that Ra may average out.
What surface finish can standard CNC machining achieve?
Standard CNC machining typically achieves Ra 1.6–3.2 μm (63–125 μin) as-machined. With finishing passes and sharp tooling, Ra 0.8 μm (32 μin) is achievable. Finer finishes require secondary operations like grinding or polishing.
How does surface finish affect part cost?
Each halving of the Ra value roughly doubles the finishing cost. An as-machined Ra 3.2 μm part is the cost baseline. Achieving Ra 0.8 μm adds 15–25% cost. Ra 0.4 μm adds 30–50%. Mirror finishes (Ra 0.1 μm or below) can double or triple the part cost.
Does surface finish affect coating adhesion?
Yes, surface finish significantly impacts coating adhesion. Too smooth (Ra < 0.4 μm) provides insufficient mechanical keying. Too rough (Ra > 6.3 μm) creates coating thickness variation. For anodizing, Ra 0.8–1.6 μm is ideal. For paint and powder coat, Ra 1.6–3.2 μm provides optimal adhesion.
How do I specify surface finish on a drawing?
Use the standard surface finish symbol (a checkmark with a tail) and specify the Ra value in micrometers or microinches. Place the symbol on the surface view where the finish is required. Include the machining process if critical (e.g., ground, lapped). Add a general note for default surface finish on non-marked surfaces.
Need a Specific Surface Finish?
We offer a full range of surface finishing options from as-machined to mirror polish. Our team can recommend the optimal finish for your application.