ABRASIVE JET MACHINING (AJM)

 Abrasive jet cutting machine tool is used to cut sheet materials or remove materials from a surface by generating a focused stream of fluid mixed with abrasive particles. Abrasive jet cutting machines are available as complete systems with all of the components required for the blasting or jet machining applications such as pressure generation / intensification, cabinets, nozzles or wheels, and dust collectors. Or they may be purchased in component form to either build a custom system from the ground up, or to replace worn out parts. Abrasive jet metal cutting machines includes four main types of device such as abrasive water jet cutting, air abrasive jet cutter and precision blaster or micro-jets. Water jet cutter are used for drilling, detailing and precision cutting of printed circuit boards and other high quality components. In addition to cutting, abrasive jet machining can perform a number of secondary procedures like sectioning, removing sharp edges or discontinuities from a part, demolition etc.

 The physics,

1.  Fine particles (0.025mm) are accelerated in a gas stream (commonly air at a few times atmospheric pressure).
2.  The particles are directed towards the focus of machining (less than 1mm from the tip).
3.  As the particles impact the surface, they fracture off other particles.

 

 

As the particle impacts the surface, it causes a small fracture, and the gas stream carries both the abrasive particles and the fractured (wear) particles away.

 

Brittle and fragile work pieces work better.

 

 Material Removal Rate (mrr) is,

 

 

Factors that effect the process are,

1. mrr
2. geometry of cut
3. roughness of surface produced
4. the rate of nozzle wear

 

 The factors are in turn effected by,

1. the abrasive: composition; strength; size; mass flow rate
2. the gas composition, pressure and velocity
3. the nozzle: geometry; material; distance to work; inclination to work

 

 The abrasive,

1.  materials: aluminum oxide (preferred); silicon carbide
2.  the grains should have sharp edges
3. material diameters of 10-50 micro m 15-20 is optimal
4. should not be reused as the sharp edges are worn down and smaller particles can clog nozzle.

 

 Gas jet,

1. mass flow rate of abrasive is proportional to gas pressure and gas flow
2.  

1. pressure is typically 0.2 N/mm2 to 1N/mm2
2. gas composition effects pressure flow relationship

 Nozzle

1. must be hard material to reduce wear by abrasives: WC (lasts 12 to 30 hr); sapphire (lasts 300 hr)
2. cross sectional area of orifice is 0.05-0.2 mm2
3. orifice can be round or rectangular
4. head can be straight, or at a right angle

 

  The relationship between head, and nozzle tip distance.

 

 

 Air drag also slows abrasive stream.

 

 

 Machines

 

 

 Summary of AJM characteristics

1.  Mechanics of material removal - brittle fracture by impinging abrasive grains at high speed
2.  media - Air, CO2
3.  abrasives: Al2O3, SiC, 0.025mm diameter, 2-20g/min, non-recirculating
4.  velocity = 150-300 m/sec
5. pressure = 2 to 10 atm.
6. nozzle - WC, sapphire, orifice area 0.05-0.2 mm2, life 12-300 hr., nozzle tip distance 0.25-75 mm
7. critical parameters - abrasive flow rate and velocity, nozzle tip distance from work surface, abrasive grain size and jet inclination
8. materials application - hard and brittle metals, alloys, and nonmetallic materials (e.g., germanium, silicon, glass, ceramics, and mica) Specially suitable for thin sections
9.  shape (job) application - drilling, cutting, deburring, etching, cleaning
10. limitations - low metal removal rate (40 mg/min, 15 mm3/min), embedding of abrasive in workpiece, tapering of drilled holes, possibility of stray abrasive action.