CNC Machining Surface Finishes: Ra, Rz, Anodizing, and When to Use Each

Written by the engineering team at Robocon CNC Pvt Ltd, Pune, India.

Surface finish is one of the most misunderstood aspects of CNC machining drawings. Engineers either don't specify it (leaving the machinist to guess) or specify Ra 0.4 μm everywhere (paying premium for surfaces where it doesn't matter). This guide explains surface roughness parameters, available finishes, and when to use each.

Understanding Ra and Rz

Ra (Roughness Average) is the arithmetic mean of all surface height deviations from the mean line, measured over a sampling length. It is the most widely used surface finish parameter globally.

Rz (Mean Roughness Depth) is the average of the five largest peak-to-valley heights in a measurement. It is more sensitive to isolated peaks or scratches and is preferred in German and European DIN standards. For a given surface, Rz ≈ 4–7 × Ra.

On your drawing: if you don't specify which parameter, Ra is assumed. If your application is sensitive to isolated high points (seals, optical surfaces), specify Rz explicitly.

As-Machined Surface Roughness Reference

Post-Machining Surface Treatments

Anodizing (Aluminum only)

Anodizing creates an oxide layer on aluminum surfaces that improves corrosion resistance, surface hardness, and appearance. Three main types:

• Type II Clear Anodize — Standard 8–12 μm coating. Transparent, shows the aluminum colour. Good corrosion resistance. Most common.
• Type II Black Anodize — Same process with black dye added. Common for electronics, camera housings, and instrument panels.
• Type III Hard Anodize (Hard Coat) — 25–50 μm coating. Much higher hardness (HRC 50+), excellent wear resistance. Used for hydraulic bores, sliding surfaces, military/aerospace. Turns the surface dark grey/black.

Anodize dimension note: Type II adds ≈0.005–0.010 mm per surface. Type III adds ≈0.013–0.025 mm per surface. Account for this in your bore and shaft tolerances — machine undersize by the expected coating thickness.

Electroless Nickel Plating

Uniform 10–25 μm nickel-phosphorus coating deposited without electrical current. Key properties: very uniform thickness on complex geometries, high hardness (500–600 HV), excellent corrosion resistance, and low friction. Works on steel, aluminum, and copper alloys. Common for precision hydraulic parts, molds, and oil & gas components.

Zinc Plating

Electrodeposited zinc, 5–15 μm thick. Good corrosion protection for steel parts, especially for interior/sheltered applications. Blue-clear, yellow, or black passivation options available. Inexpensive and widely used for structural steel brackets and hardware.

Powder Coating

Electrostatic application of dry polymer powder, cured at 180–200°C. Produces a thick (60–100 μm), durable coating with excellent impact and scratch resistance. Wide range of colours and textures. Used for external enclosures, frames, and any part requiring colour-matched appearance.

Not suitable for: tight-tolerance bores or threaded holes (mask or plug before coating). Also not suitable for parts that will see temperatures above 120°C in service.

Bead Blasting

Not a coating — a surface preparation. Glass beads or alumina grit are blasted onto the surface to create a uniform matte texture, remove tooling marks, and prepare the surface for subsequent coating. Adds no corrosion protection on its own. Very common as a final step before anodizing.

Surface Finish Decision Guide