Abrasive Jet Machining

Conventional abrasive jet machining is the use of abrasive-gas jets to propel particles against a surface and remove the material from that surface via particle impact and shear. Typically, abrasives in quantities up to 0.05 kg/min (0.1 lb/min) are propelled through a nozzle at inlet pressures ranging from 170 to 860 kPa (25 to 125 psi) in a gas (nitrogen or air) medium. Outlet velocities are generally supersonic, and the volume of material removed varies as particle velocity is raised to a power between 2 and 3. Sand blasting and deburring are the two most common applications for abrasive jet machining. Material removal rates can be enhanced by increasing the inlet pressure (experiments up to 12 MPa, or 1.8 ksi, have been carried out) and by optimizing nozzle design. However, difficulties in controlling shock-laden jet structure and particle distribution within the jet make this process impractical for precision machining. An extremely high noise level (>85 dB) is another problem.

Abrasive Waterjet Cutting. High-velocity water can be used to entrain and accelerate abrasive particles. Unlike gas jets, waterjets can be confined to a small diameter without spreading, and they can accelerate particles to desired velocities in a relatively short distance. In addition, relatively high abrasive mass flow rates (1.4 to 1.8 kg/min, or 3 to 4 lb/min) can be easily accommodated. The result is an extremely high-energy jet that can easily cut through a solid steel plate more than 102 mm (4 in.) thick.

In abrasive slurry jets, a thin slurry mixture of abrasive and water is directly pumped to a high pressure and then passed through a nozzle to form the jet. A slurry jet is more efficient because energy losses due to mixing and particle collision are much reduced. In addition, high abrasive loading can be readily achieved. However, other problems easily outweigh these advantages. Pump and component life is greatly reduced, and uniform mixing of the slurry is a problem. A continuous-batch, lower-pressure (20 to 105 MPa, or 3 to 15 ksi) system has recently been introduced that is designed primarily for portable applications.

Abrasive flow machining is a novel technique in which copious amounts of an abrasive compound (whose viscosity is between that of a compound and that of a slurry) is applied between the work material and a closely fitting mandrel. The abrasive "fluid" removes and sizes the work material as it is "extended" on the surface being finished. Some of the applications of abrasive flow machining are finishing of dies and molds, finishing of jet engine components, and cutting or trimming of sheet metals.

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