Saturday, 21 November 2015

Process Parameters of AJM

The process parameters in AJM can be grouped into the following categories. The Ishikawa cause and effect diagram depicts the effect of various process parameters on the accuracy and quality of the machining operations by the abrasive jet machine.

1. The Abrasive: types, composition, strength, size, mass flow rate

2. The Gas: composition, pressure and velocity

3. The nozzle: geometry, material, stand-off distance (SOD), feed rate, inclination to work

4. The workpiece: Type of material

The selection of abrasive particles to be used in AJM depends upon the type of work material and type of machining operation which needs to be carried out. Different machining operations such as finishing, roughing require different types of abrasive for AJM operations. Commonly used abrasive for cutting include aluminum oxide and silicon carbide. In cleaning, etching and polishing operations glass beads and dolomites are recommended. The size of the abrasive particles also plays an important role in type of machining operations of AJM. Coarse grain particles are recommended for cuttingoperations while fine grains are recommended for finishing or polishing operations

The gas used in the AJM process must be non-toxic. It should be cheap and easily available. Common types of gas used in AJM applications are air, nitrogen and carbon. The recommended velocity of gas abrasive mixture ranges between 100 m/sec to 300 m/ sec depending upon the cutting or finishing operation.

The velocity of gas abrasive mixture is a function of nozzle design, nozzle pressure, and abrasive particle size. Stand-off distance (SOD) is a very important parameter. SOD is defined as the distance between the tip of nozzle and the work surface. The larger the SOD the poorer is the quality and accuracy of the cut. The effect of SOD on the accuracy of the cut is 10 – 30 micron Cutting, grooving

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