CNC Machining Anodizing Service: What to Check Before You Order

• 1
  Key takeaways
• 2
  Anodizing types at a glance
• 3
  What a CNC Machining Anodizing Service Actually Changes
• 4
  Which Aluminum Alloys Anodize Well
• 5
  Tolerances, Masking and the Features That Bite
• 6
  Lead Time, MOQ and Quote Clarity
• 7
  Where Anodizing Is the Wrong Choice
• 8
  Step by step: releasing a machined and anodized part

Anodizing is not a coating sitting on top of your part. It is an electrochemical conversion of the aluminum surface itself into an oxide layer. Because the layer grows out of the metal, it cannot peel or flake the way a plated deposit can. For a machined part, that means the surface you inspected at the machine is not the surface the customer receives.

The oxide grows in both directions. Roughly half sits above the original surface and half eats into it. A 25 μm hardcoat layer makes the part about 12–13 μm larger per side. On a Ø20 mm shaft with a ±0.005 mm fit, that growth is the whole tolerance.

This is why the machining and the anodizing need one owner. If a machine shop cuts to the finished drawing and a separate anodizer grows the layer afterward, the fit fails. The shop has to know the anodize type and thickness before it writes the CAM program.

Anodizing also changes the finish reading. A part machined to Ra 0.8–1.6 μm will read rougher after a thick hardcoat, because the layer follows the existing surface profile and exaggerates it. If the drawing calls for a specific Ra after anodizing, say so on the drawing.

Not every aluminum alloy takes anodizing the same way. The alloy and temper change the color, the layer hardness and how evenly the oxide forms. This matters when you are choosing a material for a visible part.

6061 and 6061-T6 are the workhorses. They anodize evenly, take dye predictably and hold a clear finish without blotching. If your part does not need high strength, start here.

7075 machines beautifully and anodizes to a darker, slightly bronze or gray tone in clear. Customers expecting a bright silver clear finish on 7075 will be disappointed. For a cosmetic part, either accept the tone or pick 6061.

Cast alloys are the hard case. ADC12 and other die-cast grades contain silicon and copper that leave a mottled, dark surface after anodizing. If the part is die cast, plan on a matte or dark finish, or skip anodizing entirely.

2024 also anodizes with a visible alloy pattern. It is common in aerospace work where the mechanical properties matter more than a uniform look.

The features that cause trouble are almost always the same ones: threads, press-fit bores, sealing faces, electrical contact pads and any surface that must conduct. Anodizing is an insulator. A masked contact pad stays conductive; an unmasked one does not.

Threads are the most common failure. A hardcoat layer on a fine thread tightens the fit and can gall during assembly. Standard practice is to mask threads, or to cut them undersize before coating. Either way, the drawing has to say which.

Bores with tight tolerances need the same treatment. If a Ø10 H7 bore is anodized without masking, the bore shrinks by roughly twice the layer thickness. On a 25 μm layer that is about 50 μm of diameter, far outside an H7 band.

Masking is manual work and it costs money. A part with six masked features takes longer to rack than a simple plate. When you compare quotes, check whether masking is a line item. A quote that ignores masking usually comes back as a change order.

Anodizing adds a step, but it should not add weeks. When machining and finishing sit in the same building, the parts move to the anodize line without a shipping cycle, a receiving inspection and a second queue. That is where most of the calendar time disappears in split-supplier setups.

Ask what the quote covers. A useful quote names the anodize type, the target thickness in microns, the color or dye spec, the masking plan, and whether inspection reports come with the shipment. A quote that says anodized and nothing else is not a quote you can compare.

Minimum order quantity is a real filter. Some finishers will not run a rack for a single prototype because the setup and racking cost more than the part. A shop that machines in-house can put a one-off part on the same rack as a production run.

Certifications matter by industry. ISO 9001:2015 covers general quality systems. IATF 16949:2016 is what automotive and EV programs expect. ISO 13485:2016 applies to medical device work. ISO 27001:2022 covers information security, which matters when you send CAD files to an outside shop.

Confidentiality should be settled before files move. A mutual NDA signed at the quoting stage costs nothing and removes a later argument. Ask where the files are stored and who can open them.

Anodizing is not universal. Knowing when to skip it saves a redesign later.

If the part needs to carry current, an unmasked anodized surface will not do it. You can mask the contact areas, but if most of the part is a conductor, plating or a bare machined finish is simpler.

If the part is a wear surface running against steel, a thin Type II layer will polish through. Hardcoat helps, but at some point a hardened steel insert or a different alloy is the honest answer.

If the customer wants a specific gloss level, anodizing alone will not deliver it. The gloss comes from the mechanical finish underneath. Bead blasting gives a matte look, polishing gives a bright one, and the anodize locks in whatever is there.

If the material is die cast or contains high silicon, expect color variation. That is a material property, not a process defect. Choose a dark dye, or move the part to a wrought alloy.