Chemical Milling

Sometimes, chemical milling is referred to as chemical etching. It is a purely chemical process, not electro-chemical.

Although simple in principle, chemical milling offers a method of producing complex patterns and lightweight parts and is used for incorporating integral ribs and stiffeners in sheet metal. Tapered sections can also be easily formed – the unwanted material being eaten away by a suitable chemical.

The process is ideally suited to aluminium alloys. In this instance, the chemical is a hot alkaline solution (usually caustic soda). While it is a relatively slow process, its unique advantages make it very attractive for airframe components. The areas, which must not be eaten away by the fluid, are simply protected by a thin layer of plastic, which can be brushed or sprayed on.

Although the chemically etched surface is not very rough, a drop in fatigue strength does result. And in critical applications, restoration of fatigue strength is desirable. A light, peening operation, using glass beads or steel shot, achieves this.