CNC Machining Surface Finish Specifications Explained

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In the world of precision manufacturing, the surface finish of a CNCmachined part is far more than just an aesthetic consideration; it is a critical functional characteristic that directly impacts performance, durability, and cost. For businesses sourcing components, understanding surface finish specifications is key to ensuring their products meet exacting standards. This guide explains the common specifications and their practical implications for your projects.


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Understanding Surface Roughness

The most common measure of surface finish is roughness, typically denoted as Ra (Arithmetic Average Roughness). Ra measures the average of the peaks and valleys on a part's surface. A lower Ra number indicates a smoother surface.

Rough Machined (Ra 3.2 μm / 125 μin): Visible tool marks are present. This is suitable for noncritical surfaces, internal components without fit, and parts intended for further processing like painting.
Standard Machined (Ra 1.6 μm / 63 μin): This is the most common "asmachined" finish. Tool marks are less visible, offering a good balance between cost and performance for many functional parts and assemblies.
Smooth Machined (Ra 0.8 μm / 32 μin): Tool marks are barely visible. This finish is ideal for parts subject to stress concentration, bearing surfaces, and components that require a reliable seal.

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Very Smooth (Ra 0.4 μm / 16 μin): A highquality finish with a surface that feels smooth to the touch. It is used for highspeed bearing surfaces, precision instruments, and parts that undergo frequent movement and friction.

Beyond Ra: Other Finishing Processes



While standard CNC machining achieves a certain Ra, secondary processes are often specified to enhance properties.

Anodizing: An electrochemical process that creates a hard, protective, and corrosionresistant oxide layer on aluminum. It can be dyed in various colors. Anodizing will slightly increase the part's surface roughness.
Bead Blasting: This process uses fine glass beads to create a uniform, matte, satinlike surface. It improves aesthetics and can help mask minor tool marks, providing a consistent nonreflective appearance.
Powder Coating: A dry finishing process where a powder is electrostatically applied and then cured under heat to form a hard, durable, and attractive coating. It offers excellent corrosion resistance and is available in a vast range of colors and textures.

Why Your Choice of Finish Matters

Selecting the right surface finish is a strategic decision. An overly smooth finish on a noncritical surface unnecessarily increases machining time and cost. Conversely, a finish that is too rough can lead to premature wear, fatigue failure, or fluid leakage. The optimal specification balances functional requirements with budget and lead time.

As your onestop CNC machining partner, we possess the expertise to guide you through these specifications. We ensure your parts are not only machined to precise geometries but also finished with the optimal surface treatment for superior performance, longevity, and value. Let us help you specify the perfect finish to make your product a success.