Anodizing Thickness and How It Changes Your Dimensions

The Rule in One Sentence

For every surface, the part grows outward by roughly half the total coating thickness, and the other half is consumed from the metal that was already there. Engineers call these build-up and penetration, and the approximate fifty-fifty split holds well enough for design work on both Type II and Type III.

The consequence people miss is what happens across a dimension rather than on a face. A shaft grows on both sides, so its diameter increases by about the full coating thickness. A bore closes on both sides, so its diameter decreases by about the full coating thickness. On a flat plate, each face grows by half, so overall thickness increases by about the full amount too. The number to plan around is therefore usually the whole coating thickness, not half of it.

How to Put It on the Drawing

The cleanest approach, and the one that causes the fewest arguments at inspection, is to dimension the part after finishing and say so. A note reading "dimensions apply after anodize" tells the machinist to undercut and tells the finisher what the target is. Everybody is then working to the same number.

The alternative is to dimension before finishing and state the coating thickness, which works but pushes the arithmetic onto whoever reads the drawing next. It is fine on a simple part and a reliable source of scrap on a complicated one.

Either way, call out the features that must not be coated at all. Threads, precision bores, sealing faces and electrical contact surfaces are the usual four, and all of them can be masked or plugged to your print. That is a normal part of the job rather than an exotic request; see masking, plugging and assembly.

Threads, and Why They Cause Most of the Trouble

Threads are where Anodizing allowances go wrong most often, because the coating builds on both flanks at once and the effective pitch diameter changes by more than people expect. A tapped hole that gauged correctly before finishing can refuse the same gauge afterward, particularly on a Hard Coat.

There are three honest options. Mask the threads so they stay bare, which is the most common answer on a Hard Coat part. Machine them oversize by the expected build, which requires the machinist to know the coating thickness in advance. Or chase them after finishing, which is acceptable on some parts and unacceptable on anything where a bare thread would compromise corrosion protection.

What does not work is hoping. If threads have to gauge after finishing, say so on the request for quote and we will tell you which of the three your part needs before it runs.

Blind Holes, Internal Surfaces and the Geometry Problem

Coating thickness is not uniform across a complicated part, and the reason is electrical. The coating grows where current flows, and current does not reach deep into a narrow bore, a blind hole or a tight internal corner nearly as well as it reaches an exposed outer face.

The practical consequence is that a specification generally applies to significant surfaces rather than to every square millimeter of the part, and your drawing should say which surfaces those are. A part measured on an outside face may carry considerably less coating a couple of diameters down a blind hole, and that is normal physics rather than a shop cutting corners.

Two related traps. Deep blind holes can trap air, leaving an uncoated void where solution never reached, which is a racking and drainage question. And sharp external corners see concentrated current, so they grow faster and are where burning appears first. Generous radii finish more evenly than sharp edges, which is worth knowing while the part is still being designed.

Measuring the Finished Coating

Coating thickness on a non-conductive layer over a conductive substrate is normally measured with an eddy current gauge, which is quick, non-destructive and adequate for most production checks. Where a specification demands it, a cross-section examined under a microscope is the definitive method and the one used to resolve disputes.

One practical note. Measurements are taken on the significant surfaces the drawing names, and never on a rack contact point, because that spot is not coated and a reading there means nothing.

Common Questions