Surface treatment is one of the last lines added to a drawing and one of the most common reasons a CNC machined part is rejected at incoming inspection. A model can be dimensionally perfect and still fail because the anodize came back the wrong shade, the plating closed a tapped hole, or a bore grew out of tolerance after hardcoat. None of those are machining errors. They are specification errors.
A usable surface treatment callout tells the supplier four things: which standard applies, which type and class within that standard, where the coating must and must not appear, and how the finished part will be accepted. Everything else — bath chemistry, rack points, cure schedule — belongs to the finisher. When those four items are missing, the finisher guesses, and the guess is usually the cheapest process that matches the words on the drawing.
At Gran Industries we review surface treatment together with material, tolerances, threaded features, and the inspection plan before a quote is issued, because the treatment changes the machining that comes before it. This guide covers how to write that callout so the part you receive matches the part you designed.
A surface treatment callout is a specification, not a color note
“Black anodized” is not a specification. It does not say whether the coating is decorative sulfuric anodize a few microns thick or a hard anodic coating that will add 25 microns per surface and shift every dimension it touches. It does not say whether the color must match a reference sample, whether the part will be used outdoors, or whether an electrical ground path has to survive the process.
Two suppliers can both deliver a black anodized part that satisfies that note and produce results that behave completely differently in service. One may pass 336 hours of salt spray; the other may chalk in a season. The drawing has to remove that ambiguity, because a note that can be satisfied two ways will eventually be satisfied the cheaper way.
The fix is short. Write the standard, the type, the class, the thickness if the standard allows a range, and the color or appearance requirement. Five items, one line.
Name the standard, the type, and the class
Most industrial surface treatments are already defined by a published standard. Referencing one is faster than describing a process in your own words and gives both sides a common acceptance basis.
Anodizing on aluminum
MIL-A-8625 remains the most widely used anodizing specification in custom manufacturing, and ISO 7599 and ISO 10074 cover the same ground for decorative and hard anodic coatings respectively. Within MIL-A-8625, Type II is conventional sulfuric acid anodize, typically 5–25 µm (0.0002–0.0010 in), and Type III is hard anodize, typically 25–75 µm (0.0010–0.0030 in). Class 1 is undyed, Class 2 is dyed.
That gives a complete callout in one line: ANODIZE PER MIL-A-8625 TYPE II CLASS 2, BLACK, 10–15 µm. A part that needs wear resistance instead of appearance reads differently: ANODIZE PER MIL-A-8625 TYPE III CLASS 1, 50 µm NOMINAL. The two parts will be machined to different pre-treatment sizes, so the distinction has to exist before the first cut, not after.
Alloy matters here as well. Anodize color and consistency vary noticeably between 6061 and 7075, and high-silicon casting alloys will not take a clean cosmetic anodize at all. If appearance is critical, the alloy has to be locked down at the same time as the finish.
Chemical conversion coatings
Where a conductive, paintable, or low-cost corrosion barrier is needed on aluminum, chemical conversion coating per MIL-DTL-5541 is the usual answer. Type I uses hexavalent chromium and Type II uses trivalent or other non-hexavalent chemistry, which matters for RoHS and REACH compliance. Class 1A is specified for maximum corrosion protection; Class 3 is specified where a low-resistance electrical path is required, with contact resistance limited to 5,000 microhms per square inch initially and 10,000 microhms after salt spray exposure, tested at 200 psi electrode pressure.
If the part is a chassis, bracket, or enclosure that carries a ground, Class 3 is not optional and should be stated explicitly. Chassis parts are the single most common place we see a Class 1A note applied to a component that actually needs Class 3.
Passivation on stainless steel
Passivation is not a coating. It removes free iron and other surface contamination so the natural chromium oxide layer can reform, and it changes no dimensions. ASTM A967 and AMS 2700 are the standards to cite, and both allow nitric or citric acid treatments with several numbered variants for different alloy families.
Because ASTM A967 also defines acceptance testing — salt spray, water immersion, copper sulfate, high humidity, and potassium ferricyanide-nitric acid among them — naming the test you want is what makes the requirement enforceable. PASSIVATE PER ASTM A967, CITRIC, COPPER SULFATE TEST is a complete requirement. “Passivate” alone is not.
Electroless nickel and electroplated coatings
ASTM B733 covers autocatalytic nickel-phosphorus and classifies coatings by phosphorus content: Type II at 1–3% for solderability and conductivity, Type III at 2–4% for hardness and wear, Type IV at 5–9% as a general-purpose choice, and Type V above 10% for the best corrosion resistance and non-magnetic behavior. Service classes define thickness, and heat treatment classes define whether the deposit is used as-plated or baked for hardness or adhesion. ISO 4527 is the equivalent international standard.
Electroless nickel deposits uniformly, including inside bores and blind holes, which is exactly why it needs a thickness number on the drawing. A 25 µm deposit closes a small clearance hole measurably.
Quick reference for common callouts
| Treatment | Standard to cite | What to add | Typical build-up per surface |
|---|---|---|---|
| Sulfuric anodize | MIL-A-8625 Type II / ISO 7599 | Class 1 or 2, color, thickness | ~50% of coating thickness |
| Hard anodize | MIL-A-8625 Type III / ISO 10074 | Class, thickness, masked areas | ~50% of coating thickness |
| Conversion coating | MIL-DTL-5541 | Type I or II, Class 1A or 3 | Negligible |
| Passivation | ASTM A967 / AMS 2700 | Nitric or citric, acceptance test | None |
| Electroless nickel | ASTM B733 / ISO 4527 | Type, service class, heat treat class | Full deposit thickness |
Account for dimensional growth before you set the tolerance
Anodizing is a conversion process: the coating grows roughly half outward and half into the base metal. A 50 µm hard anodic coating therefore adds about 25 µm per surface, which means a shaft diameter increases by roughly 50 µm and a bore diameter decreases by roughly 50 µm. Plated coatings behave differently — the full deposit thickness is added on top of the existing surface.
