Anodizing and CNC Dimensions: What Designers Should Specify

Author：admin Views：59 Source：This Site Time：2026-10-08

Summary：

If you machine to the drawing size and then anodize, the finished part can miss a tight tolerance even though nothing on the machine was set wrong. Anodizing grows the oxide layer partly outward, so the surface the inspector measures is not the surface the machinist cut. The buyer’s job is not to run the tank — it is to state, in writing, the coating type, at what point dimensional change applies, where masking is required, how threads and fits must behave, and what happens at inspection after finishing. This article explains each decision and the drawing language that carries it.

Why anodizing changes size at all

Anodizing is an electrolytic passivation process that converts the aluminium surface into aluminium oxide. In typical commercial processes the oxide grows both into the substrate and out from it in roughly equal amounts, so the coating pushes the surface outward. According to the general process description in the Wikipedia Anodizing entry (consulted October 2026), the oxide is grown down into the surface and out from the surface by equal amounts; therefore a coating 2 µm thick increases the part dimension by about 1 µm per coated surface. If every side is anodized, total linear growth is approximately equal to the oxide thickness. That is the rule of thumb buyers should carry into tolerance decisions — it is a geometric relationship, not a guarantee of any particular shop’s tank control.

The practical number therefore depends on the specification you call out. The same reference notes that chromic acid (Type I) films are typically thin, sulfuric acid Type II coatings are moderate, and Type III hard anodize can reach far thicker layers, with the film thickness span running from under 0.5 µm for bright decorative work up to 150 µm for architectural applications. Growth per surface is half of whatever thickness is actually produced, so a 25 µm hard coat moves a surface about 12.5 µm. Put another way, the wider the coating window you accept, the wider the finished-size window you must design around.

Choosing the coating type first

Coating type drives thickness, growth, hardness, color, and sealing, so it should be fixed before tolerances are finalized. The widely used U.S. military specification MIL-A-8625 defines three aluminium anodizing types: Type I chromic acid, Type II sulfuric acid, and Type III sulfuric acid hard anodize. Aerospace and general industry specifications such as AMS 2469, AMS 2470, AMS 2471, AMS 2472, ASTM B580, ASTM D3933, and ISO 10074 are commonly referenced alongside it. None of these specifications define a detailed process chemistry; they define tests and quality assurance measures the finished coating must satisfy. That distinction matters to buyers: naming the specification tells the finisher which properties to verify, not exactly what thickness you will receive on every part.

A sensible callout names the standard and the targeted thickness range, for example a Type II sulfuric coating for decorative or corrosion work, or a Type III hard coat where wear resistance and insulation are the priority. Because higher voltages and refrigerated baths are used for thicker films, Type III parts need stricter process control and generally a wider finishing tolerance to accommodate that. Where the design must avoid measurable growth — for example a light cosmetic finish — a thin chromic or Type II film may be the better trade.

Dimensional change: drawing language that removes ambiguity

Most anodizing disputes trace to one missing note: does the nominal dimension apply before or after finishing? The established engineering convention is to state that dimensions apply after all surface finishes. That single sentence forces the machine shop to offset the final cut by the expected coating growth before the part goes to the tank. Without it, the shop may reasonably machine to the drawing and let the finisher add the coating on top, and a part that measured perfectly at the machine can drift out of tolerance afterward.

Three workable conventions exist, and the buyer should choose one explicitly:

Finish-size specification: all tolerances apply after anodizing. The shop plans an undersize pre-finish dimension equal to the expected growth per surface. Best for tight-tolerance fits.
Pre-finish specification: tolerances apply as machined, and the coating is allowed to add thickness on top. Simpler to inspect at the machine, but the finished part will be larger than nominal.
Split or asymmetric: critical surfaces are masked or later re-machined; non-critical surfaces are allowed to grow.

When tolerances are tight, confirm that the expected growth is realistic for the actual coating thickness window. A ±5 µm finished tolerance on a surface receiving a 20–30 µm hard coat may be unachievable because the thickness window itself is wider than the tolerance. Where this conflict appears, the honest options are to widen the finished tolerance, mask the surface, or re-machine it after finishing.

