Anodizing is one of those finishes that looks like a checkbox on the print — until you get the parts back and realize you specified the wrong one. Type II and Type III aluminum anodizing solve different problems, cost different amounts, and behave very differently in service. Mixing them up is one of the most common — and expensive — finish mistakes in CNC machining.

Here’s a working machinist’s breakdown of when to specify which, what callouts your shop actually needs to see on the drawing, and the dimensional traps to plan for.

Quick Refresher: What Anodizing Actually Does

Anodizing is an electrochemical conversion process that grows an aluminum oxide layer on the surface of an aluminum alloy part. It is not a coating in the paint-on sense — the oxide layer is the part’s own surface, restructured. That’s why anodized surfaces remain dimensionally tied to the substrate and don’t chip the way painted finishes do.

The military specification that governs both common types is MIL-A-8625 (now superseded by MIL-PRF-8625, but the “Type” nomenclature persists everywhere). The Type tells you the process and the resulting characteristics:

• Type I — chromic acid anodize (used mostly in aerospace, low-thickness, dimensional stability)
• Type II — standard sulfuric acid anodize
• Type III — hard sulfuric acid anodize (“hardcoat”)

Most CNC shops in the U.S. work with Type II and Type III. Type I is a niche aerospace process and usually farmed out to specialty plating houses.

Type II: The Cosmetic and Light-Duty Default

Type II anodizing produces a thin oxide layer — typically 0.0001 to 0.001 in (2.5 to 25 µm) per surface. It accepts dye well, which is why nearly every colored aluminum consumer product (laptops, water bottles, optics housings) is Type II.

Specify Type II when:

• The part is cosmetic, or you need a specific color
• Service environment is mild — indoors, low-abrasion, no chemical exposure
• Tight dimensional tolerances are required (the thinner layer adds less stack)
• Cost matters — Type II is usually 1/2 to 1/3 the cost of Type III

Hardness: approximately 200–400 HV (Vickers). It will survive normal handling but will scratch under metal tooling.

Type III: The Wear-and-Corrosion Workhorse

Type III (hardcoat) is processed in colder, more concentrated sulfuric acid at higher voltage. The result is a denser, harder, thicker oxide layer — typically 0.001 to 0.004 in (25 to 100 µm) per surface, with the most common spec being 0.002 in ±0.0005 in.

Specify Type III when:

• The surface sees abrasive wear (sliding contacts, bushings, guide rails, cam followers)
• The part operates in a corrosive environment (marine, industrial, outdoor)
• You need dielectric isolation (Type III provides up to ~900 V breakdown)
• Fatigue life matters — Type III actually improves fatigue resistance modestly on some alloys

Hardness: approximately 600–1100 HV — comparable to tool steel in the upper range. Type III on 6061 routinely outlasts the underlying aluminum at the contact face.

The Dimensional Trap Everyone Trips Over

Here’s the one fact every designer should have tattooed somewhere visible: anodize layer growth is roughly half outward, half inward.

If you specify a 0.002 in Type III layer, the part grows approximately 0.001 in on each external surface and shrinks approximately 0.001 in on each internal surface. On a precision bore or a slip-fit pin, that 0.001 in matters.

For tight bores, mating shafts, and precision pin holes, machinists pre-compensate by opening up the bore (or shrinking the shaft) by the expected growth. Your shop will do this automatically if you specify the type and thickness on the drawing — but only if you give them the information.

Alloy Matters More Than You Think

Not all aluminum anodizes equally well:

• 6061-T6: excellent in both Type II and Type III. Default choice for general anodized parts.
• 6063: excellent finish quality. Common in extrusions.
• 2024: can be anodized but the copper content reduces hardness and produces a darker, mottled finish. Often left unfinished or finished by other means.
• 7075: anodizes acceptably but the zinc content can cause “burning” in Type III if the process isn’t controlled tightly. Talk to your anodizer before committing.
• Cast alloys (A356, etc.): patchy, blotchy appearance. Acceptable for function, rarely acceptable for cosmetics.

If the print doesn’t say which alloy, the shop will choose for you — usually 6061 — and that may or may not match what the rest of the part needs to do.

What to Put on the Drawing

A useful anodize callout has four elements:

FINISH: ANODIZE PER MIL-PRF-8625
TYPE III, CLASS 2 (DYED), BLACK
THICKNESS: 0.002 IN MIN
SEAL: NICKEL ACETATE

Class 1 is undyed (natural). Class 2 is dyed. The seal type matters for parts that will see hot water or solvents — deionized water seal is more thermally stable, nickel acetate gives better color retention.

Also specify any mask areas: threaded holes, contact patches for grounding, mating bores. Anodize is a dielectric; if you need electrical continuity through the surface, you have to mask it during the process.

Color Tip Nobody Mentions

Color batch-to-batch matching is hardest in red, blue, and purple. Easiest in black, clear, and gold. If you’re building a product line where color consistency between production runs matters, design around that constraint or budget for tight batch control.

Pricing Reality

Rough order of magnitude for a typical part the size of a deck of cards, low-volume production through a U.S. anodizer:

• Type II clear: $5–15 per part
• Type II dyed: $7–20 per part
• Type III: $15–40 per part
• Selective/masked anodizing: add 30–100% depending on mask complexity

Volume discounts are real. Tank loading favors batches of identical parts, so consolidating multiple part numbers into a single anodize run can cut per-part cost significantly.

Get a Quote With the Right Finish Spec

If you’re machining aluminum and you’re unsure whether your part needs Type II or Type III — or whether you should be specifying alloy, mask zones, or fastener masking — we’ll review the drawing as part of the quoting process. Most quote rounds we do at DFW Machine include a finish review, because the shop floor cost of getting the anodize wrong is always higher than the engineering minute it takes to specify it correctly.

Send us a print and we’ll come back with a real quote and any finish notes that matter. Contact us or use the quote form on any service page.