Definition
A manufacturing process used to remove metal from selected areas of an aircraft part by controlled chemical etching rather than mechanical cutting. The areas to remain at full thickness are coated with a chemical-resistant masking material, and the part is immersed in an etching solution that dissolves metal from the unmasked areas, producing precisely thinned sections without inducing mechanical stress.
Plain English
A way of shaving metal off parts of a structure by dipping them in a chemical bath that eats away the exposed metal, while protected areas stay untouched. It lets manufacturers make parts thinner in some spots and thicker in others without cutting or grinding.
Context Anchor
Seen in aircraft manufacturing, structural repair information, and maintenance discussions about thin metal parts such as aircraft skin panels.
Derivation
"Milling" originally meant grinding grain between stones, then came to mean machining metal by cutting. "Chemical milling" borrows the word because the result is similar — material removed in controlled amounts — but the method uses chemistry instead of cutting tools.
Why Pilots Care
Chemical milling is how manufacturers create variable-thickness skins that save weight while keeping strength where it's needed. Knowing the term helps when reading maintenance manuals or structural repair documents that reference chemically milled areas, which often have specific repair restrictions.
Analogy
It is like covering part of a surface with tape before cleaning or painting it: the covered area is protected, and only the exposed area is affected. In chemical milling, the exposed metal is dissolved away in a controlled way.
Intuition Check
Chemical milling is not ordinary cutting or grinding with chemicals added. The chemical solution itself removes the metal from the exposed areas.
Example Sentence 1
The wing skin's variable thickness was achieved by chemical milling, leaving thicker sections along the spar attachments and thinner sections between ribs.
Example Sentence 2
Inspectors checked the chemically milled fuselage panels for uniform thickness after the etching process.