CNC Surface Finishes: Complete Comparison Guide
Ra values, cost impact, appearance, and practical selection advice for 12 common post-machining finishes. What works on aluminum may ruin stainless steel.
I've been managing finishing lines for 18 years. The most expensive mistake I see is picking a finish that doesn't match the material. Hard anodize on stainless doesn't work. Passivation on aluminum does nothing. Black oxide on aluminum is a waste of money — it only works on steel.
Here's every finish we offer, what it costs, and when to use it.
Surface Finish Comparison Table
Cost is relative to as-machined baseline. Ra values are achievable range, not minimum.
| Finish | Ra (µm) | Appearance | Cost Impact | Best For | Notes |
|---|---|---|---|---|---|
| As-Machined | 0.8-3.2 | Tool marks visible | Baseline | Non-cosmetic internal parts, prototypes | Cheapest. Mill or lathe finish. |
| Bead Blast | 0.4-1.6 | Matte, uniform gray | +5-10% | Cosmetic parts, aluminium housings | Hides minor surface defects. 60-120 grit glass beads. |
| Media Tumble | 0.2-0.8 | Satin, edge radius | +5-10% | Deburring, small parts in batches | Ceramic media in vibratory finisher. |
| Brushed / Line | 0.4-0.8 | Linear grain pattern | +10-15% | Decorative panels, enclosures | Directional finish. Consistent grain direction matters. |
| Clear Anodize Type II | 0.4-1.6 | Natural aluminum tint | +15-25% | Aluminium corrosion protection | 0.005-0.015mm coating. Adds hardness. |
| Hard Anodize Type III | 0.4-1.6 | Dark gray to black | +25-40% | Wear surfaces, hydraulic components | 0.025-0.100mm coating. Very hard (60-70 HRC). |
| Chem Film / Alodine | 0.8-3.2 | Gold/yellow tint | +8-12% | Electrical conductivity, primer base | Thin conversion coating, conductive. |
| Passivation | 0.4-1.6 | No visible change | +5-8% | Stainless steel corrosion resistance | Citric or nitric acid bath. Removes free iron. |
| Electroless Nickel | 0.2-0.8 | Bright silver | +30-50% | Wear + corrosion, tight tolerances | Uniform coating 0.005-0.050mm. ±0.002mm on coating thickness. |
| Black Oxide | 0.4-1.6 | Deep black | +8-12% | Steel parts, reduced light reflection | Magnetite layer. Minimal dimensional change. |
| Powder Coat | — | Various colors, textured | +20-35% | Enclosures, frames, outdoor parts | 0.050-0.150mm thick. May affect thread fit. |
| Grinding | 0.1-0.4 | Mirror-like | +50-100% | Sealing surfaces, gasket faces | Surface grinder or jig grinder. Slow process. |
Matching Finishes to Materials
| Material | Recommended Finishes | Avoid |
|---|---|---|
| 6061/7075 Aluminum | Bead blast + anodize, chem film, hard anodize | Black oxide, electroless nickel (aluminum needs zincate first) |
| Stainless Steel | Passivation, electroless nickel, bead blast, as-machined | Anodize (aluminum only), black oxide (steel only) |
| Steel / Alloy Steel | Black oxide, electroless nickel, powder coat, phosphate | Anodize, passivation |
| Titanium | Bead blast, chemical etch, anodize (Type II), as-machined | Hard anodize, black oxide |
| Brass / Copper | Bead blast, clear coat, tumbled, as-machined | Anodize, black oxide |
| Plastics (PEEK, Acetal) | As-machined, bead blast (low pressure), tumbled | Anodize, passivation, black oxide, powder coat (heat damage) |
Surface Finish vs Tolerance — The Interaction
A common question I get: can I hold ±0.005mm on a bore that needs hard anodize? Answer: yes, but we machine it 0.025mm under final size, apply the coating, then ream to print. That's a two-step process that adds cost. Coatings are not perfectly uniform — they build up more on corners and edges than on flat surfaces.
If you need both tight tolerance and a coating, tell us during quoting. We quote it differently than a raw-machined part.
See also: CNC Machining Tolerances Guide, DFM Guidelines
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Get a Quote →What the Ra numbers actually mean in the shop
As-machined Ra 1.6 is what you get from a good carbide tool at reasonable feed — free, reliable, fine for 95% of parts. Ra 0.8 needs a finishing pass or a wiper insert: slightly more machine time, still cheap. Ra 0.4 and below is grinding or honing territory — a different process entirely, with its own setup, wheel dressing and inspection cost.
The drawing convention matters: Ra (arithmetical mean) is the default in most packages, but Rz (max height) appears on sealing faces and DIN drawings. A Ra 0.8 specification is roughly Rz 6.3 — mixing the two on a drawing is the most common finish error we see from design departments.
When to finish, when to leave it alone
Anodize on aluminium: functional (corrosion, hardness, cosmetic) and cheap as a batch process — but it changes dimensions slightly and hides surface defects, so inspect before you anodize. Black anodize in a specific shade is a slower, more expensive batch. Passivation on stainless is fast, cheap and does nothing visible — it is about corrosion resistance and medical compliance, not looks.
The parts that should stay as-machined: anything with a press fit (interference fits change with coating thickness), threads (chasing threads after plating is a cost nobody budgets), and any surface where a coating would make behaviour unpredictable. When a customer asks us whether to anodize, the answer is usually about where the part sits in the assembly — not about looks.