ABRASIVE JET MACHINING:
Abrasive micro-blasting, pencil blasting and micro-abrasive blasting is also called as abrasive jet machining.
In short it can also be called as AJM.
Abrasive blasting machining process consist of abrasives which are propelled by a high velocity gas to erode material from the workpiece.
Cutting heat-sensitive, brittle, thin, or hard materials are the common uses of the abrasive jet machining.
In manufacturing of large quantity, the Abrasive jet machining process is used to remove unwanted material from a workpiece.
Special purpose of it is used to cut intricate shapes or form specific edge shapes.
The effect of the particle develops a concentrated force to perform operations which are mentioned below,
(a) Brittle metallic and non metallic materials for cutting small holes, slots, or intricate patterns is very hard .
(b) Deburring or removing small flash from parts.
(c) trimming and beveling.
(d) Some surface films and oxide films are removed.
(e) generally cleaning components with irregular surface.
The gas-supply pressure is on the order of 850 kPa (125 psi).
The abrasive jet velocity can be controlled by a valve as it is high as 300 m/s (100 ft/s).
From the tungsten carbide or sapphire, both of which have abrasive wear resistance are used to prepare nozzles.
The abrasive size is in the range from 10 to 50 mm.
Designs for abrasive jet machining should avoid sharp corners due to the flow of free abrasives.
Because of airborne particulates, there is some hazard which is included in this process.
The problem related to hazard can be neglected using the abrasive water-jet machining process.
By fine abrasive particles, usually about 0.001 inch (0.025 mm) in diameter the material is removed.
This is driven by a high velocity fluid stream.
The common gases which are required that are the air or inert gases.
The gas pressure range is from 25 to 130 psig (170-900 kPa or 4 bars).
Speeds can be also as high as 300 m/s (1,000km/h).
When the carrier gas enters in closed chamber then this is known as the mixing chamber.
PHYSICS OF THE PROCESS:
Fine particles (0.025mm) are accelerated in a gas stream.
The particles are sent in the direction of focus of machining.
As soon as the particles get impact on the surface, then it causes a micro fracture, and gas carries fractured particles away.
Material removal rate – 0.015 cm3/min.
Narrow slots – 0.12 to 0.25 mm ± 0.12 mm.
Surface finish – 0.25 micron to 1.25 micron.
Sharp radius up to 0.2 mm is possible.
Steel up to 1.5 mm , glass up to 6.3mm is possible to cut.
It is possible and easy to machine thin sectioned hard and brittle materials .
AJM machines are generally self contained bench top units.
First of all it compresses the gas and then mixes it with the abrasive in a mixing chamber.
The gas passes through convergent or divergent nozzle before entering the mixing chamber, and then exits through a convergent nozzle.
Nozzles need to be typically made of tungsten carbide or synthetic sapphire and it should be highly resistant to abrasion.
The tungsten carbide nozzles have a useful life around of 12 to 30 hours, and sapphire nozzles last about 400 hours.
Affections of the size of the machined area and the rate of material removal is due to the distance of the nozzle from the workpiece.
Flexibility, low heat production, and ability to machine hard and brittle materials plays an important role in it.
Its flexibility owes from its ability to use hoses to transport the gas and abrasive to any part of the workpiece.
By this process inaccessible portion can be also machined with good accuracy.
But one problem we have to face here is that it has slow material removal rate.
Due to this problem usually this is used as a finishing process.
Also this process produces a tapered cut.
Following are the AJM process criteria:
Material removal rate.
Geometry and surface finish of work piece
Wear rate of the nozzle.
Due to the process parameters, process criteria are generally influenced.
ABRASIVE JET MACHINING APPLICATIONS:
This is used for abrading and frosting glass more economically as compared to etching or grinding .
Cleaning of metallic smears on ceramics, oxides on metals, resistive coating etc.
For the manufacture of electronic devices , drilling of glass wafers, de burring of plastics, making of nylon and Teflon parts permanent marking on rubber stencils, cutting titanium foils AJM plays important role.
Deflashing small castings, engraving registration numbers on toughened
glass used for car windows.