How Much Stock Should You Leave on a Surface That's Going to Be Anodized?

Anodizing removes almost nothing from the dimension and adds a thin oxide layer on top. For most type II anodizing, don't leave extra stock at all, just know the surface grows by roughly the coating thickness. For type III hardcoat, plan on the surface growing by about half the total coating thickness per side, since roughly half a hardcoat layer builds outward and half converts the base material inward.

Why it's not a simple stock allowance

Anodizing is an electrochemical conversion of the surface, not a plating process that piles material on top of a fixed part. Type II coatings typically run a few thousandths of an inch thick, thin enough that most drawings never call out a growth allowance for them. Type III hardcoat is the one that actually matters dimensionally. Hardcoat thicknesses commonly land in the 0.001-0.003 in (0.025-0.076 mm) range, and because it builds both outward and inward, a part with a hardcoat spec needs its critical dimensions adjusted before anodizing, not after.

The practical move: machine any hole or shaft that needs a mating tolerance after hardcoat undersize or oversize by the growth amount before it goes to anodize. Confirm that growth number with the anodizer you're using. Coating thickness varies by alloy and by tank chemistry.

That variance is the whole problem.

Where this actually bites people

The failure mode isn't usually the flat surfaces. It's threaded holes and close-fit bores. A tapped hole that anodizes clean can come out too tight to run a bolt through, because the coating builds up inside the thread flanks. Many shops mask threads entirely rather than anodize them, or chase the threads after coating. Bore fits for pins or dowels have the same problem in miniature. A bore anodized to spec can close down enough to interfere with a pin that fit before coating.

Sharp inside corners and blind holes are the other trouble spot. Anodize thickness isn't perfectly uniform. It tends to run thinner in deep recesses and tight corners where the electrolyte doesn't exchange as freely, and thicker on edges and points where the electric field concentrates. A part that needs uniform coating on a deep pocket floor may come back with visibly different anodize color or thickness there than on the open faces.

What to actually do about it

Call out the anodize type and thickness on the drawing, not just "anodize per spec." For type II, don't add stock. The layer is thin enough that ISO 2768-m tolerances absorb it. For type III hardcoat, get the actual growth number from whoever's doing the coating, for your specific alloy, then adjust critical dimensions in the model before it goes to CNC, not after the part measures out.

Mask or plan to chase any thread that has to accept a standard fastener. And if a feature has a tight-fit tolerance and sits in a deep pocket or blind hole, flag it. That's where coating thickness is least predictable, and where a first article check earns its keep before a whole batch goes through the tank.