Working Principle of Abrasive Jet Machining (AJM) Process in modern machineries

Are you looking for detailed notes on working principle of Abrasive Jet Machining (AJM) Process? In this article, you will find the basic principle, equipments used for, its descriptions, process parameters, material removal rate (MRR) and its applications with merits and demerits. This AJM process will be useful for those students who are studying B.E Mechanical 3rd year under Anna University and M.E first year manufacturing engineering students. It is included in the syllabus of "Unconventional Machining Processes" or "Modern Machining Processes". Let go to the descriptive details of working principle of Abrasive Jet Machining process.

Abrasive Jet Machining process - Working Principle:

In this AJM system, a focused stream of abrasive grains of Al2O3 or Sic carried by high pressure gas or air at a high velocity is made to impinge on the work surface through a nozzle of 0.3 to 0.5 mm diameter. The work-piece material is removed by the mechanical abrasion (MA) action of the high velocity abrasive particles.

Equipments used:

The following set up is being used for the abrasive jet machining process.

Fig 1. Equipment Set up for AJM Process

Description of working principle of AJM:

In the machining system shown in the above figure, a gas (nitrogen, CO2 or air) is supplies under a pressure of 2 to 8 KG/cm^2. Oxygen should never be used because it causes violent chemical reaction with work-piece chips or abrasives. After filtration and regulation, the gas is passed through a mixing chamber that contains abrasive particles and vibrates at 50 HZ. From the mixing chamber, the gas along with the entrained abrasive particles (10-40 micro meter), passes through a 0.45 mm diameter tungsten carbide nozzle at a speed of 150 to 300 m/s. Aluminium Oxide (Al2O3) and Silicon Carbide powders are used for heavy cleaning, cutting and deburring. 

Magnesium carbonate is recommended for use in light cleaning and etching while sodium bicarbonate is sued for fine cleaning and the cutting of soft materials. Commercial grade powders are not suitable because their sizes are not well classified. They may contain Silica dust, which can be a health hazard. It is not practice to reuse the abrasive powder because contaminations and worn grit will cause a decline of the machining rate. The abrasive powder feed rate is controlled by the amplitude of vibrations in the mixing chamber. The nozzle stand off distance is 0.81 mm. The relative motion between the work-piece and the nozzle is manually or automatically controlled by using computer control according to the cut geometry required. Masks of copper, glass, or rubber may be used to concentrate the jet stream of abrasive particles to a confined location on the workpiece. Intricate and precise shapes can be produced by using masks with corresponding contours. Dust removal equipment is incorporated to protect the environment.

Process Parameters:

Abrasives:

Type : Al2O3 or Sic (used once)

Size: Around 25 micro metre

Flow rate: 3-20 g/min

Medium:

Type: Air or CO2

Velocity: 150 - 300 m/s

Pressure: 2 - 8 Kg/cm^2

Flow rate: 28 L/min

Nozzle:

Material: Tungsten Carbide or Sapphire

Shape: Circular with 0.3 - 0.5 mm diameter, Rectangular (0.08 x 0.51 mm - 6.61 x 0.51 mm)

Tip distance: 0.25 - 0.15 mm

Life: WC (12 - 30 hours), Sapphire (300 hours)

Operating angle: Vertical to 60 deg off vertical

Area: 0.05 - 0.2 mm^2

Tolerance: + or - 0.05 mm

Surface roughness: 0.15 - 0.2 micro metre (10 micro metre particles), 0.4 - 0.8 micro metre (25 micro metre particles), 1.0 - 1.5 micro metre (20 micro metre particles)

Material Removal Rate:

As shown in figure, the abrasive particles from the nozzle follow parallel paths for a short distances and then the abrasive jet flaws outward like a narrow cone.

Fig 2. AJM Terminology

When the sharp edged abrasive particles of Al2O3 or SiC hit a brittle and fragile material at high speed, tiny brittle fracture are created from which small particles dislodge. The lodged out particles are carried away by the air or gas. The material removal rate (MRR) is given by the following equation shown in figure 3.

Fig 3. Material Removal Rate

 Where,

             K = Constant

             N = Number of abrasive particles impacting /unit area

             da = Mean diameter of abrasive particles in micro metre

             Pa (Read as rhro a) = Density of abrasive particles in Kg/mm^3

             Hw = Hardness number of the work-piece material

              v = Speed or velocity of the abrasive particles in m/s

The material removal rate, cut accuracy, surface roughness and nozzle wear are influenced by the size and distance of the nozzle. The MRR is mainly dependent on the flow rate and size of abrasives. Large grain sizes produce grater removal rate. The typical material removal rate is 16.4 mm^3/min when cutting glass. MRR for metals vary from 1.6 to 4.1 mm^3/min. For harder ceramics, cutting rates are about 50% higher than those for glass .

Applications:

The Abrasive Jet Machining Process are used for the following applications:

1. Drilling holes, cutting slots, cleaning hard surfaces, deburring, polishing
2. Deburring of cross holes, slots and threads in small precision parts that require a burr free finish such as hydraulic valves, aircraft fuel systems and medical appliances
3. Machining intricate shapes or holes in sensitive brittle thin or difficult to machine materials
4. Insulation stripping and wire cleaning without affecting the conductors
5. Micro deburring of hypodermic needles
6. Froasting glass and trimming of circuit boards, hybrid circuit resistors, capacitors, silicon and gallium.

Advantages (Merits):

1. Ability to cut fragile, brittle, hard and heat sensitive materials without damage
2. Ability to cut intricate hole shapes in the materials of any hardness and brittleness
3. No heat is generated in the work-piece 
4. Very low capital cost
5. No part chatter or vibrations
6. Good for difficult to reach areas
7. The energy transfer media (water) is cheap, non-toxic and easy to dispose off
8. The work remains clean and dust free
9. The system has no moving part and therefore its operating and maintenance costs are low and the process is very safe
10. Intricate contours can be cut
11. There is no thermal damage to the work surface as little heat is generated during cutting operation.
12. The process is very convenient for cutting soft and rubber like materials for which the teeth of the conventional saw get clogged

Disadvantages (Demerits):

1. Low material removal rate (MRR)
2. Due to stray, cutting accuracy is affected
3. Particles can be imbed in work-piece
4. Abrasive powder cannot be reused
5. Requires a separate dust collecting system.

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