CNC Machined Surface Finishes: How to Choose the Right Treatment Without Overpaying

A surface finish spec is one of the fastest ways to add cost to a machined part, and one of the easiest to get wrong. The right finish protects the part and makes it look intentional. The wrong finish adds operations, delays delivery, and sometimes hides the surface quality the part actually needs. The trick is knowing what each finish really does, and what it does to your dimensions.

Machined Finishes: The Surface as It Comes Off the Tool

Most parts do not need any treatment at all. The machined surface is defined by Ra, the average roughness:

• 3.2 Ra: the standard machined finish. Fine for most structural and internal parts.
• 1.6 Ra: a finer cut, visible improvement, a small extra cost.
• 0.8 Ra and below: a precision finish, used on sealing faces, bearing seats and appearance surfaces. Costs more because the tool, feeds and inspection all have to earn it.

Two details often missed: the direction of lay (the tool marks have a direction, and on sealing surfaces it matters), and the fact that a low Ra number does not remove waviness or edge burrs.

Mechanical Finishes

• Bead blasting: a uniform matte look that hides minor tool marks. It also rounds sharp edges slightly, so it can affect edge geometry and, on fine tolerances, the edges that matter.
• Wire drawing and brushing: a linear grain, mostly cosmetic, used on enclosures and visible panels.
• Polishing: removes material, which is exactly the problem when tolerances are tight. Use it on appearance or sealing surfaces, and account for the material removed.

Conversion and Coating Finishes

This is where finish and tolerance interact most:

• Anodizing (Type II and Type III): grows oxide into the aluminum surface. Holes shrink and external features grow by roughly 10 to 20 microns per surface for Type II. Type III (hard anodize) builds a thicker layer and changes dimensions more. It is a wear and corrosion treatment first, a color option second.
• Passivation: for stainless steel. Removes free iron and restores the passive layer. No significant dimensional change, and it is cheap insurance for corrosion resistance.
• Plating (nickel, zinc, chrome): adds material. The coating thickness matters to fits, and hardened parts need to be checked for hydrogen embrittlement risk.
• Powder coating and painting: thick, protective and cosmetic. They cover fine tolerances; threads and precision bores must be masked, which is cost and risk.

How to Specify Without Overpaying

Four habits keep finish cost under control:

• Only specify finish where it matters: appearance, corrosion, wear or sealing. An internal hidden bracket does not need anodize; a visible housing might.
• Give a clear callout. “Anodize, black, Type II” or “Ra 0.8 on sealing faces only” beats “smooth surface”.
• Understand the dimension change. If anodizing is specified, the machining allowance has to be set for it, or the fit changes.
• Ask the supplier what the finish will do to your critical dimensions before you order it, not after.

How KGL Coordinates Surface Treatment

KGL coordinates surface treatment in-house: anodizing, chromate conversion coating, sandblasting, blackening, wire drawing, polishing, electroplating, electrophoretic coating and painting. Parts that need treatment are machined with the coating allowance built in, and re-verified after finishing, so the part that ships is the part that fits.

Conclusion

The right finish is the one matched to function and verified on the part. Specify it clearly, budget the dimension change, and let the supplier confirm the process before it runs. The cheapest finish is the one you did not need; the second cheapest is the one specified once, correctly.

FAQ

Q: Does anodizing change part dimensions?

A: Yes. Anodizing grows into the aluminum surface: holes shrink and external dimensions grow, typically by roughly 10 to 20 microns per surface for Type II. If the part is machined to final size and then anodized, the fit changes.

Q: What is passivation and when do I need it?

A: Passivation is a chemical treatment for stainless steel that removes free iron from the surface and restores the corrosion-resistant passive layer. It is inexpensive, changes dimensions negligibly, and is standard practice for stainless parts that see moisture, chemicals or cleaning.

Q: Does bead blasting affect my tolerances?

A: On edges, yes. Blasting rounds sharp edges slightly, which can matter on fine edge tolerances and sharp-corner requirements. If edge geometry is critical, specify whether blasting is acceptable before the finish is applied.