Understanding Surface Roughness (Ra)
Surface roughness is measured as Ra (Roughness Average) in micrometers (μm) or microinches (μin). Standard CNC machining produces Ra 1.6-3.2μm. Finer finishes require additional operations.
Common Ra Values
| Ra (μm) | Ra (μin) | Appearance | Typical Method |
|---|---|---|---|
| 6.3 | 250 | Visible tool marks | Rough machining |
| 3.2 | 125 | Standard machined | Standard CNC |
| 1.6 | 63 | Smooth machined | Fine CNC pass |
| 0.8 | 32 | Very smooth | Grinding/polishing |
| 0.4 | 16 | Mirror-like | Lapping/polishing |
Post-Machining Finishes
Anodizing (Aluminum Only)
Type II: decorative, 5-25μm thickness, many colors available. Type III (Hard Anodize): functional, 25-100μm, excellent wear resistance.
Electroless Nickel Plating
Uniform thickness regardless of geometry, good corrosion protection, can be applied to steel and aluminum.
Powder Coating
Thick, durable coating available in any color. 50-100μm typical thickness. Good for external housings and covers.
Passivation (Stainless Steel)
Chemical treatment that enhances the natural oxide layer. Does not change dimensions or appearance but significantly improves corrosion resistance.
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