Surface Finish Guide: Ra Values, Anodizing, Plating and Powder Coating
Roughness values by preparation
| Preparation | Ra (µm) | Typical use |
|---|---|---|
| As-machined finish | 1.6 - 3.2 | General surfaces, non-sealing |
| Fine machined | 0.4 - 1.6 | Sealing faces, sliding fits |
| Ground | 0.2 - 0.8 | Precision fits, seal seats |
| Polished / lapped | 0.05 - 0.4 | Optics, seals, low friction |
| Bead blasted | 0.8 - 3.2 | Matte decorative, pre-paint |
Finishing options compared
| Finish | Thickness | Appearance | Best for |
|---|---|---|---|
| Anodizing (Type II) | 5 - 25 µm | Clear, black, dyed colours | Aluminium, corrosion and wear |
| Hard anodizing (Type III) | 25 - 50 µm | Dark grey to black | Aluminium, heavy wear |
| Zinc plating | 5 - 12 µm | Blue, yellow, black | Steel fasteners and brackets |
| Nickel plating | 5 - 25 µm | Bright metallic | Wear and corrosion, electrical |
| Powder coating | 60 - 120 µm | Wide colour range | Outdoor steel and aluminium parts |
| Passivation | None | Bare, clean | Stainless, removes free iron |
| Electropolishing | Removes 5 - 20 µm | Mirror, bright | Stainless hygiene and finish |
Choosing a finish
- Aluminium that must stay conductive or be masked for grounding should be chromate conversion coated, not anodized.
- Anodizing adds a few microns - allow for it on threads and close fits.
- For stainless parts, specify passivation after machining to remove embedded free iron and stop rust spots.
- Powder coating is for appearance and weather, not for a precision fit.
Frequently asked questions
Which finish should I specify for an aluminium part?
Clear anodizing (Type II) for general protection, hard anodizing (Type III) for sliding or wear surfaces, and chromate conversion coating where the part must stay conductive or grounded.
Does anodizing change the part dimensions?
Yes. Anodizing adds 5 to 25 µm per surface, and hard anodizing up to 50 µm. On tight fits and threads, allow for the coating in the drawing or specify masking.
How do I prevent rust on stainless parts?
Specify passivation after machining. It removes free iron left by tooling and prevents the surface rust spots that appear on otherwise correct stainless steel.
Can you provide the finish as well as the machining?
Yes. Machining and finishing are arranged together so the coating thickness is planned into the tolerances, and the parts ship finished in one delivery. Email sc@bquq.com.
Talk to an engineer
CNC machining · metal stamping · custom springs · heat sinks — ISO9001 source factory in Dongguan, China. ±0.005mm process control, quote within 12 hours.
Email sc@bquq.com | WhatsApp +86 13713157787 | www.bquq.com