In this article we will discuss about the meaning and advantages of chemical milling.
Meaning of Chemical Milling:
This is a form of etching process. In this process job is first cleaned properly and then some sort of preventive coating is applied (masking) on that particular portion which is not to be machined. The preventive coating is of vinyl plastic. This is applied with the help of a template. Then the job is exposed to the etching material.
Time depends upon metal to be removed and strength of chemical reagent. Chemical etching reagent depends upon work material. For aluminium it is caustic soda, and for steel it is acid (Nitric). The metal is removed by the chemical conversion of the metal into metallic salt.
Material removal rate is mainly dependent on the selected etchant. If metal is removed at fast rate with certain etchant then undercutting increases, surface finish deteriorates and more heating takes place. The etch rate is, therefore, limited to 0.02 to 0.04 mm per minute.
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Etching rate and depth of cut are high for hard metals (0.13 mm/ min. and 0.5 to 1.5 mm/min respectively for titanium alloys, stainless steel and heat resistant alloys) and low for softer materials (0.025 mm/min and upto 0.50 mm/min for aluminium). The accuracy is of the order of ± 0.01 mm with optimum time, temperature and solution control. Surface finish of the order of 5 pm is usually obtained.
Hydrogen embrittlement may occur in materials like steel, stainless steel, copper alloys and nickel alloys owing to absorption of hydrogen in chemical machining. If it occurs, it can be overcome by heating the workpiece around 120°C for 1 to 4 hours.
Sometimes chemical reagent removes more metal from one place and less from other. Therefore, to make the metal removal even, after some time take out the job and protect that portion which is more machined by applying protective coating (masking).
Advantages of Chemical Milling:
This process can give very complicated shaped pattern on work material, but much depends on the skill of operator.
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It is mostly used in aircraft industry. However depth of etching must be very less otherwise long time will be required and at the same time surface may not be fine finished and even also. It is mainly used for embossing, coining, engraving operations.
The tooling set up costs are considerably low. Machining is done without any production of burrs and even thin sheets of metal can be processed with ease without distortion. Materials which are brittle in nature can be processed with ease.
This process is best suited for production of printed circuits, where the basic connections of the circuit consist of thin metal strips attached to an insulating base. These circuits are produced from insulating board faced with a thin layer of copper.
The copper is coated with photosensitive resist and an image of the required circuit is printed photographically on the surface. Etching removes all the unwanted copper. Line thickness upto 0.02 mm can be controlled.
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The etchant vapours are very corrosive in nature and, therefore, the process equipment must be kept safe from the etching and other operating equipments.