ABRASIVE JET MACHINING (AJM)
In abrasive jet machining method, the material is removed from the surface of a workpiece, by impinging a focused jet of fine abrasive particles carried by a compressed gas which imparts kinetic energy to the stream of fine abrasives. The stream leaves through a nozzle at a velocity of the order of 300 m/s and strikes the surface of the workpiece, producing impact loading on it. Severe plastic deformation or micro-cracks occur in the vicinity of the impact. Due to repeated impacts, small chips of material get loosened and a fresh surface gets exposed to the jet.
The carrier gas can be air, nitrogen or carbon dioxide, but never oxygen. The air must be filtered to remove water, oil and other contaminants. Nozzle pressure can range between 0.20 N/mm2 and 0.85 N/mm2, but is usually about 0.5 N/mm2. Higher pressure results in rapid nozzle wear, low pressure gives slow metal removal rates.
To resist abrasion and wear of nozzles, these are made of hard materials such as tungsten carbide and synthetic sapphire. The useful life of sapphire nozzles is about 10 times that of tungsten carbide nozzles. The sizes of the nozzles are usually:
Tungsten Carbide
Round : 0.0125 to 0.80 mm diameter
Rectangular : 0.075 mm x 0.50 mm to 0.175 mm x 3.75 mm
Square : 0.65 mm square
Sapphire nozzles are made only round :- 0.20 to 0.65 mm diameter.
The various abrasive materials used in AJM include: aluminium oxide, silicon carbide, glass powder or beads, dolomite (calcium magnesium carbonate) and specially prepared sodium bicarbonate. When sodium bicarbonate is used, it must be kept dry. The cutting performance will depend upon: hardness, strength, particle size and particle shape of the abrasive. Aluminium oxide is a general purpose abrasive and is used in sizes of 10, 25 and 50 microns. SIC is used for faster cutting on extremely hard materials. It is used in 25 and 50 microns sizes. Dolomite is used in 200 mesh size for light cleaning and etching. Sodium bicarbonate is used for extra fine cleaning and glass beads (0.635 to 1.27 mm diameter) are used for light polishing and fine deburring.
In general, larger sizes are used for rapid removal rate while smaller sizes are used for good surface finish and precision.The metal removal rate will depend upon the diameter of nozzle, composition of abrasive gas mixture, hardness of abrasive particles and that of the work material, particle size, velocity of jet and distance of workpiece from the jet. The typical metal removal rates vary from 0.016 to 0.02 cm3/min.Masks defining the machining area are sometimes used to prevent stray cutting. Copper is a good all purpose masking material. Glass gives excellent definition but has short life. Rubber has long life but gives poor definition.
Applications of AJM include:
Abrading, cutting, or cleaning (for example electrical contacts) and for machining semi-conductors such as silicon, gallium or germanium, for making holes and slots in glass, quartz, sapphire, mica and ceramics. Other applications are: cleaning hard surfaces, deburring, scribing, grooving, polishing and radiusing. A dimensional tolerance of ± 0.05 mm can be obtained. Surface finish obtained is in the range of 0.5 to 1.2 µ m CLA. Typical power input is 0.25 kW. The method is used chiefly to cut materials that are sensitive to heat damage and thin section of hard materials that chip easily and to cut indicate holes that would be more difficult to produce by other methods. The other advantage is its low capital cost.
The disadvantages of the method are: low metal removal rate, possibility of stray cutting, embedding of abrasive particles in soft workpiece and dust control requirements.
Schematic diagram of AJM
A schematic diagram of AJM is shown in Figure above. Dry and filtered gas (in this case air) is raised to a high pressure in the compressor. The pressurised air flows to the vibrating mixing chamber containing abrasive powder. The mixture of pressurised air and abrasive powder then flows to the nozzle. It then impinges on the workpiece after shooting out of the nozzle exit. The pressure regulator regulates the gas flow and its pressure. The feed rate of abrasive powder is controlled by amplitude or vibrations of the mixing chamber. The movement of the nozzle towards the workpiece or vice versa is controlled by a cam mechanism or a pantograph mechanism.
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