A CNC machined part is not always finished when it comes out of the machine.

Depending on the application, the part may require additional surface treatment to improve:

• Corrosion resistance
• Wear resistance
• Appearance
• Surface texture
• Hardness
• Electrical properties
• Cleanability
• Friction
• Dimensional stability

This is why CNC machining surface finish should be considered during the design and quoting stage—not after the parts have already been machined.

A finish that looks excellent may not be suitable for a tight-tolerance assembly.

A coating that provides excellent corrosion protection may add thickness to critical surfaces.

And a highly polished surface may be unnecessary for an internal component.

The right approach is to choose the finish based on the function, environment, material, dimensional requirements, and appearance of the part.

This guide compares the most common CNC surface finishes and explains how to specify the right one for your custom parts.

What Is CNC Surface Finishing?

CNC surface finishing refers to processes performed on a machined part to modify its surface properties or appearance.

These processes can be broadly divided into several categories:

Mechanical finishing

• Bead blasting
• Brushing
• Polishing
• Tumbling
• Grinding

Chemical or electrochemical finishing

• Anodizing
• Passivation
• Electropolishing
• Chemical conversion coating

Coatings and plating

• Powder coating
• Electroplating
• Electroless nickel plating
• Black oxide
• Other protective coatings

The appropriate process depends heavily on the material.

For example, anodizing is commonly associated with aluminum, while passivation is primarily used for stainless steel. Powder coating can be applied to a range of metal substrates.