Abrasive Jet Machining

Abrasive jet machining is a process that uses abrasive particles that are carried at high air pressure or gas and is used to drive impinged on the work piece surface through a nozzle. The work material is removed by erosion process with the help of high velocity abrasive particles.

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Process:

In the process of abrasive machining the abrasive particles are impinged on to the work material at high velocity jet of carrier gas abrasive particles. The velocity streams carried by abrasives are generated by the conversion of carrier gas pressure energy into the kinetic energy that in-turn produces high velocity jet. The nozzle controls the direction of abrasive jets on the work piece surface. These high velocity particles of abrasives remove the material by a technique of micro cutting and brittle fracture on the work piece.

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  • This is a process where the material removal takes place by the impact high velocity concentrated stream of abrasives through erosion. This process is different from sand blasting and shot as the finer abrasives in AJM provide effective control over product quality. The abrasives grit size is around 50 microns that impinges at a velocity of 200 m/s from a nozzle that has internal diameter of 0.5 mm, which is located at a standoff distance of 2 mm. The particles are focused towards machining of the work surface.

Components of AJM process:

The figure shows the overall layout of the AJM process.  The stream of gas is passed into the hose where the gas stream is passed. The velocity of the abrasive stream that is ejected from the nozzle is upto 330m/sec.

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The major systems in the abrasive jet machining process are:

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  • Gas propulsion system:

This system provides clean and dry air, nitrogen and oxygen to boost up the abrasives. This gas is supplied through by using a compressor or else a cylinder. In case of compressor the air filter or drier is used to evade the water or oil contamination. The gas that is used in this system is cheap, easily available and also non-toxic. It necessarily need not be spread excessive when it is discharged from the nozzle. The nozzle pressure is maintained at 5bar; flow rate varies in between 2-4 gm/min.

  • Abrasive feeder:

Abrasive feeder supplies required quantity of abrasive a particles that are required for machining. From the propellant a sieve feds it into the mixing chamber. The sieve is seen to be vibrating at 50-60 Hz frequency. It is controlled by the vibration amplitude of sieve. The abrasive air mixture will further get processed to the nozzle. The nozzle imparts high velocity to the mixture and directs it towards the work piece.

  • Machining chamber: 

It is a chamber enclosing the working of the abrasive mixture over the work piece in order to get rid of harmful effects during working process.  Vacuum Dust collector is equipped in the machining chamber. The dust collection should be given a special consideration because the toxic materials like beryllium are used in the machining process.

  • AJM Nozzle:

The nozzle is generally made of Tungsten Carbide material and the other choice is sapphire. These have higher wear resistance. The nozzle cross section is either circular or rectangular in shape. These will be straight plus on at right angles to avoid the pressure loss that occurs due to bends or friction. The cutting accuracy is increased as the divergence increases at the nozzle wear. Types of nozzle are:

  • Round Shaped Slot:
  • Right angled nozzle
  • Straight edge nozzle
  • Rectangular shaped slots
  • Rectangular nozzle
  • Straight edged nozzle
  • Abrasives:

Aluminum oxide  (Al_2 O_3),Silicon carbide (SiC), Glass beads, sodium bicarbonate, crushed glass is the most common types of abrasives use in the process of Abrasive Jet Machining. Metal Removal Rate, work material type and the accuracy of machining depends on the abrasives selected for the process.

Aluminum oxide          –           12, 20, 50 microns       –  cleaning, cutting, deburrying

Silicon carbide              –           25, 40 microns             –  hard material processes

Glass Beads                  –           0.635 to 1.27 mm         –  surface finish

Dolomite                      –           200 mesh                      –  polishing, etching

Sodium Bicarbonate     –           27 microns                    – cleaning, soft metal cutting                                                                  Deburrying, soft material                                                                                                 Finishing < 50^o c

Process parameters and capability:       

Following parameters are to be considered in order to analyze successful operation the AJM process:

  • Material Removal Rate (MRR)
  • Geometry and Surface Finish of the work piece
  • Nozzle wear rate.

The capabilities of the process:

  • MRR              –           015 cubic centimeters / minute
  • Narrow Slots –           12 to 0.25 mm  0.12 mm
  • Surface Finish – 25 microns to 1.25 microns
  • Sharp Radius –           2 mm
  • Cut possibility – 5 mm (steel), 6.3 mm (Glass)
  • Thin sections of hard and brittle materials are possible.

