Chemical machining (CHM)

Chemical machining is the process of removing metal from the workpiece through a controlled chemical attack or etching with acids or alkalies (etchant solution). The metal is gradually transformed into metallic salt by chemical reaction and is finally removed in this form During operation, areas of the workpiece from where material is not to be removed are protected by some etchant resistant material. Chemical machining is used for machining nearly all the materials, from metals to ceramics.

Characteristics:

The workpiece to be machined is first cleaned. It is required to ensure good adhesion of etchant solution with the work. For removing dust and oil, the workpiece is first dipped in a solution of mild alkaline (or in trichlorethylene vapours). After cleaning, the workpiece is dried and later coated with the etchant resistant material (called maskant) either with a cut and peel maskant or by brushing, dipping or spraying (up to 0.2 mm thickness).

When this has dried, the desired shape to be processed on the work material is cut on the maskant with a scribe knife. The workpiece is later dipped completely in a tank of chemicals which dissolve (etch) away the exposed metal. Accuracy in the range of ±0.01 mm can be achieved on relatively shallow depths of cut with surface finish about 5 microns. On aluminium, surface finish may be about 1.6 microns with metal removal rate about 140 cm³/min.

Types of CHM:

Chemical machining processes can be classified into the following categories:

(a) Chemical blanking is used for cutting out parts from thin sheet metal.

(b) Chemical milling or chemical contour machining is employed for selective or overall metal removal from thicker work-pieces for weight reduction.

Application:

CHM finds important usages where the depth of the material removal is critical to a few microns, for example, removing unwanted material from the surface or skin of an aircraft, forgings, castings, extrusions, or formed wrought stock, making of burr-free stampings, etc.

Advantages and limitations:

CHM does not distort the workpiece and both the faces of the workpiece can be machined simultaneously and machining is burr free. It can be used on materials which are considered most difficult to machine. The process is, however, slow. manufacturing cost is high and metal thickness that can be machine is limited.

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