How To Choose Surface Finishes For CNC Milled Parts

The machining is done. Parts come off the machine looking decent, maybe even pretty good. But here’s the thing — that as-machined surface isn’t always the final answer. Sometimes it’s perfect. Other times it’s just the starting point before finishing transforms those components into something that truly meets the application requirements.

Surface finishing decisions get overlooked surprisingly often during the design phase. Then reality hits when parts need to resist corrosion, wear better, look presentable to customers, or meet some industry spec. This guide walks through the common finishing options for CNC milled parts and helps clarify when each makes sense.

Why Surface Finishes Matter for CNC Milled Parts

A surface finish does more than make parts look nice. Although aesthetics certainly play a role in many applications, the functional benefits often matter more.

Functional Reasons to Finish Parts

Several practical considerations drive finishing decisions:

Corrosion protection for parts exposed to moisture, chemicals, or outdoor conditions
Wear resistance on sliding or contact surfaces
Reduced friction for moving components
Improved paint or adhesive bonding
Electrical conductivity or insulation requirements
Easier cleaning for food-grade or medical applications

A raw aluminum part might work fine in a controlled indoor environment. Put that same part outside and oxidation starts immediately. Context determines whether finishing is optional or essential.

Common Surface Finish Options for CNC Milled Parts

Finish Type	Process	Best For	Typical Materials	Relative Cost
As-Machined	None (direct from machine)	Prototypes, internal components	All machinable materials	Lowest
Bead Blasting	Abrasive media impact	Uniform matte appearance	Metals, some plastics	Low
Anodizing	Electrochemical conversion	Corrosion and wear resistance	Aluminum only	Medium
Powder Coating	Electrostatic application + heat cure	Durable colored finish	Metals	Medium
Electroplating	Metal deposition in solution	Hardness, conductivity, appearance	Metals	Medium-High
Polishing	Mechanical abrasion	Mirror finish, decorative use	Metals, some plastics	Variable
Passivation	Chemical treatment	Corrosion resistance	Stainless steel	Low-Medium

Detailed Look at Popular Finishes for CNC Milled Parts

As-Machined Finish

Straight off the CNC machine, parts have visible tool marks — those characteristic patterns left by the cutting tool’s path. The roughness depends on machining parameters, tooling condition, and material. Typical as-machined surfaces fall in the Ra 1.6 to 3.2 μm range, though finer finishes are achievable with adjusted feeds and speeds.

When does as-machined work? For internal components nobody sees, prototypes being tested for fit, or parts that receive additional finishing anyway. No point paying for a finish that adds no value.

Bead Blasting

Glass beads or other media get propelled at high velocity against the part surface. The result is a uniform, satin-like matte texture that hides minor machining marks and fingerprints. Bead blasting is quick, affordable, and works on most metals.

It doesn’t add corrosion protection on its own though. Many shops combine bead blasting with anodizing or other protective finishes — the blasting provides the texture, the secondary process provides protection.

Anodizing (Type II and Type III)

Anodizing converts the aluminum surface into a hard aluminum oxide layer through an electrochemical process. It’s not a coating that sits on top — it’s actually integrated into the base material.

Type II anodizing creates a thinner layer (typically 8-25 μm), accepts dyes well, and provides good corrosion resistance
Type III hard anodizing produces a thicker, harder layer (25-75 μm) with excellent wear properties

The dimensional change from anodizing needs consideration during design. Hard anodizing in particular adds noticeable thickness that can affect tolerances on precision CNC milled parts.

Powder Coating

Dry powder gets electrostatically applied to a grounded metal part, then cured in an oven where it melts and flows into a continuous film. The result is a durable, even coating available in countless colors and textures.

Powder coating excels for:

Consumer products requiring specific brand colors
Outdoor equipment needing UV and weather resistance
Industrial components where chip and scratch resistance matters

It’s thicker than most finishes — typically 50-100 μm — which rules it out for tight-tolerance features. Masking critical surfaces is standard practice.

Electroplating

Plating deposits a thin metal layer onto the part surface. Different plating metals serve different purposes:

Nickel — hardness, corrosion resistance, attractive appearance
Chrome — extreme hardness, low friction, distinctive look
Zinc — sacrificial corrosion protection for steel
Gold — conductivity, corrosion immunity for electronics

The base material must be electrically conductive. Plating CNC milled parts from plastic requires a conductive undercoat first, which adds complexity and cost.

Factors to Consider When Selecting a Finish

Key Selection Criteria

Environment — Will the part face moisture, chemicals, salt spray, UV exposure?
Function — Does the surface need to be hard, slippery, electrically conductive?
Appearance — Is this customer-facing or hidden inside an assembly?
Material compatibility — Not every finish works with every material
Dimensional impact — Will the added thickness affect fit or function?
Budget — Finishing can easily double part cost if not planned carefully

Sometimes the cheapest path is designing around the need for finishing altogether. A corrosion-resistant base material might eliminate the need for protective coatings. If you want to know more about CNC milled parts, please read 5 Most Common Materials for CNC Milled Parts.

الأسئلة الشائعة

What surface finish is best for aluminum CNC milled parts?

Anodizing is generally the most popular choice for aluminum. It provides corrosion resistance, wear resistance, and color options in a single process. Type II works for most applications while Type III hard anodizing suits high-wear situations. Bead blasting plus clear anodize creates an attractive matte finish common on consumer electronics.

Do surface finishes affect the dimensions of CNC milled parts?

Yes, most finishes add some thickness. Anodizing penetrates partially into the material while also building outward — roughly half and half. Powder coating adds 50-100 μm. Plating thickness varies by type and specification. Critical dimensions often require masking or post-finish machining to maintain tolerance.

Can the same part have multiple different finishes?

Absolutely. It’s common to see CNC milled parts with masked areas to create finish zones — perhaps anodized body with uncoated threads, or plated contacts on an otherwise powder-coated housing. Masking adds labor and cost but allows optimization of different surfaces for different requirements.