What are the types of surface finishes for CNC machining?

CNC machining is a subtractive manufacturing process that can produce parts with tight tolerances (up to ± 0.025 mm) and fine detail from an enormous range of metals and plastics. However, due to CNC’s subtractive nature— in contrast with 3D printing and additive manufacturing—parts often come off milling and turning machines with visible tool marks. 

This is where surface finishing comes in. Applying post-processing and the right surface finishes can improve surface roughness, cosmetic and visual properties and wear resistance of CNC-machined parts. Surface finishing, applied correctly, improves functionality and aesthetics, and in many cases both. 

In this article, we discuss the most common surface finishes for metal CNC parts to help you select the right ones for your application.

What CNC machining surface finishes do we offer?

Here are the most common surface finishes available on our platform. Please note that these are all for metal parts.

Surface finishes explained: As machined

All CNC machined parts will have marks that follow the path of the cutting tool used during machining. The quality of the surface is measured in terms of average surface roughness (Ra). Ra is a measure of the average deviation of the machined profile from the ideal surface.

The standard as-machined surface roughness is 3.2 μm (125 μin). A finishing cutting pass can be applied to reduce the roughness of a surface down to 1.6, 0.8 or 0.4 μm (63, 32 or 16 μin). This will increase the cost of producing a part, as extra machining steps and tighter quality control are required.

As machined parts can be smoothed or polished to improve their surface quality and visual appearance by reducing their surface roughness. Smoothing and polishing will remove some material affecting the dimensional tolerances of the part.

Surface finishes explained: Bead blasting

Bead blasting adds a uniform matte or satin surface finish to a machined part, which removes the tool marks. It works by bombarding your part with small glass beads using a pressurized air gun. This removes excess material and smoothes out the surface of the part. Critical surfaces or features (like holes) can be masked to avoid any dimensional changes.

In general, bead blasting is used more for part aesthetics than functionality. It is a manual process, so the result will depend to some extent on the skill of the operator. The amount of air pressure and the size of the glass beads are the main process parameters. Glass beads come in different sizes (from course to very fine) just like sandpaper comes in different sizes and grades. Protolabs Network generally bead blasts parts with a grit size of #120.

Surface finishes explained: anodizing (Type II and Type III)

Anodizing adds a thin ceramic layer to the surface of metal parts. This layer protects against corrosion and wear. The anodic coating is electrically non-conductive, has high hardness (type III) and can be dyed in different colors. Anodizing is only compatible with aluminum and titanium.

With Type II & Type III anodizing, the part is submerged in a diluted sulfuric acid solution and an electric voltage is applied between the component and the cathode. An electrochemical reaction consumes the material on a part’s exposed surface, converting it into hard aluminum or titanium oxide. A mask can be applied to surfaces with critical dimensions (like threaded holes) or surfaces that must remain electrically conductive to prevent them from anodizing. The anodized parts can also be dyed various colors before sealing (such as red, blue, black or gold).

By varying the electric current, anodizing time and consistency and temperature of the solution, coatings of different thicknesses and densities are created.

What is Type II (clear or color) anodizing?

Anodizing Type II is also called "standard" or "decorative" anodizing and can produce coatings with a thickness of up to 25 μm. The typical coating thickness depends on the color and can vary between 8-12 μm for parts dyed black and 4-8 μm for clear (un-dyed) parts.

Type II anodizing is mainly used to produce parts with a smoother surface and provides good corrosion resistance and limited wear resistance.

What is Type III (hardcoat) anodizing?

Anodizing Type III is also called "hardcoat" anodizing and can produce coatings up to 125 μm thick. If not otherwise specified, the typical type III anodic coating is 50 μm thick.

Type III anodizing produces thick, ceramic coatings with high density that provide excellent corrosion and wear resistance, suitable for functional applications. Be aware that it requires closer process control compared to type II anodizing (higher current density and constant solution temperature near 0 degrees Celsius), so the cost is higher.

Pro-tip for anodizing

The anodic coating grows in both directions equally. This means that a 50 μm thick coating will protrude approximately 25 μm above the original surface and that a cylinder with 1 mm diameter before anodizing, will have a diameter of 1.05 mm after anodizing. More details on anodizing can be found in the MIL-A-8625 standard.

Surface finishes explained: Powder coating

Powder coating adds a thin layer of protective polymer to the surface of the part. Powder coating is a strong, wear-resistant finish that is compatible with all metal materials and can be combined with bead blasting to create parts with smooth and uniform surfaces and excellent corrosion resistance.

The powder coating process is similar to spray painting, but the "paint" in question is a dry powder instead of a liquid. The parts are first primed with an optional phosphating or chromating coating to increase their corrosion resistance, then coated with dry powder using an electrostatic "spray" gun and then cured at high temperature (usually in an oven at 200 degrees Celcius).

Multiple layers can be applied to create a thicker coating and the typical thickness varies from approximately 18 μm up to 72 μm. A wide range of colors is available.

What are Protolabs Network top tips & tricks for choosing surface finishes for CNC machining? 

Every surface finish has its advantages and disadvantages, so choosing the right one depends on what features match your part requirements and application. 

It’s essential to evaluate the functional and aesthetic characteristics you need, and to consider how your part will interact with adjoining components. Whether or not your part will be subject to repeated or intense friction may sway your decision. 

Also, remember to take into account your part operational environment. You may believe your part doesn’t need robust resistances, but the environmental facts might require a more intensive surface finish than you previously thought during the design phase. 

Best practices per surface finish

• Bead blasting: This surface finish works well if you’re not worried about dimensional tolerances. It leaves the part with a matte uniform finish.

• Anodizing (type II): Use this to give your aluminum and titanium parts a protective layer that’s also cosmetic—parts come out with natural matte colors. 

• Anodizing (type III): Perfect for advanced engineering applications, anodizing (type III) gives parts high wear resistance and surface hardness.

• Powder coating: Use this surface finish for applications where you need high-impact strength. Powder coating is also suitable if your parts can’t be anodized.