Masking: protecting features that must not coat

Masking stops the coating from forming on selected areas. Its importance is practical as well as dimensional. Because the part must conduct current while it is the anode, the area where it is hung or gripped generally cannot anodize; the general process description notes that the surface where the contact is made will not form an oxide layer, so holding points should be placed on non-critical surfaces or designed in deliberately.

Buyers should declare on the drawing which surfaces are coated and which must remain conductive or uncoated. Typical masking needs include:

Electrical contact and grounding surfaces that must stay conductive, since the anodic layer is insulative.
Precision bores, bearing seats, and alignment features that must retain exact machined size.
Threaded holes and studs where coating growth would change engagement (covered below).
Cosmetic surfaces where a specific finish must not be disturbed.
Sealing faces and mating surfaces where coating thickness would break a joint.

Masking adds operations and cost, so listing masks only where function requires them keeps the quote realistic. Note also that if a masked surface is later machined to final size, the exposed aluminium under the mask is bare and unprotected; state whether that is acceptable.

Threads and fits under a coating

Threaded features are where uneven growth bites hardest. On a coated external thread, the oxide grows outward and the thread becomes effectively larger and often rougher, so a nut can bind. On a coated internal thread the minor and pitch diameters shrink, so a screw may not start. The general guidance from the Wikipedia process description is that anodizing can cause screws to bind, and threaded holes may need to be chased with a tap after coating to restore the original dimensions, or a special oversize tap may be used beforehand to pre-compensate for the growth.

For threads and fits, the buyer has four credible strategies, each with a cost consequence: mask the threads and leave them bare; call out an oversize or undersize thread to pre-compensate for growth; chase or re-tap after coating; or specify that the thread tolerance applies after finishing and let the finisher own it. Whichever is chosen must appear on the drawing, because the finisher cannot infer intent.

Unthreaded features deserve the same attention. Holes that receive a fixed-diameter pin or shaft may need to be made slightly oversized to allow for the dimension change, since a nominally correct hole can become undersized after coating. Preferred limits and fits (H7/h6 and similar) become far less reliable when the coating layer is inside the tolerance band, so fits that must assemble after finishing should be called out with the finish condition stated.

Inspection after finishing

The measurement plan has to match the drawing convention. If dimensions apply after finishing, the final inspection is performed on the coated part, and that is the number that governs acceptance. If dimensions apply pre-finish, the machine-stage inspection governs, and the coating is checked for thickness and appearance rather than size. Measuring an anodized part with contact instruments can also read differently from measuring bare aluminium, since the oxide layer is hard and slightly porous; the inspection method should be agreed, not assumed.

For coating quality, the referenced specifications define the tests a coating must pass rather than a fixed recipe, so the inspection content follows from the standard named on the drawing. Thickness, seal quality, color match, and appearance are typical verifications for decorative and corrosion coatings; thicker hard coats additionally involve wear and insulation considerations. On critical programs, first-article and CMM documentation should identify clearly whether dimensions were recorded before or after finishing, because a report that omits the finishing state can be technically accurate yet useless for resolving a fit problem.

Practical specification checklist for buyers

Before sending an RFQ for an anodized machined part, confirm the following are stated on the drawing or in the technical notes:

Coating type and, where relevant, the governing specification (for example MIL-A-8625 Type II or Type III, or an AMS / ASTM / ISO equivalent).
Targeted coating thickness and its allowable window, since growth per surface is half the thickness.
A clear statement of whether dimensional change is absorbed before or after finishing — the “dimensions apply after all surface finishes” convention or its alternative.
Masking requirements, feature by feature, plus the location and size of holding/contact points.
Thread and fit treatment for each threads and fits interface: masked, pre-compensated, chased after coating, or tolerance-after-finish.
The inspection after finishing plan: what is measured, at what stage, with what method, and how coating quality is verified.
Any color, dye, or sealing requirement, noting that sealing improves corrosion resistance while reducing abrasion resistance for Type III, and that some seals are unsuitable for adhesive bonding.

None of this asks the buyer to dictate the electrolyte or the tank schedule. It asks the buyer to remove the two ambiguities that cause most rework — where the nominal size lives and which surfaces must stay bare — and to name the specification that the finisher will verify against. Get those onto the drawing before quoting, and the finished part matches the part the designer intended.