Finishing · Anodizing · Bead blasting · Powder coating
Surface finishes for machined parts
A practical guide to CNC surface finishes: what each finish does, which materials it suits, how much thickness it adds and when to use it. Every finish below can be ordered together with your machined or sheet metal parts.
CNC surface finishes chart
The finish affects appearance, corrosion resistance, wear, electrical conductivity and, often overlooked, final dimensions. Use this surface finish chart to shortlist options, then put the finish on your drawing with any masked areas marked. Thickness values are typical industry ranges, not guarantees; if a dimension must hold after finishing, say so on the drawing.
| Finish | What it does | Materials | Added thickness | Typical use |
|---|---|---|---|---|
| As machined | No treatment; visible tool marks | All | 0 (Ra 3.2 µm typical) | Internal parts, prototypes, fit checks |
| Bead blasted | Uniform matte texture, hides tool marks | Aluminum, stainless, steel, titanium, brass | ≈0 (slight surface change) | Cosmetic parts, pre-treatment before anodizing |
| Anodized Type II | Thin oxide layer; dyeable clear, black or colors | Aluminum | ≈5–25 µm total | Consumer housings, brackets, corrosion protection |
| Hard anodized Type III | Thick, hard oxide layer | Aluminum | ≈25–75 µm total | Wear surfaces, sliding parts, harsh environments |
| Powder coating | Baked polymer coat in RAL colors | Steel, aluminum, stainless | ≈50–150 µm | Sheet metal enclosures, frames, outdoor parts |
| Passivation | Removes free iron, restores the passive layer | Stainless steel | 0 | Medical, food, marine, any stainless part |
| Zinc plating | Sacrificial corrosion protection | Carbon and alloy steel | ≈5–25 µm | Steel brackets, fasteners, sheet metal |
| Nickel plating | Corrosion and wear resistance, bright or satin look | Steel, brass, copper, aluminum | ≈5–25 µm | Connectors, decorative and wear parts |
| Polishing | Smooth, reflective surface | Aluminum, stainless, brass, plastics | Removes material | Cosmetic surfaces, sealing faces |
| Laser marking | Permanent text, logos, serials, QR codes | Metals, many plastics, anodized aluminum | 0 | Part numbers, traceability, branding |
Finishes can be combined. The most common stack is bead blasted plus black anodized aluminum, followed by laser marking through the anodize. Powder coating with masked threads is the usual choice for steel sheet metal enclosures.
Surface roughness: what Ra 3.2 µm means
Ra is the arithmetic average deviation of the surface profile from its mean line. An as-machined surface at Ra 3.2 µm (125 µin) shows faint tool marks you can see and just feel with a fingernail. It is fine for most non-cosmetic and non-sealing faces.
| Ra (µm) | Ra (µin) | Typical meaning |
|---|---|---|
| 3.2 | 125 | Standard as-machined finish |
| 1.6 | 63 | Fine finish; slower cuts; good for mating and light sealing faces |
| 0.8 | 32 | Bearing fits, O-ring grooves, dynamic seals; may need extra passes or polishing |
| ≤0.4 | ≤16 | Polished or lapped surfaces; specify only where it matters |
Finer finishes are available on request. Call them out on the specific faces that need them, not as a general note, because a smoother finish everywhere adds machine time. Note that bead blasting increases roughness, and anodizing slightly roughens and then reproduces the surface underneath; it does not hide deep tool marks.
Bead blasting vs anodizing
These are not alternatives so much as partners. Bead blasting is a mechanical texture: glass beads hit the surface and leave a uniform matte look that hides machining marks. It offers no corrosion protection on its own, and blasted bare aluminum fingerprints and stains easily. Anodizing is an electrochemical conversion that grows an oxide layer, protecting the part and allowing color.
- Bead blast only: cheap matte look for steel or stainless parts, or a prep step.
- Anodize only: keeps the machined texture visible under the color; tool marks show through.
- Bead blast + anodize: the familiar matte consumer-electronics look, usually black or clear.
Thin walls and tight features can be affected by blasting, so mask precision bores and sealing faces. Our anodizing guide covers Type II vs Type III, thickness and color matching in detail.
Powder coating machined parts
Powder coating is a dry polymer powder applied electrostatically and cured in an oven. It gives a thick, durable, colorful coat and is the standard finish for steel enclosures and frames. On precision machined parts, its thickness is the main concern: at roughly 50–150 µm per surface, it will close up threads, shrink holes and change fits. Mask threads, bores and mating faces, or open up the CAD dimensions to allow for the coating. Specify the color by RAL code and the gloss level (matte, satin or gloss). Sharp edges coat thinly, so a small edge break improves coverage.
Passivation for stainless steel
Machining can leave free iron from tools on a stainless surface, which shows up later as rust spots. Passivation is an acid treatment (nitric or citric, per standards such as ASTM A967) that removes this free iron and lets the chromium-rich passive layer re-form. It does not change appearance or dimensions, and it is good practice on almost any stainless part, especially 303, which has lower corrosion resistance than 304 or 316. See stainless steel machining for grade selection.
Plating, polishing and laser marking
Zinc plating is the low-cost corrosion protection for carbon and alloy steel, often with a clear or black passivate. Nickel plating gives a harder, more decorative surface and is common on brass and copper. Plating builds up on edges more than in recesses, so allow for it on threads. Polishing is quoted per surface and depends on access to the face. Laser marking adds part numbers, logos or serials without changing dimensions; on anodized aluminum it produces a crisp white or light gray mark.
Related
Finishes by material
FAQ
Surface finish questions
What surface finish do CNC parts have by default?
As machined, about Ra 3.2 µm (125 µin), with light tool marks visible. Finer finishes such as Ra 1.6 or 0.8 µm are available on request for the faces you call out.
Does anodizing or powder coating change part dimensions?
Yes. Type II anodizing adds roughly 5–25 µm, Type III roughly 25–75 µm, and powder coating roughly 50–150 µm per surface. Mask threads and precision bores, and state whether drawing dimensions apply before or after finishing.
Should I bead blast before anodizing?
If you want a uniform matte look that hides tool marks, yes. If you want a smoother, slightly reflective anodized surface, leave it as machined or polished before anodizing.
Do stainless steel parts need passivation?
It is recommended for most stainless parts. Passivation removes free iron left by machining and improves corrosion resistance without changing dimensions or appearance.
Can I powder coat aluminum parts?
Yes. Powder coating works on aluminum, steel and stainless. For aluminum, anodizing is thinner and keeps tighter dimensions, while powder coating offers more color choice and a thicker protective layer.