How anodize thickness is predicted
A sulfuric anodize film grows in proportion to the charge passed, so current density and time are enough to estimate thickness. US anodizing shops use the well-known 720 rule: thickness (mil) = ASF × minutes ÷ 720. Running 12 ASF for 60 minutes therefore builds 1.0 mil, and a 24 ASF hardcoat cycle for 60 minutes builds the 2.0 mil that many prints call out.
Thickness classes come from MIL-A-8625. Type II conventional sulfuric anodize normally falls between 0.0002 and 0.001 in (5 to 25 µm), while Type III hardcoat defaults to 0.002 in (50 µm) unless the drawing specifies otherwise. Class 1 is undyed and Class 2 is dyed; always work from the print callout rather than a shop habit.
For dimensional control, remember that roughly half the film grows into the base metal and half grows outward. Each anodized surface gains about half the film thickness, so a shaft diameter or a plate thickness with both faces coated grows by the full film thickness. Machine tight fits undersize by that amount before the parts go in the tank.
Frequently asked questions
Bath temperature and alloy matter most. A warm bath dissolves film as fast as it forms and slows growth, and high-copper 2000 series alloys build more slowly. Tune the correction factor from measured coupons.
The same charge relationship applies, but hardcoat runs cold at 24 to 36 ASF and higher voltage. Growth efficiency tends to drop as the film gets thick, so reflect that in the correction factor.