Chemical milling or chemical machining is a process of removing metal by applying strong chemical solutions to a metal surface. It is used to remove large amounts of metal to obtain parts that cannot be machined easily through traditional machining methods. Parts that require precision engineering, like miniaturized microcomponents or those containing deep internal cavities, are just some of the components produced with chemical milling. While it has numerous applications in automotive manufacturing and electronics, it is very extensively used in the aerospace industry.
This machining method is considered to be one of the oldest nontraditional machining methods in existence. Chemical milling was used in the 19th century for decorative etching, and its application in industry came about much later. Manuel C. Sanz, an engineer with the North American Aviation Company, is credited with solving a critical weight problem on a missile casing by utilizing the process. The company patented the process in 1956.
The entire process is quite simple and involves cleaning and masking the parts that don't require etching. The metal is then submerged in large tanks of etching solution. The amount of material removed is controlled by the concentration of the chemical solution, the type of etchant used, the time spent in the tank, and the temperature. An ultrasonic thickness tester helps the operator to regularly assess the thickness of the part until it meets the thickness specified in the blueprint. The etched parts are cleaned and inspected for quality control.
A large number of parts can be milled simultaneously, making it an extremely cost-effective method of manufacturing parts. The primary use of chemical milling is to reduce the weight of the metal in contoured parts. It is possible to even etch complex shapes and protrusions with the utmost precision. Many parts in commercial aircraft, launch vehicles, missiles, and turbines are produced with chemical milling. Both small parts such as cover plates or large parts such as fuselage skins can be produced very easily.
It is one of the most popular machining methods because it is quite simple, inexpensive, and well established. The advantages of using this process over other machining methods are low capital and tooling costs, quick weight reduction, and minimal need for skilled manpower. Besides making it easy to implement design changes, it does not expose the metal to additional stresses. The surface quality of the metal stays good, and no burrs are formed in the process. The disadvantages are that it can be difficult to machine very thick material or get to sharp corners, and etching solutions can be dangerous to work with.