ABRASIVE JET MACHINING (AJM)

In AJM, a focused jet or stream of abrasive particles carried by high pressure gas (carrier) is made to impinge on the work surface through a nozzle (Fig.). The metal cutting occurs due to erosion caused by the abrasive particles impacting the work surface at a high speed. As a result of repeated impact, small bits of material get loosened and separated from the workpiece surface, exposing a fresh surface to the jet. AJM is different from sand blasting as the latter is a surface cleaning process and AJM is a metal cutting process.

Characteristics: 

Abrasive particles of aluminium oxide, silicon carbide, or glass powder with particle size 10 to 50 microns are used in this process, larger size particles used for rough machining and smaller size for finish work. Abrasives are contained in a hopper and fed into the mixing chamber. Compressed air or high pressure gas is supplied to the mixing chamber at a pressure 2 to 8 kgf/cm² and the chamber is vibrated. The amplitude of these vibrations controls the flow of abrasive. 

These abrasives mixed in the gas stream travel through the hose and finally pass through the nozzle at a very high speed (150 to 300 m/min). Since nozzles are subjected to very high abrasion wear, they are made of hard materials such as tungsten carbide or ceramic (synthetic sapphire). Metal removal rate in AJM depends on the nozzle diameter, composition of abrasive gas mixture, hardness of abrasives, velocity of jet, distance of workpiece from jet and the workpiece material. A typical example may be cutting of glass with a rate of 16 mm/min. A tolerance of ± 0.05 mm in diameter can be attained.

Application: 

AJM finds use in fine drilling, cutting thin section of glass and refractory, machining intricate profiles on hard and fragile materials, contouring, aperture drilling for electronic microscope, machining of semi-conductors, frosting and abrading of glass article, cleaning and polishing of plastics, etc. 

Advantages and limitations: 

The process is capable of cutting intricate holes and shapes in materials of any hardness and brittleness; it has the ability to cut fragile and heat-sensitive materials; it has low capital cost, etc. 

Material removal rate is slow, machining accuracy is poor, nozzle wear rate is high, abrasives may stick on to the surface of soft material and hence additional work of cleaning the surface is involved. AJM is therefore not suitable for machining ductile materials. Also, the abrasive used cannot be reclaimed.

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