Introduction :-
Although some nontraditional process can be used in such conditions, they are not economical due to high capital and tooling costs involved. These disadvantages are overcome with the use of chemical machining process.
Fig 1 :-
a. Brief illustration of chemical machining process
b. Stages of producing a profile of cavity by the help of machine called agitator
● Types of Chemical Machining Process :-
1. Chemical blanking method :-
Chemical blanking is a process of producing a part from thin sheet metal by chemically etching the periphery of the desired shape. The material is removed by chemical dissolution.
Fig 2 :-
Chemical blanking employs the following four steps :-
a) Pre-process - Material cleaning - Metal surface is cleaned by degreasing, pickling and grinding.
b) Masking. Metal portion not to be removed by etching is covered with chemical resistant coating.
c) Through-material etching.
d) Post-process.
2. Chemical milling or contour machining method :-
Fig 3 :-
Chemical milling is the subtractive manufacturing process of using baths of temperature-regulated etching chemicals to remove material to create an object with the desired shape. It is mostly used on metals, though other materials are increasingly important.
● Procedure for chemical machining
Workpiece pre cleaning process :-
The surface of workpiece metal is cleaned thoroughly, degreased and pickled by acid or alkalis. Pre cleaning is the most important method to remove oil, grease, dirt, rust, or any foreign substance from the work surface to produce a good adhesion of masking material.
Masking and scribing mask:-
Masking involves covering the portion of the workpiece metal where material is not to be removed by the chemical action .masking with adhesive types or paints is a common practice although rubber (elastomers) and plastics are also used. Since it is difficult to apply maskant on small surface, the maskant is initially applied on a large surface.
Etching:-
The unmasked surface of the workpiece is machined chemically with selected etchant. Etching is carried out by immersing the work material in a tank of agitated etchant. The process is carried out at high temperature depending on the etched material. Temperature control and agitation during chemical machining. Erosion of the work material takes place from the exposed surface. The work piece is converted into metallic salt, which is then dissolved and carried away in the etchant solution.
Damasking:-
When etching is completed the mask is removed either through mechanical or chemical means. any etchant on the work material is also removed by cold water to clean. A deoxidizing bath may also be required in order to remove the oxide coating or films left on the surface of the work material.
● Maskants
Masking material is known as Maskant which is used to protect workpiece surface from chemical etchant.
In another words maskants protect the portion of workpiece metal where material is not to be removed by chemical action of the etchant.
Polymer or rubber based materials are generally used as a maskant material. Various maskant application methods can be used such as dip, brush, spray, roller, and electro coating as well as adhesive tapes.
The type of maskant to be selected for machining is based on the following factors :-
• Be inert to the chemical reagent used.
• Chemical resistance required.
• Be tough enough to withstand handling.
• Adhere well to the work piece.
• Allow itself to be scribed easily.
• Be removed easily after etching.
• Be inexpensive after etching.
• Be able to withstand the heat generation by etching.
• Availability and low cost.
● Masking material for various work material :-
1. Work material = Aluminium & Alloys
Maskant = Polymer, butyl rubber & neoprene
2. Work material = Copper & alloys
Maskant = Polymer
3. Work material = Iron based alloys
Maskant = Polymer, poly vinyl chloride, polyetilien butyl rubber
4. Work material = Nickel
Maskant = Neoprene
5. Work material = Magnesium
Maskant = Polymer
6. Work material = Titanium
Maskant = Polymer
● Etchants :-
Etchants are acid or alkaline solutions maintained within a controlled range of chemical composition and temperature.
The workpiece material to be removed is sprayed or immersed in a suitable etchant.
various etchant are available for machining different material as listed in table.
Conclusion :-
Material removal rate, accuracy, surface finish and environmental issues are the major challenges what chemical machining face.
Rate of removal of material is depended on the type of etchant we are using for machining process. Etchant which remove metal in a faster manner will have many side effects including surface finish, higher heating etc.
So the etchant should be selected in such a way that it should balance among all these problems. Undercutting is the issue which reduces the accuracy of machining. Hydrogen embrittlement also causes alterations.
The surface produced by CHM process is otherwise stress free and show no thermal effects. Chemical machining greatly affects the environment since it make use of chemical etchants.
To improve the material removal rate of chemical machining is to combine both laser machining and chemical machining.
References :-
1. Benedict, G. (2017). Nontraditional manufacturing processes. Routledge.
2 Çakir, O., Yardimeden, A., & Ozben, T. (2007). Chemical machining. Archives of Materials Science and Engineering, 28(8), 499-502.
3 Langworthy, M. (1994). Chemical Milling By Nontraditional machining process. Machining ASM Handbook.
4 Tehrani, A. F., & Imanian, E. (2004). A new etchant for the chemical machining of St304. Journal of materials processing technology, 149(1-3), 404-408.
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