Abrasive-jet Machining (AJM)

 

In abrasive-jet machining (AJM) material is removed by the impact of a high velocity stream of gases and abrasive mixture focused on to the workpiece. High Velocity impact of an abrasive particle causes a tiny brittle fracture on the work surface and the flowing gases carries away the dislodged small workpiece particle.

Abrasive-jet machining process is similar to convectional sand blasting, but differs in the way that the abrasive is much finer and the process parameters are effectively controlled.

Features of Abrasive-jet Machining:

• Size of abrasive particle is in the range from 10 to 50 µm.
• Mechanism of material removal is due to brittle fracture by impinging abrasive grains at high speed.
• This process is more suitable when the work material is brittle and fragile.
• Normally number of grains per unit time is 2 to 20 g/min with non recirculation type and size of grains is 15 to 20µm.
• Pressure of air/gas normally used is 2 to 10 MPa.
• Nozzle is made by WC with orifice area of 0.05 to 0.2 mm² and its life is 12 to 300 hours.

Materail Removal Rate (MRR) characteristics:

• Nozzle is generally having contact with abrasives, so it should be made by very hard material to avoid wear.
• Shape of orifice is circular or rectangular.
• Nozzle tip distance is the distance between nozzle tip and work piece, it affects not only the MRR but also shape and size of cavity produced.
• As the nozzle lip distance increases velocity of abrasive particles impinging on work surface increases due to their acceleration after they leave the nozzle, which in turn increases the MRR.
• But with further increase in nozzle tip distance velocity decreases due to drag of the atmosphere.

Limitations:

• Low MRR (40 mg/min) embedding of abrasive in workpiece, tapering of drilled holes.
• Tapping is about 7° angle if the nozzle top distance is 15 mm.

Applications:

• AJM is used for cutting, cleaning and for machining of semi-conductors such as silicon, gallium or germanium, for making holes and slots in glass, quartz, mica and ceramics.
• A dimensional tolerance of 0.05 mm can be obtained with surface finish of 0.5 to 1.2 µm.