In Abrasive Jet Machining, the gases employed are usually air and nitrogen. Abrasives used are aluminium oxide, silicon carbide, glass powder and sodium carbonate. The principle size ranges from 10 meters to 50.
The smaller size particles are used for obtaining good surface finish and precision, where as the larger size particles give rapid removal rate of material. Nozzles are made up of hard material such as Tungsten Carbide or Ceramic, so as to withstand high degree of abrasion wear. The abrasive gas mixture, jet pressure, abrasive particle size and hardness are mainly responsible for the metal removal rate. The metal removal rate for this type of process is usually 16mm^3 /min in cutting glass and the velocity of the jet ranges from 150 to 300 meters per minute.
The basic working principle employed in abrasive jet machining is shown in the above figure. This method utilizes high speed stream of abrasive particles which are carried by a high pressure air or gas on the work material through a nozzle device. The filtered gas is supplied that is between the pressure limits of 1.96 bar to 7.85 bar to the mixing chamber that comprises abrasive powder. The mixing chamber is made to be vibrated at about 50 Hz for proper mixing of abrasive particles with the gas. Thus the mixture is passed into a connecting hose that emerges from a nozzle with high velocity. The pressure form regulator is also employed to regulate the gas flow while the abrasive powder feed rate is regulated by amplitude of vibration of mixing chamber. The cams, pantograph or other suitable mechanisms are responsible to move the work piece or nozzle for controlling the cutting action for cutting size and shape.
When the abrasive particles hit the work piece with a high velocity, then this impact causes a fracture. There are many parameters that influence material removal rate.
Metal removal rate depends upon the composition, strength, size and mass flow rate of abrasive particles. The material removal rate increases with an increase in mixing ratio. Further increase in mixing ratio decreases the Metal Removal Rate. The mass flow rate of abrasive particle depends on the pressure of the gas. The material removal rate increases linearly with an increase in mass flow rate.
As the velocity pressure depends on the composition of gas, the material removal rate is mainly affected by the composition of the gas.
In a nozzle, the abrasive grains continuously flow with a very high speed. Thus nozzle is the most important element for controlling the process characteristics. The important parameter is the distance between the work surface and tip of the nozzle known as nozzle tip distance. When nozzle tip distance increases, the velocity of abrasive particles impinging on work piece also increases. the increase in the velocity of abrasives not only effects the metal removal rate but also the shape and size of the hole or cavity produced in the work piece. The metal removal rate increases with an increase in nozzle tip distance up to a maximum value. Further increase in nozzle tip distance decreases the metal removal rate since the velocity will get reduced due to drag of atmosphere.
Advantages of Abrasive Jet Machining-
1. This method facilitates to machine the complex holes and intricate cavities of harder materials of desired shape.
2. The fragile metals which are very difficult to machine in the conventional machining process are easily machined by this process with better accuracy.
3. Machining can be performed easily for brittle type of materials of thin sections.
4. Capital investment if very low.
5. No direct contact occurs between the work piece and the tool.
6. Heat generated in this process is very less.
Disadvantages of Abrasive Jet Machining-
1. Material removal rate is very slow.
2. This process is not applicable for ductile materials.
3. Poor machining accuracy.
4. Abrasive powder cannot be recycled or reclaimed.
5. Cleaning of work material is necessary after the operation, as there is a danger of abrasive particles sticking to the material.
Applications of Abrasive Jet Machining-
1. This method is employed in micro welding, fine drilling and aperture drilling for electronic microscope.
2. Used in machining of intricate profiles on the fragile and hard metals.
3. Used for machining semi conductors.
4. This method is also employed to perform cleaning and cutting operations on the materials like silicon, germanium, quartz, mica and many more.
5. Abrading and frosting of brittle materials such as glass, ceramics, refractories and many more.
This is all about Abrasive Jet Machining process.