6.3 Chemical Machining (CM)

Chemical machining (CM) is a well known non-traditional machining process in which metal is removed from a workpiece by immersing it into a chemical solution. The process is the oldest of the nontraditional processes and has been used to produce pockets and contours and to remove materials from parts having a high strength-to-weight ratio. Moreover, the chemical machining method is widely used to produce micro-components for various industrial applications such as microelectromechanical systems (MEMS) and semiconductor industries. [3]

Fig.6.1 Basic electro-discharge system

In CM material is removed from selected areas of workpiece by immersing it in a chemical reagents or etchants, such as acids and alkaline solutions. Material is removed by microscopic electrochemical cell action which occurs in corrosion or chemical dissolution of a metal. Special coatings called maskants protect areas from which the metal is not to be removed. This controlled chemical dissolution will simultaneously etch all exposed surfaces even though the penetration rates of the material removal may be only 0.0025–0.1 mm/min. The basic process takes many forms: chemical milling of pockets, contours, overall metal removal, chemical blanking for etching through thin sheets; photochemical machining (pcm) for etching by using of photosensitive resists in microelectronics; chemical or electrochemical polishing where weak chemical reagents are used (sometimes with remote electric assist) for polishing or deburring and chemical jet machining where a single chemically active jet is used. A schematic of chemical machining process is shown in Fig.6.2a. Because the etchant attacks the material in both vertical and horizontal directions, undercuts may develop (as shown by the areas under the edges of the maskant in Fig.6.2b). Typically, tolerances of ±10% of the material thickness can be maintained in chemical blanking. In order to improve the production rate, the bulk of the workpiece should be shaped by other processes (such as by machining) prior to chemical machining. Dimensional variations can occur because of size changes in workpiece due to humidity and temperature. This variation can be minimized by properly selecting etchants and controlling the environment in the part generation and the production area in the plant.

Fig.6.2 Schematic illustration of chemical machining process