CNC Surface Finish Guide: Ra Values and Anodizing

By CHX Metal Engineering Team · Published 2026-10-09

A guide to CNC surface finish - what Ra means, as-machined finishes, anodizing types (II and III), plating, passivation and how to specify finish on a drawing. Engineering teams review drawings and return manufacturability feedback within 24 hours.

Surface finish is where a good part can look expensive for no reason - or fail because a callout was missing. Finish is specified in Ra for the machined surface, then in the name of a coating or treatment. Here is how the two layers work together.

What Ra actually means

Ra is the arithmetic average roughness of a surface, expressed in micrometres (or micro-inches). Lower Ra means a smoother surface. It is measured across a short sampling length, so it describes texture rather than waviness or form.

As a rough reference for CNC machining: a standard turned or milled finish sits around Ra 1.6-3.2 um; a fine finish with a light finishing pass sits around Ra 0.8 um; a ground or lapped surface can reach Ra 0.4 um or better. The exact value depends on the tool, the feeds and speeds, and the material.

Do not specify a low Ra across a whole part. Specify it only on the faces that touch something, seal something, or are seen. A blanket polish adds cost without function.

As-machined finishes

A well-set CNC machine produces a clean as-machined finish on its own, with visible tool marks that follow the cut direction. For most internal and hidden surfaces this is fine, and it is the most economical option because it adds no operation.

Where a decorative or sealing surface is needed, a finishing pass with a smaller stepover or a finer feed produces a visibly smoother face. Because it is done on the same machine, it does not disturb the tolerance chain the way a separate operation can.

Anodizing: Type II vs Type III

Anodizing is an electrochemical conversion of the aluminium surface into a hard oxide layer. It is not a paint or a plating - it grows out of the metal, so it does not chip or peel like a coating.

• Type II (sulphuric anodizing) - the common decorative and protective finish. Clear or dyed in a range of colours. The oxide layer is typically around 5-25 um and adds mild corrosion and wear resistance.
• Type III (hard anodizing) - a thicker, denser layer, typically 25-50 um, with noticeably higher wear resistance. Used on wear surfaces and sliding parts. It is usually darker and can shift dimensions, so tight-tolerance parts must account for the growth.

Two practical notes: anodizing only applies to aluminium, and because the layer grows both outward and inward, a tight bore may need to be masked or pre-compensated. Colour matching across batches is good but not guaranteed, so if several parts must match, ask for them to be anodized together.

Plating, passivation and other treatments

• Passivation - a chemical clean of stainless steel that removes free iron and restores the passive oxide layer, improving corrosion resistance. Common on 303/304/316 parts.
• Electroless nickel plating - a uniform, hard, corrosion-resistant layer that works on complex shapes and is often used where a conductive, wear-resistant surface is needed.
• Zinc / tin / silver / gold plating - used on copper and steel connector and contact parts for solderability, conductivity or corrosion resistance.
• Black oxide - a thin conversion coating for steel, mostly cosmetic and lightly protective; it holds a light oil film well.
• Powder coating and painting - a thicker cosmetic and protective layer; it is a masking operation, so specify which surfaces must stay bare.
• Bead blasting - a mechanical texture used to even out a surface or give a matte look; it is often applied before anodizing.

How finish and tolerance interact

Any coating that adds thickness can move a dimension. Anodizing, plating and powder coating all affect the final size, and the effect is not always uniform on threads and bores. If a part has both a tight fit and a coating, mask the fit or compensate the pre-finish dimension.

The other interaction is order of operations. Finishing usually comes after machining, so a surface that must be flat after anodizing has to be planned for, and a laser-marked serial number should be applied before or after the coating depending on whether it must survive.

How to specify finish on a drawing

• Give a general note for the default finish, then call out Ra only on the faces that need it.
• Name the treatment precisely: "clear Type II anodize", "Type III hard anodize, 25-50 um", "passivate per ASTM A967".
• Mark masking requirements - threads, bores, electrical contact areas, datum faces.
• Say whether colour matching across parts matters, so they can be processed together.

At CHX Metal, anodizing, plating, passivation, black oxide and laser marking are handled in-house, with powder coating and painting coordinated as part of the same order. Keeping finishing close to machining keeps the tolerance chain short.

Frequently asked questions

What Ra is normal for CNC machining?

A standard turned or milled finish is around Ra 1.6-3.2 um; a fine finishing pass reaches about Ra 0.8 um; ground surfaces can reach Ra 0.4 um or better. Specify Ra only where the function needs it.

Will anodizing change my part dimensions?

Yes. The oxide layer grows both outward and inward, typically 5-25 um for Type II and 25-50 um for Type III. Tight bores and threads should be masked or pre-compensated.

What is the difference between Type II and Type III anodizing?

Type II is a thinner decorative and protective finish, often dyed. Type III (hard anodizing) is thicker and denser, with higher wear resistance, and is typically darker.

Can you anodize and machine in the same shop?

Yes. Anodizing, plating, passivation, black oxide and laser marking are handled in-house at CHX Metal, which keeps the finishing step in the same tolerance chain as the machining.

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