ABRASIVE JET MACHINING (AJM)
Abrasive water jet cutting is an extended version of water jet cutting; in which the water jet contains abrasive particles such as silicon carbide or aluminium oxide in order to increase the material removal rate above that of water jet machining. Almost any type of material ranging from hard brittle materials such as ceramics, metals and glass to extremely soft materials such as foam and rubbers can be cut by abrasive water jet cutting. The narrow cutting stream and computer controlled movement enables this process to produce parts accurately and efficiently. This machining process is especially ideal for cutting materials that cannot be cut by laser or thermal cut. Metallic, nonmetallic and advanced composite materials of various thicknesses can be cut by this process. This process is particularly suitable for heat sensitive materials that cannot be machined by processes that produce heat while machining.
In Abrasive Jet Machining (AJM), abrasive particles are made to impinge on the work material at a high velocity. The jet of abrasive particles is carried by carrier gas or air. The high velocity stream of abrasive is generated by converting the pressure energy of the carrier gas or air to its kinetic energy and hence high velocity jet. The nozzle directs the abrasive jet in a controlled manner onto the work material, so that the distance between the nozzle and the work piece and the impingement angle can be set desirably. The high velocity abrasive particles remove the material by micro-cutting action as well as brittle fracture of the work material.
In AJM, air is compressed in an air compressor and compressed air at a pressure of around 5 bar is used as the carrier gas.
Figure also shows the other major parts of the AJM system. Gases like CO2, N2 can also be used as carrier gas which may directly be issued from a gas cylinder. Generally oxygen is not used as a carrier gas.
The carrier gas is first passed through a pressure regulator to obtain the desired working pressure. To remove any oil vapour or particulate contaminant the same is passed through a series of filters.
Then the carrier gas enters a closed chamber known as the mixing chamber. The abrasive particles enter the chamber from a hopper through a metallic sieve.
The sieve is constantly vibrated by an electromagnetic shaker. The mass flow rate of abrasive (15 gm/min) entering the chamber depends on the amplitude of vibration of the sieve and its frequency.
The abrasive particles are then carried by the carrier gas to the machining chamber via an electro-magnetic on-off valve.
The machining enclosure is essential to contain the abrasive and machined particles in a safe and eco-friendly manner.
The machining is carried out as high velocity (200 m/s) abrasive particles are issued from the nozzle onto a work piece traversing under the jet. Process Parameters and Machining Characteristics.
The process parameters are listed below
Material – Al2O3 / SiC / glass beads
Shape – irregular / spherical
Size – 10 ~ 50 μm
Mass flow rate – 2 ~ 20 gm/min
Composition – Air, CO2, N2
Density – Air ~ 1.3 kg/m3
Velocity – 500 ~ 700 m/s
Pressure – 2 ~ 10 bar
Flow rate – 5 ~ 30 lpm
Velocity – 100 ~ 300 m/s
Mixing ratio – mass flow ratio of abrasive to gas
Stand-off distance – 0.5 ~ 5 mm
Impingement Angle – 600 ~ 900
Material – WC / sapphire
Diameter – (Internal) 0.2 ~ 0.8 mm
Life – 10 ~ 300 hours
The important machining characteristics in AJM are
The material removal rate (MRR) mm3/min or gm/min
The machining accuracy
The life of the nozzle
|Effect of process parameters MRR|
Parameters of Abrasive Jet Maching (AJM) are factors that influence its Metal Removal Rate (MRR). In a machining process, Metal Removal Rate (MRR) is the volume of metal removed from a given work piece in unit time. The following are some of the important process parameters of abrasive jet machining:
1. Abrasive mass flow rate
2. Nozzle tip distance
3. Gas Pressure
4. Velocity of abrasive particles
5. Mixing ratio
6. Abrasive grain size
Abrasive mass flow rate
Mass flow rate of the abrasive particles is a major process parameter that influences the metal removal rate in abrasive jet machining.
In AJM, mass flow rate of the gas (or air) in abrasive jet is inversely proportional to the mass flow rate of the abrasive particles.
Due to this fact, when continuously increasing the abrasive mass flow rate, Metal
Removal Rate (MRR) first increases to an optimum value (because of increase in number of abrasive particles hitting the work piece) and then decreases.
However, if the mixing ratio is kept constant, Metal Removal Rate (MRR) uniformly increases with increase in abrasive mass flow rate.
Nozzle tip distance
Nozzle Tip Distance (NTD) is the gap provided between the nozzle tip and the work piece.
Up to a certain limit, Metal Removal Rate (MRR) increases with increase in nozzle tip distance. After that limit, MRR remains constant to some extent and then decreases.
In addition to metal removal rate, nozzle tip distance influences the shape and diameter of cut.
For optimal performance, a nozzle tip distance of 0.25 to 0.75 mm is provided.
Air or gas pressure has a direct impact on metal removal rate.
In abrasive jet machining, metal removal rate is directly proportional to air or gas pressure.
Velocity of abrasive particles
Whenever the velocity of abrasive particles is increased, the speed at which the abrasive particles hit the work piece is increased. Because of this reason, in abrasive jet machining, metal removal rate increases with increase in velocity of abrasive particles.
Mixing ratio is a ratio that determines the quality of the air-abrasive mixture in Abrasive Jet Machining (AJM).
It is the ratio between the mass flow rate of abrasive particles and the mass flow rate of air (or gas).
When mixing ratio is increased continuously, metal removal rate first increases to some extent and then decreases.
Abrasive grain size
Size of the abrasive particle determines the speed at which metal is removed.
If smooth and fine surface finish is to be obtained, abrasive particle with small grain size is used.
If metal has to be removed rapidly, abrasive particle with large grain size is used.
Applications of Abrasive Jet Machining (AJM)
Abrasive water jet cutting is highly used in aerospace, automotive and electronics industries.
In aerospace industries, parts such as titanium bodies for military aircrafts, engine
components (aluminium, titanium, heat resistant alloys), aluminium body parts and interior cabin parts are made using abrasive water jet cutting.
In automotive industries, parts like interior trim (head liners, trunk liners, door panels) and fibre glass body components and bumpers are made by this process.
Similarly, in electronics industries, circuit boards and cable stripping are made by abrasive water jet cutting.
|Steel gear and rack cut with an abrasive water jet|
Advantages of abrasive water jet cutting
In most of the cases, no secondary finishing required
No cutter induced distortion
Low cutting forces on workpieces
Limited tooling requirements
Little to no cutting burr
Typical finish 125-250 microns
Smaller kerf size reduces material wastages
No heat affected zone
Localises structural changes
No cutter induced metal contamination
Eliminates thermal distortion
No slag or cutting dross
Precise, multi plane cutting of contours, shapes, and bevels of any angle.
Limitations of abrasive water jet cutting
Cannot drill flat bottom
Cannot cut materials that degrades quickly with moisture
Surface finish degrades at higher cut speeds which are frequently used for rough cutting.
The major disadvantages of abrasive water jet cutting are high capital cost and high
Noise levels during operation.