This is where most surface treatment problems actually originate. A bore toleranced H7 in the model and hard anodized after machining will not be H7 when it arrives. Either the machining size is offset to compensate, or the feature is masked, or the tolerance is stated as an after-treatment requirement and the supplier is told to work backwards from it.
The clearest way to write this is to state which dimensions apply after treatment. A note such as DIMENSIONS MARKED (AT) APPLY AFTER SURFACE TREATMENT, with those dimensions flagged on the drawing, removes the ambiguity entirely. Related guidance on where tolerance genuinely pays for itself is covered in our article on in che modo le tolleranze ristrette influiscono sui costi e sui tempi di consegna della lavorazione CNC.
Say where the treatment must not go
Masking is a manual operation with a real cost, and it cannot be inferred from a model. Surfaces that commonly need to be excluded include bearing seats and press-fit bores, ground contact points, sealing faces where a coating would affect compression, threads that must accept a mating fastener at nominal size, and datum surfaces used for inspection.
Masked areas should be shown on the drawing with a boundary, not described in words alone. “Mask bore” on a part with four bores generates a question at best and a wrong part at worst. Where the transition between coated and uncoated areas matters cosmetically, say how sharp the boundary has to be, since hand masking always leaves some variation.
Threads deserve particular attention. A tapped hole that is anodized or plated after tapping will lose clearance, and on small threads that can be enough to bind a screw. Options are to mask the hole, to tap after treatment, or to tap oversize with the coating allowance built in — but the drawing has to say which. Our guide on how to specify threaded holes for CNC machined parts covers the thread side of that decision.
Define appearance in terms that can be reviewed
Color, gloss, and cosmetic acceptance are subjective unless they are anchored to something physical. For production work, the practical approach is an approved first-article sample retained by both parties, with the drawing stating that production parts must match it within an agreed tolerance under specified lighting.
It is equally important to define which surfaces are cosmetic. Most parts have one or two visible faces and several that are never seen. Marking the cosmetic surfaces on the drawing lets the finisher rack the part so that unavoidable contact marks land where they do not matter. If no cosmetic surface is identified, rack marks may appear on the face the customer sees.
Where a specific texture is required before coating — bead blast, brushed, or as-machined — that belongs in the callout too, because anodize reproduces whatever surface it is applied to. A full treatment of the underlying roughness requirement is in our article on how to specify surface finish for CNC machined parts.
State the acceptance tests you actually need
Every test added to a drawing adds cost and lead time, so specify the ones that reflect real service conditions. A salt spray requirement per ASTM B117 or ISO 9227 is reasonable for outdoor or marine hardware and unnecessary for an internal bracket. Coating thickness verification by eddy current or cross-section is appropriate where dimensional growth or wear life matters. Adhesion and contact resistance testing apply where the coating carries a load or a current.
Note also that many of these tests are destructive and are run on witness coupons processed with the batch rather than on the parts themselves. If you require testing on the delivered parts, say so, and expect the sample quantity to be quoted separately. What a first article should actually establish is covered in our article on what first article inspection means in CNC machining quality control.
Send the treatment requirement with the RFQ, not after
Surface treatment affects machining allowances, masking labor, batch scheduling, and lead time. Adding it after a quote has been issued usually means re-quoting, and adding it after machining has started can mean scrapping parts that were cut to the wrong pre-treatment size.
Include the finish requirement in the same package as the model, the drawing, the material specification, and the quantity. A drawing review before production, discussed in our article on why drawing review matters before CNC machining quotes and production, is where remaining ambiguity gets resolved at no cost to either side.
Frequently asked questions about surface treatment for CNC parts
Does anodizing change the dimensions of a machined part?
Yes. Anodizing grows roughly half into the base metal and half outward, so about half the coating thickness is added per surface. A 50 µm hard anodic coating increases a shaft diameter by approximately 50 µm and reduces a bore diameter by approximately the same amount. Plated coatings such as electroless nickel add the full deposit thickness per surface instead.
What is the difference between Type II and Type III anodizing?
Type II is conventional sulfuric acid anodize, typically 5–25 µm, used for corrosion protection and color. Type III is hard anodize, typically 25–75 µm, used where wear resistance matters. Type III is thicker, harder, more dimensionally significant, and limited in the colors it can produce.
Do I need to specify passivation on stainless steel parts?
If the part must resist corrosion in service, yes. Machining leaves free iron and tooling residue on the surface that will rust even on a corrosion-resistant grade. Citing ASTM A967 with the acid type and an acceptance test makes the requirement verifiable rather than assumed.
Should threads be masked before anodizing or plating?
It depends on the thread size and the coating thickness. Fine threads and thick coatings usually require masking or tapping after treatment; coarse threads with thin coatings often do not. State the intended approach on the drawing so the fit condition is agreed before production.
Make the finish requirement explicit before quotation
Surface treatment sits at the end of the process, but the decisions it drives happen at the beginning — alloy selection, machining allowance, masking, thread strategy, and inspection. A callout that names the standard, type, class, thickness, masked areas, and acceptance basis lets all of that be planned once instead of corrected later.
Gran Industries supplies precision CNC machined components with anodizing, conversion coating, passivation, and plated finishes to specification for OEM customers across Southeast Asia and beyond. If you have a drawing where the finish requirement is still unresolved, contact our engineering team with your model and drawing and we will review the treatment together with the machining plan before quoting.
