Type 3 Hard Coat Anodizing: Drawing Notes That Prevent Scrap

Anodizing Specification Notes

Type 3 hard coat anodizing is rarely lost in the anodizing tank. It is lost on the drawing. A purchase order that says "hard anodize per MIL-A-8625 Type III" leaves the coater to decide film thickness, which surfaces get coated, whether threads stay functional, and what color comes out. Those four decisions change part dimensions, thread fit, and appearance—and they are the four that most often send a batch to the scrap bin.

This article is written for buyers and engineers specifying hard coat on machined aluminum parts. It lists the drawing notes that decide success, with the failure mode each one prevents.

What Type 3 Hard Coat Anodizing Does to a Part

Type III anodizing is an electrochemical conversion of the aluminum surface into an aluminum oxide layer. Because the coating grows partly into the substrate and partly outward, every coated surface gains dimension. A nominal 50 µm (0.002 in) coating does not add 50 µm to one side; roughly half grows inward and half grows outward, and the split varies with alloy, electrolyte, and process control.

Two consequences follow directly from that growth:

• Any dimension that must be held after coating has to be specified as an after-coating dimension, or the pre-coat dimension has to be adjusted by a known allowance.
• Any feature that must not grow—threads, dowel bores, bearing seats, contact faces—has to be masked or stopped off, and the drawing has to say so.

Color is a separate variable. Hard coat anodizing is normally supplied in natural (clear to grey-brown, darkening with thickness) or black. Because the coating is thicker and denser than decorative Type II, color is less uniform from lot to lot, particularly on natural finish where alloy chemistry shows through.

Film Thickness Callouts: One Number Is Usually Not Enough

A single note such as "anodize 50 µm" forces the coater to guess whether that is a minimum, a nominal, a target on a specific face, or an average across the part. The result is often a part that meets the note on one surface and misses a mating surface by the same amount.

Workable notes separate the function of each surface:

• Wear surfaces: specify a minimum, e.g. "50 µm minimum on external wear faces, after coating." A minimum protects function; a tight bilateral range on a rough part only raises cost.
• Mating and interference surfaces: specify a maximum, or mask the surface entirely. A maximum protects fit.
• Cosmetic surfaces: specify a range and note that color uniformity is judged visually, not measured.
• Non-functional surfaces: explicitly release them from the thickness requirement so the coater does not chase an impossible target.

If the drawing does not distinguish these, the shop will typically apply one target thickness everywhere. On a shaft with a press-fit journal and a sliding wear band, that single number cannot satisfy both.

Example: one callout, two failures

A turned aluminum bushing carries an internal bore that presses onto a shaft and an external band that runs against a seal. The drawing said "hard anodize Type III 50 µm." After coating, the external band passed; the bore would not accept the shaft because the coating had grown inward as well as outward. Adding a masking note to the bore and a minimum-only callout on the band would have solved both sides before the parts reached the tank.

Masking Threads, Bores, and Contact Points

Threads are the single most common hard coat casualty. A 1/4-20 thread with 50 µm of coating on the flanks will not accept its mating fastener without re-tapping, and re-tapping removes the coating the drawing asked for. The standard fix is to mask the thread so the coating stops at the first full thread, or to specify that the thread be cut after coating. Both are acceptable; a silent thread is not.

Drawings should state masking explicitly for:

• All threaded holes and studs, including pipe threads
• Precision bores used for bearing seats, dowel pins, and slip fits
• Electrical grounding and contact points, where the oxide layer is an insulator
• Sealing faces, O-ring grooves, and any surface where a gasket must seat
• Areas that will be welded, soldered, or bonded after coating

Masking is not free—it adds handling, fixturing, and touch labor. Where a feature can instead be machined to size after coating, that is often cheaper than masking a complex geometry. The drawing should state which approach applies so the shop is not choosing the process for you.

What happens when masking is omitted

The coater coats the whole part, the buyer discovers the thread or bore is out of tolerance, and the part is either re-machined—losing coating and thickness in the process—or scrapped. On a machined part with hours of turning in it, that is an expensive way to learn a note was missing. For parts that go through turning before anodizing, review the feature list against the Custom CNC Turning Services drawing set before release.

Color Limits: Natural and Black on a Hard Coat

Hard coat anodizing is supplied in two practical colors: natural and black. Both are produced by the anodizing process itself rather than by a separate dye step, which is why the color range is wider than most buyers expect.

Two drawing notes prevent most color disputes. First, state the color and whether it is integral or dyed, because the two behave differently under abrasion and UV. Second, state that color is judged against an approved sample or a defined appearance range, not against a numeric color standard that the process cannot hold. A natural hard coat on 6061 and the same coating on 7075 will not match; if a batch spans alloys, the drawing should say which surfaces must match each other.

Datum Changes from Coating Growth

Coating grows on every unmasked surface, so datums defined on coated faces move. When the inspection plan does not account for this, parts measure out of tolerance even though the coater hit the specified thickness.

Three notes keep the datum scheme coherent:

• State whether each critical dimension is pre-coat or post-coat. A press-fit diameter specified as post-coat is a production requirement; the same dimension specified as pre-coat is an inspection requirement. They are not interchangeable.
• Choose datums on masked or uncoated features where possible. A masked bore or an uncoated end face is a stable reference before and after coating.
• Apply the allowance in the shop, not in the inspection room. If the drawing says the pre-coat dimension must be reduced to hit the post-coat target, the machining program has to carry that allowance.

Where the drawing is silent, the common default is to inspect the part as machined and hope the coating falls inside the unstated allowance. That works on loose tolerances and fails on tight ones. For parts that also require a specific cosmetic and functional finish, an anodizing supplier who works from the same drawing revision as the machine shop avoids the gap entirely—see Custom Aluminum Anodizing Services.

Film Inspection and Documentation

The drawing should also say how the coating will be verified, because different methods answer different questions. Thickness gauges, microsection, and visual inspection each have a place, and the acceptance criteria should match the surface function.

Documentation expectations should be realistic. An IATF 16949:2016 quality system, where it applies to the supplier and the part, governs process control and traceability; it does not replace the drawing as the source of dimensional and thickness limits. Buyers in automotive or similar chains typically require the coating supplier to work to the same revision level as the part drawing and to record thickness readings per specified surface. Confirm with the supplier which thickness method, sampling frequency, and record format apply to your part before release.

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