What is Chemical Machining – Chemical Milling

Chemical Machining (CM)

Chemical machining (CM) was developed from the observation that chemicals attack and etch most metals, stones, and some ceramics, thereby removing small
amounts of material from the surface. The CM process is carried out by chemical dissolution using reagents or etchants, such as acids and alkaline solutions.
Chemical machining is the oldest of the advanced machining processes and has been used in engraving metals and hard stones, in deburring, and in the production of printed-circuit boards and microelectronic devices.

Introduction
Chemical machining (CM) is the controlled dissolution of workpiece material (etching) by means of a strong chemical reagent (etchant). 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, as occurs in corrosion or chemical dissolution of a metal. 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

Chemical milling
In chemical milling, shallow cavities are produced on plates, sheets, forgings and extrusions. The two key materials used in chemical milling process are etchant and maskant. Etchants are acid or alkaline solutions maintained within controlled ranges of chemical composition and temperature. Maskants are specially designed elastomeric products that are hand strippable and chemically resistant to the harsh etchants.

Steps in chemical milling
· Residual stress relieving: If the part to be machined has residual stresses from the previous processing, these stresses first should be relieved in order to prevent warping after chemical milling.
· Preparing: The surfaces are degreased and cleaned thoroughly to ensure both good adhesion of the masking material and the uniform material removal.
· Masking: Masking material is applied (coating or protecting areas not to be etched).
· Etching: The exposed surfaces are machined chemically with etchants.
· Demasking: After machining, the parts should be washed thoroughly to prevent further reactions with or exposure to any etchant residues. Then the rest of the
masking material is removed and the part is cleaned and inspected.

Application Of Chemical Milling :

Chemical milling is used in the aerospace industry to remove shallow layers of material from large aircraft components, missile skin panels , and
extruded parts for airframes. Tank capacities for reagents are as large as 3.7 >< 15 m. The process is also used to fabricate microelectronic devices and often is referred to as Wet etching for these products (see Section 28.8.1). The ranges of surface finish and tolerance obtained by chemical machining and other machining
processes .
Some surface damage may result from chemical milling because of preferential etching and intergmnular attack, which adversely affect surface properties. The
chemical milling of welded and brazed structures may result in uneven material removal. The chemical milling of castings may result in uneven surfaces caused by
porosity in, and nonuniformity of, the material.