Anodized CNC Machining Service: How to Pick a Supplier

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  Key takeaways
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  Which anodize type fits your part
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  Alloy choice drives the finish you get
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  How anodize changes your dimensions
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  Machining marks show up after anodize
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  What to verify before you place the order
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  Where the cost actually sits
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  Step by step: from drawing to anodized part

Anodize is not a coating you spray on. It is an oxide grown from the aluminum itself, so the alloy and its heat treat decide how the layer looks and how hard it gets. The 6xxx family is the safe default. 6061, 6063, 6082 and 6005 anodize evenly, hold a dyed black well, and machine cleanly at the same time.

7075 gives you strength and still anodizes, though the copper and zinc content pushes the color slightly darker and less uniform than 6061. Hardcoat on 7075 is common for wear parts. 2024 is the one to question. Its high copper level makes the oxide darker, patchy and less corrosion resistant, and many shops will steer you to 6061 or 7075 instead.

Castings are a separate story. ADC12 and similar die-cast alloys contain silicon and copper that anodize unevenly, produce a muddy gray, and sometimes show black smut in the pores. If the part must be die cast and anodized, expect a matte or textured look and approve a sample before the production run.

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  Best cosmetic result6061-T6 or 6063 for clear and dyed finishes
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  Strength plus hardcoat7075-T6, accept slightly darker clear
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  Risky finish2024 and high-copper castings, sample first
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  Smallest grain, best sheenExtruded 6063 profiles over cast material

Type II anodize grows the surface by roughly 5–25 μm. About half of that is oxide sitting above the original surface and half is conversion of the metal below it. On a 25 μm layer, plan for about 12 μm of growth per side. On a Ø20 mm shaft that is 0.024 mm across the diameter, enough to break a press fit.

Hardcoat Type III is the bigger problem. A 50 μm layer can add about 25 μm per side. Sharp corners build faster than flat faces because the current concentrates there, so an edge that was chamfered 0.2 mm can end up rounded or even chipped after hardcoat. Break every edge to at least 0.3 mm and radius internal corners where you can.

The practical rule is to finish critical features after anodize, or to pre-machine them undersize. Threads are the classic case. A 1/4-20 thread anodized without masking will not accept the mating screw. We mask threads, dowel bores, bearing seats and electrical contact pads, then inspect those features after the tank. General machining tolerance is ±0.005 mm where the drawing calls for it.

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  Type II growthAbout half of 5–25 μm sits above the surface
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  Type III growthPlan roughly 25 μm per side on a 50 μm layer
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  Edge prepChamfer 0.3 mm minimum before hardcoat
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  Critical featuresMask or machine after anodize, then verify

A clear anodized layer is transparent. Whatever the cutter left behind is still visible, only now it is locked under a hard oxide. That is why two parts machined to the same drawing can look completely different after the tank. Tool marks, chatter and uneven stepovers do not disappear with anodize. They get amplified.

Your finish callout sets the baseline. As-machined at Ra 1.6–3.2 μm is fine for internal brackets. Ra 0.8–1.6 μm is a good target for visible covers and front panels. Ra 0.2–0.8 μm is possible but costs more, and the smoother the surface the more any later scratch or fingerprint shows. For matte housings, bead blasting before anodize hides tool paths and gives a uniform low-gloss look.

Cleaning matters as much as cutting. Cutting fluid, chips, fingerprints and oxidation from sitting overnight all cause streaks, white spots or weak adhesion. We run ultrasonic cleaning and chemical pre-treatment before the anodize line, and parts go into the tank the same day they come off the machine whenever the schedule allows.

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  Visible coversTarget Ra 0.8–1.6 μm, then bead blast if matte is wanted
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  Hidden bracketsRa 1.6–3.2 μm as-machined is enough
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  Avoid overnight storageFreshly cut aluminum oxidizes and stains
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  HandlingGloves from deburr to packing on cosmetic parts

Ask who runs the anodize line. Some machine shops outsource the tank to a nearby plater, which adds transit, handling and a second schedule you do not control. A shop with both machining and finishing under one roof can hold the part to the same traveler and inspect after the tank. That matters when a cosmetic reject means re-machining, not just re-dipping.

Ask for the anodize thickness range in writing, not just the type. Type II, Type III, clear, black or a specific dye should be named on the purchase order, along with the masking plan and the color target. If color matters, request a first-article sample in the actual alloy and finish before the run starts.

Check the paperwork side too. ISO 9001:2015 covers the general quality system. IATF 16949:2016 applies to automotive and EV work. ISO 13485:2016 is what medical device buyers expect. ISO 27001:2022 covers information security, which matters if you are sending CAD files for parts that are not public yet. In-process inspection records, material certificates and dimensional reports should be available on request.

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  One roofMachining and anodize in the same plant reduces handling risk
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  Written specType, thickness range, color, masking plan on the PO
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  First articleSample in the real alloy before the run
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  CertificatesISO 9001, IATF 16949, ISO 13485, ISO 27001 as applicable

Machining time usually dominates the price of an anodized CNC machining service order. The anodize itself is a small line item until you add masking. Hand masking of many small threads and bores can cost more than the tank time, so simplify the design where you can. A single masked face with a clear boundary is cheap. Twenty masked M3 holes are not.

Volume changes the picture. There is no minimum order quantity here, and a single prototype is fine for checking fit and finish. But dye lots behave differently at volume. If you need 10,000 parts in the same black, splitting the order across several days of tank time risks visible shade steps. Book the run as one lot, or accept that a tight color band costs more to hold.

Cosmetic grading is the other hidden cost. If your drawing says no visible scratches at all, expect a higher reject rate and higher price. Most suppliers grade cosmetic surfaces as A, B or C by how visible the face is when assembled. Agree on that grading before quoting, because a strict cosmetic call on a hidden face buys nothing.

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  Masking is the swing costFewer masked features, lower price
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  Prototype friendlyOne part to 10,000+ parts, no minimum order quantity
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  Color at volumeKeep the run in one dye lot for a tight shade band
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  Cosmetic gradingDefine A, B and C faces before quoting