Abrasive Jet Machining (AJM) Principles
In abrasive jet machining process, a focused steam of abrasive particles (of size 10 to 40 microns) carried by high pressure gas or air at a velocity of about 150 to 300 m/sec is made to impinge on the work surface through a nozzle, and the work material is removed by erosion by the high velocity abrasive particles. The inside diameter (ID) of the nozzle through which abrasive particles flow is about 0.18 to 0.80 mm and the stand-off distance (i.e. distance between nozzle tip and workpiece) is kept about 0.3 to 20.0 mm. The process can be easily controlled to vary the metal removal rate which depends on flow rate and size of abrasive particles. This process is best suited for machining super alloys and refractory type of materials, and also machining thin sections of hard materials and making intricate hard holes. The cutting action is cool because the carrier gas serves as coolant.
When an abrasive particle (like Al2O3 or SiC) having sharp edges hits a brittle and fragile material with a high speed, it makes dent into the material and lodges a small particle from it by a tiny brittle fracture. The lodged out or wear particle is carried away by the air or gas. The operating elements in AJM are abrasive, carrier gas and the nozzle as schematically shown in the following Figure
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Abrasive Jet Machining (AJM) Principles
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The distance between the nozzle tip and the work surface has great influence on the diameter of cut, its shape and size and also rate of material removal. The following Figure shows the variation in the diameter of cut with change in the stand off distance (SOD). It is evident that the SOD changes the abrasive particles spreads (i.e. covers wider area) on the work surface and consequently increases the diameter of the cut.
The basic Units of AJM
The basic unit is schematically shown in following Figure. It consists of gas supply system (compressor), filter, pressure regulator, mixing chamber, nozzle assembly and the work holding device. In the mixing chamber, the abrasive is allowed to flow into the gas stream. The mixing ratio is generally controlled by a vibrator. The particle and gas mixture comes out of the nozzle inside the machining chamber of the machine tool unit. The feed motion can be given either to the work holding device or to the nozzle.
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| AJM setup |
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Abrasive Jet Machining (AJM) Advantages:
- This process is quite suitable for machining brittle, heat resistant and fragile materials like, glass, ceramic, germanium, mica etc.
- It can be utilized for cutting, drilling, polishing, deburring, cleaning etc. of the materials.
- The depth of damage to the surface is very little.
- Holes of intricate shapes could be produced efficiently.
- The surface machined can have good finish (by controlling the grain size mainly).
AJM Disadvantages:
The materials removal rate is low. For example, for glass, it is 0.0164 cm3/min.
- The tapering of hole especially, when the depth of the hole is more, becomes almost inevitable.
- A dust collecting chamber is a basic requirement to prevent atmospheric pollution to cause health hazards.
- The abrasive particles may remain embedded in the work surface.
- Abrasive particles cannot be reused.
AJM Applications:
- Abrasive jet machining is best suited for machining brittle and heat sensitive materials like glass, quartz, sapphire, ceramics etc,
- It is used for drilling holes, cutting slots, cleaning hard surfaces, deburring, polishing etc.
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