The process makes use of an abrasive jet with high velocity, to remove material and provide smooth surface finish to hard metallic workpieces. It is similar to Water Jet Machining (WJM).
Process
Material is removed by fine abrasive particles, usually about 0.001 in (0.025 mm) in diameter, driven by a high velocity fluid stream; common gases are air or inert gases. Pressures for the gas range from 25 to 130 psig (170–900 kPag) and speeds can be as high as 300 m/s.PROCESS CAPABILITIES AND LIMITATIONS
Abasive Jet Machining utilises the presure of fluid stream to remove material from the surface of the job.When using air as a medium the mixture of air and abrasives are allowed to impinge on the work surface at about 200 to 400m/s through the nozzle and work material is eroded by the high velocity abrasive particles.The inside diameters of the nozzle is about 0.04mm and standoff distance is kept about 0.7 to1.0mm. The process can be easily controllled to vary the metal removal rate which depends on flow rate and size of abrasive particles .The cutting action is cooled because the carrier gas serves as a coolant.
Gas used are nitrogen or carbondioxide or even air which are supplied underpressure(2-8 kg/cm^2) filtered through regulator is passed to a mixing chamber(containing abrasive particles) vibrating at 50c/s.From the mixing chamber, the gas alongwith the entrained abrasives particles of size 10-50 micro meter passes on to nozzle having its tip tungsten carbide and diameter of around 0.45mm,with a velocity of 150 to 300 m/s.The air consumption is of order of 0.6m^3/hr.The nozzle tip distance is of order of 0.81mm.The abrasive powder feed rate is controlled by the amplitude of vibration of mixing chamber.The relative motion between the nozzle and the workpiece is obtained by the programable torch or by cams and pantographs to control the size and shape of the cut.Dust removal equipment is incorporated to protect the enviroment. The material removal rate, geometry of cut, surface roughness, and nozzle wear rate are influenced by the size and distance of nozzle, composition,strength,size and shape of abrasive flow rate;and composition,pressure and velocity of carrier gas.
The abrasive particles should have irregular shape and consist of short edges rather than having rounded shapes.Abrasives generally used are Aluminium Oxide,Silicon Carbide, Sodium bicarbonate,dolomite,glass beads;their selection and their grain size depending on the machining operation.
The material removal rate is mainly dependent on the flow rate and size of the abrasive particles .High grain size will always produce more metal removal.At a paticular pressure metal removal rate increases with the abrasive flow rate but after reachining a optimum value ,the material removal rate decreases with increase in abrasive flow rate .This is because mass flow rate of the gas decreases with increase of abrasive flow rate and mixing ratio increases causing a decrease in material removal rate because of less energy available for erosion.The abrasive particles are generally not used again and again.
The material removal rate first increases with the increases of tip distance from work upto a certain limit after which it remains unchanged for a certain tip distance and then falls gradually.In this process the limitations are that the material removal rate is low,stray cutting can't be avoided,tapering effect may be found because of unavoidable flaring of the abrasive jets ,abrasives may get embedded i the work surface,and suitable dust collecting system has to be provided.
The advantages of this process are that it can be used to cut intricate hole shapes in hard and hard and brittle materials;even fragile and sensitive materials can be cut without damage ,and the initial cost is low.It's disadvantages are that it removes material at very low rate,stray cutting can occur resulting in poor accuracy, and soft materials can't be machined by this process.
Advantages of Abrasive Jet Machining:
- Surface of the workpiece is cleaned automatically.
- Smooth surface finish can be obtained.
- Equipment cost is low.
- Hard materials and materials of high strength can be easily machined.
Disadvantages of Abrasive Jet Machining:
- Metal removal rate is low
- In certain circumstances, abrasive particles might settle over the workpiece.
- Nozzle life is less. Nozzle should be maintained periodically.
- Abrasive Jet Machining cannot be used to machine soft materials.
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