Anodizing Tolerance: What the Coating Does to a Bore, a Thread and a Face
An anodized part is not a machined part with a coat on top. Roughly half of the coating grows into the aluminium and half stands above the original surface, which means every dimension on the drawing moves by an amount that scales with the coating thickness — and the coating thickness is chosen for wear or colour, not for the tolerance.
That is how a part that measured perfectly at the machine fails at assembly. A 50 µm hard anodize closes a bore by 50 µm on diameter, which is the entire standard ±0.05 mm band consumed by a finishing decision. This note covers what actually moves, by how much, and how to write the drawing so the shop and the finisher are working to the same number.
The anodizing tolerance rule: half in, half out
Anodizing converts aluminium at the surface into aluminium oxide. The conversion front moves inward and the oxide occupies more volume than the metal it replaced, so the coating ends up partly below and partly above the original surface — about half each for Type II sulfuric anodize.
Anodizing tolerance follows from that geometry, and two consequences are easy to get backwards:
- A bore closes. Both walls grow inward, so the diameter shrinks by the full coating thickness.
- An outside diameter grows by the full coating thickness, because both sides move outward.
A 20 µm coating therefore takes a 20.000 mm bore to about 19.980 mm and a 20.000 mm shaft to about 20.020 mm. The change is symmetric and predictable, which is good news: it can be designed around. It is only a problem when nobody accounts for it.
The bore is the feature that fails
| Coating | Nominal thickness | Bore or slot change | OD change | Where it is used |
|---|---|---|---|---|
| Type II, clear | 10 µm | −0.010 mm | +0.010 mm | General corrosion protection |
| Type II, black | 20 µm | −0.020 mm | +0.020 mm | Housings, visible aluminium |
| Type II, heavy | 30 µm | −0.030 mm | +0.030 mm | Outdoor parts, light wear |
| Hard anodize (Type III) | 50 µm | −0.050 mm | +0.050 mm | Wear faces, sealing bores |
| Hard anodize, heavy | 150 µm | −0.150 mm | +0.150 mm | Abrasive wear, masking essential |
The 50 µm row is the one that catches people, and it is where anodizing tolerance stops being a finishing detail. A housing bored to H7 for a bearing, then hard anodized at 50 µm, ends up about 50 µm undersize: the bearing will not enter, and pressing it in damages the coating that was specified to protect the bore.
There are three honest ways to handle it, and the drawing has to pick one:
- Mask the bore. No coating on the feature, no dimensional change, but masking is manual work and it leaves an uncoated ring that will corrode first.
- Leave stock. Bore oversize by the coating thickness, then anodize, then accept the coated bore as the finished size. This is normal for wear surfaces and for bores that are not a precision fit.
- Finish after coating. Only possible where the coating is thin enough to ream or hone without breaking through, and it is rarely worth it.
Small holes deserve a separate mention. A 2 mm hole with a 20 µm coating becomes 1.96 mm, which is a 2% restriction and enough to change the flow through a vent or the fit of a pin. If the hole is a fluid path or a locating feature, size it before coating and state the finished requirement.
See also: Bearing Bore Tolerance: Fit Classes, Roundness and Thermal Growth and Keyway Machining: Broaching, Slotting or Milling?.
Threads: mask, chase or accept
Threads are the feature most often damaged by anodizing, because a coated thread is a thread with a different pitch diameter and a brittle flank.
Type II at 5 to 10 µm usually survives on an external thread and can be tolerated on an internal thread gauged to 6H, provided the thread is cut at the top of its tolerance. It feels rougher and it galls more, which matters on parts that are assembled repeatedly.
Hard anodize at 25 µm and above should not be applied to a functioning thread. The coating thickness is comparable to the thread clearance, so the nut either will not start or strips the coating on the way on. Mask the thread, or specify a thread insert, or move the thread to a masked boss.
The practical rule we apply: any thread that carries load, seals, or is assembled more than once gets masked. Cosmetic threads on non-functional features can be coated if the drawing says so.
Masking is a drawing decision
Masking is often treated as a finishing preference. It is a design decision with three costs: labour per feature, a corrosion-prone uncoated area, and a visual break in the coating.
Four features earn masking on most parts:
- Precision bores and bearing seats where the coating would consume the fit.
- Threads as above.
- Electrical contact and grounding points. Anodize is a dielectric; a grounding pad that gets coated is a grounding pad that does not conduct.
- Sealing faces where a gasket or O-ring needs a flat, uncoated land rather than a brittle oxide edge.
Everything else can usually be coated. The mistake is masking nothing and then re-machining a bore after finishing, which removes the coating from the very surface that needed it.
Edges, flatness and surface finish
Anodizing does not move a flat face much, but it does change edges and it does change the surface the seal sees.
Edges. Coating builds on a sharp corner and cracks when it bends. A 0.5 mm edge break before anodizing is worth more than any post-finish inspection, and the chamfer and fillet guide covers how to call it out.
Flatness. A thin wall can bow slightly as the oxide forms, particularly on hard anodize at 50 µm and above. If flatness is functional, specify it after finishing and expect the finisher to hang the part rather than stack it.
Surface finish. Anodizing reproduces the surface underneath rather than smoothing it. A tool mark at Ra 1.6 µm stays visible through a clear coating; a matte etch before anodizing hides it. The surface finish callout note explains how Ra and Rz are specified per surface.
Fatigue. Anodic coatings reduce fatigue strength on cyclically loaded parts, more so as thickness rises. On a rotating aluminium component, that is a reason to keep the coating thin on the highly stressed region.
Type II or hard anodize
The choice is usually made on wear, not on colour.
Type II (sulfuric, 5 to 30 µm) is the default. It takes dye well, gives good corrosion protection, and moves dimensions by an amount that is easy to allow for. Black Type II is the standard finish for visible aluminium housings.
Type III (hard anodize, 25 to 150 µm) is chosen for wear resistance and for dielectric strength. It is darker, more brittle, harder to dye, and it moves dimensions by up to 0.15 mm — so any bore, thread or fit has to be designed around it from the start.
A part designed for Type II and then switched to hard anodize at the finishing stage will not fit. That decision belongs on the drawing, next to the tolerance, not in a purchase order note.
Frequently asked questions
Does anodizing change the dimensions of a part?
Yes. The coating grows about half into the surface and half above it, so a bore closes by the coating thickness and an outside diameter grows by the same amount. At 20 µm that is 0.020 mm on diameter, which is measurable on any part with a fit.
How much stock should I leave for hard anodize?
For a bore that must end at a specific size, leave the coating thickness plus a finishing allowance, or mask the bore. For a wear surface where the coated size is acceptable, machine to nominal and let the coating define the final dimension.
Can I anodize a thread?
Thin Type II coatings of 5 to 10 µm can be tolerated on threads if they are cut near the top of tolerance, but hard anodize should not be applied to a working thread. Mask it, or specify an insert.
Why did my black anodized part come back tight on the pins?
Because the coating grew into the holes. A 20 µm coating reduces a hole by 0.020 mm, which is enough to turn a slip fit into a press fit. Size the holes before coating and state whether the dimension applies before or after finishing.
Does anodizing hide machining marks?
It reproduces them. Anodizing does not fill or smooth a surface, so scratches and tool marks remain visible, and a matte etch is the usual way to make them uniform before coating.
Next step
Send the drawing with the finish, the thickness and the features that must stay conductive or stay in tolerance marked on it. The surface finishing page lists the processes and thickness ranges we coordinate, and the bearing bore note covers what happens to an H7 seat when a coating is added on top of it.
Related reading
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