Chemical machining is the material removal process for the production of desired shapes and dimensions. It is done by selective or overall removal of material by a controlled chemical attack with acids or alkalies.
The metal is slowly converted into metallic salt by chemical reaction and is finally removed in this form. Areas from where the material is not to be removed are protected by an etching resistant material, known as ‘maskant or ‘resist’.
Almost all the materials, from metals to ceramics, can be chemically machined. The component to be machined is first cleaned in trichloroethylene vapour or in a solution of mild alkaline solution at 80 to 90 °C, followed by washing in clean water.
One of the roughest methods is to coat the component all over by spraying or dipping. This removes dust and oil. The cleaning ensures good adhesion of the coating or masking agent.
After cleaning the component is dried and coated with the maskant material which may be cut and peel, photoresist or screen-print, type. Finally, the metal is removed by etching.
Chemical Machining Working Principle
The chemical machining working principle is chemical etching. The part of the workpiece whose material is to be removed is exposed to a chemical known as enchant. The enchantment removes the metal from the chemical attack. The method of making contact of material by the enchant is masking.
Chemical Machining Process
The process can be applied to different types of operations such as milling, blanking, and engraving. The different chemical machining processes can be classified as:
Chemical milling
Chemical blanking
Chemical engraving
Chemical machining for some special purposes can also be achieved by using a jet of reactive gas, e.g., chlorine on the machining zone. This is known as Gaseous Chemical Machining or Hot Chlorine Machining and can be used for deburring of metal parts.
Chemical Milling
Chemical milling sometimes called Chem milling or contour machining or etching. It is used to produce shapes by selective or overall removal of metal parts from relatively large surface areas.
The main purpose is to achieve shallow but complex profiles, reduction in weight by removing unwanted material from the surface as in the skin of an aircraft. The components are cleaned and degreased by immersion in trichloroethylene vapour. Or some alternative chemical cleaner followed by washing in clean water.
Chemical Milling
The component is then coated with a cut and peel maskant by brushing, dipping or spraying (up to 0.2 mm). This can be a suitable fluid with a neoprene base. Or some alternative plastics solution impervious to the action of the etching agent (permitting etching depths up to 10 mm).
When this has dried, by mild heating otherwise, the desired shape to be processed on the work material is cut on the maskant with a scribing knife and the unmachined portions of the maskant are peeled away. Usually, a template is used to portray. The desired machining shape within tolerance.
The parts are then dipped completely into a tank of chemicals which will dissolve (etch) away from the exposed metal. After etching to the required depth, and washing to remove all traces of the etchant, the entire masking is stripped from the component and their surfaces are anodised or treated with a temporary protective agent as necessary.
Chemical Blanking
Chemical blanking, chem-blanking, photo forming, photofabrication or photo etching is a variation of chemical milling. In this process, the material is completely removed from several areas by chemical action. The process is used chiefly on their sheets and foils. Almost any metal can be worked by this process, however, it is not recommended for material thinner than 2 mm.
The workpiece is cleaned, degreased and pickled by acid or alkalis. The cleaned metal is dried and photoresist material is applied to the workpiece by dipping, whirl coating or spraying. It is then dried and cured. The method of photography has been used to produce etchant resistant images in photoresist materials.
This type of maskant is sensitive to light of a particular frequency, normally ultraviolet light, and not to room light. This surface is now exposed to the light through the negative i.e., a photographic plate of the required design, just as in developing pictures. After exposure, the image is developed. The unexposed portions are separated out during the developing process showing the bare metal.
The used metal is next placed into a machine which sprays it with a chemical etchant, or it is drawn into the solution. The etching solution may be hydrofluoric acid (for titanium), or one of several other chemicals. After 1 to 15 min, the unwanted metal has been eaten away, and the finished part is ready for immediate rinsing to remove the etchant.
Printed circuit cards, other engraving operations and blanking of intricate designs can be suitably made by chemical blanking by using photoresist maskants.
Very thin metal (0.005 mm) can be well etched.
High accuracy of the order of +0.015 mm can be maintained.
High production rate can be met by using an automatic photographic technique.
Application of Chemical Machining
CHM has been applied in a number of usages where the depth of metal removal is crucial to a few microns, and the tolerances are close.
The surface finish obtained in the process is in the range of 0.5 to 2 microns.
Besides, it removes metal from a portion of the entire surface of formed or irregularly shaped parts such as forgings, castings, extrusions or formed wrought stock.
One of the major applications of chemical machining is in the manufacture of burr-free, intricate stampings.
Advantages and Disadvantages of Chemical Machining
The advantages are that this process does not distort the workpiece, does not produce burrs, and can easily be used on the most difficult-to-machine materials.
However, the process is slow, and thus it is not usually used to produce large quantities or to machine material thicker than 2 mm.
Some small parts are made 10 to 100 at a time on a single plate, which speeds up production