29 December 2016

Abrasive Jet Machining

The principle of Abrasive jet machining involves the use of a high-speed stream of abrasive particles carried by a high-pressure gas or air on the work surface through a nozzle. The metal removal occurs due to erosion caused by the abrasive particles impacting the work surface at high speed.

Abrasive jet machining

Working Principle :

The filtered gas supplied under a pressure of 2 or 8 kgf/cm2 to the mixing chamber containing the abrasive powder and vibrating at 50 Hz and is then passed into a connecting hose.  

This abrasive and gas mixture emerges from a small nozzle mounted on a fixture at a high velocity ranging from 150 to 300 m/min. The abrasive powder feed rate is controlled by the amplitude of vibration of the mixing chamber. A pressure regulator controls the gas flow and pressure.

The carrier gas should be cheap, nontoxic and easily available. Air and nitrogen are two of the most widely used gas in AJM.

The abrasives generally employed are aluminium, oxide, silicon carbide, glass powder or specially prepared sodium bicarbonate.

The average particle sizes vary from 10 to 50 microns. Larger sizes are used for rapid removal rate while smaller sizes are used for good surface finish and precision work.

Since nozzles are subjected to a great degree of abrasion wear, they are made of hard materials such as tungsten carbide or synthetic sapphire to reduce the wear rate. 

Water jet machining is another variation where a high-pressure jet of water is directed on a surface of removal material.

The metal removal rate depends upon :
  • Diameter of nozzle
  • Composition of the abrasive-gas mixture 
  • Jet pressure 
  • The hardness of abrasive particles
  • Particle size
  • Velocity of Jet 
  • The distance of workpiece from jet

Accuracy :
With close control of the various parameters a dimensional tolerance of 
(+0.05 mm to -0.05 mm) can be obtained. on normal production work accuracy of
(+0.01 mm to -0.01 mm) is easily held.

Applications :
  • Cutting slots
  • Thin sections
  • Contouring
  • Drilling 
  • For producing shallow crevices
  • Deburring 
  • Producing intricate shapes in hard and brittle materials
  • Cleaning and polishing of plastics, nylon and Teflon components
  • Frosting of the interior surface of the glass tubes 
Advantages :
  • AJM is suitable for materials of any hardness and brittleness like ceramics, germanium, glass.
  • Ability to cut fragile and heat-sensitive materials without damage as no heat is generated due to the passing of gas or air.
  • Low capital cost
  • Holes of intricate shapes could be produced efficiently.
  • It can be utilized for cutting, drilling, polishing, deburring, cleaning of the materials.
Disadvantages :
  • The material removal rate is slow 
  • The machining accuracy is poor and the nozzle wear rate is high.
  • Additional cleaning of the work surface may occur because the abrasive particles may remain embedded in the work surface
  • Abrasive particles cannot be reused.
  • A dust collecting chamber is a basic requirement to prevent atmospheric pollution to cause health hazards.