Abrasive jet machining

Machining characteristics

The abrasive jet machining consists of following process criteria they are

  • Metal removal rate
  • Geometry of work piece
  • Surface finish of the work piece
  • Wear rate of the nozzle

Process criteria are influenced by the process parameters:

Effect of grain size and abrasive flow rate on Metal removal rate:

With increase in the abrasive flow rate, there is an increase in the metal removal rate and sizes of the abrasive particles are influenced on the MRR. After the reach of optimal level the abrasive flow rate increases, with decrease in metal removal rate. The gas flow rate decreases with increase in the abrasive flow rate. Increasing the mixing ratio causes decrease in the metal removal rate because the energy might decrease for erosion.

Effect of abrasive particle density and exit gas velocity on metal removal rate:

If the density of the abrasive particles increases, at that condition we observe there is a decrease in the exit velocity at the same pressures. Kinetic energy of gas is used for the abrasive particle transportation.

Effect of mixing ratio on metal removal rate:

Mass ratio is defined as, the ratio of the mass flow rate of the carrier gas to the mass flow rate of the carrier gas and the abrasives. If there is increase in the abrasive mass flow rate then there is decrease in the fluid velocity and also available energy. By standardizing the mixing ratio and increasing the abrasive flow rate we observe an improvement in the material removal rate.

\alpha= mass flow

m_a= mass flow rate of the carrier gas

m_{a+g}= mass flow rate of the carrier gas and the abrasive

Mass flow: \alpha=\frac{m_a}{m_{a+g}}

Effect of nozzle pressure on metal removal rate:

By increasing the carrier gas flow rate the abrasive flow rate is be increased; by increasing the internal gas pressure the cycle completion is possible. If the pressure of the internal gases increases then abrasive flow rate increases, the metal removal rate is also increases. If the pressure of the gas increases then there is an increase in the metal removal rate. The kinetic energy present in the abrasive particles is responsible for the metal removal by erosion process. The abrasives move with a minimum velocity onto the work piece material for machining process.

Standoff distance

The distance between the nozzle tip and the surface of the work piece is known as the standoff distance. If the distance between the nozzle tip and the surface of the work piece increases then the flow of the material is also increases. So it results in the poor accuracy and surface finish. For cleaning and frosting operations the standoff distance is 12.5 mm to 75 mm.

Metal removal models in Abrasive jet machining

In the metal removal models there are following assumptions

  • Abrasives are circular in shape and inelastic.
  • To cut the material kinetic energy is used
  • In the hemispherical due to the fracture of volume and brittle fracture the brittle materials are consider to fail
  • Due to the particulate impression the metal removal is supposed to be equal for the ductile material volume.

For brittle materials:

MRR=\left [ \frac{M_a}{{(\rho_g)^{\frac{1} {4}}(\sigma_w)^{\frac{3} {4}}}}\right ]

For ductile materials:

MRR=\left [ \frac{M_a V^2}{2\sigma_w}\right ]

Operations performed by AJM process:

  • Drilling
  • Milling
  • Boring
  • Trimming
  • Surface finishing
  • Etching
  • Cutting
  • Deburring
  • Cleaning

Advantages:

  • High surface finish can be easily obtained in accordance to the grain size.
  • Damage depth is very low.
  • The process is heat sensitive, delicate and it can machine cool cutting action.
  • The process is free from vibrations and chatters as there is no direct contact between the tool and work piece.
  • The initial capital cost is very low and is easy to operate and maintain the AJM.
  • Mica, silicon, glass and ceramics type thin sections of the hard and brittle materials can be machined easily.
  • Intricate shape hole cutting for hard materials is easy.

Limitations:

  • The MRR is limited upto 40gm/min.
  • Abrasives are embedded in the work spaces that have soft surfaces as like elastomers or soft plastics.
  • The tapering of the hole is hampered in its accuracy; this is due to the flares in abrasive jets.
  • The stray cut is difficult to avoid.
  • Dust collection system is a basic need to prevent the atmospheric pollution and health hazards.
  • The life of the nozzle is very limited upto 300hours approximately.
  • The abrasive powder is restricted be reused because the sharp edges may worn out there is a chance to clog small particles in the nozzle.
  • The short standoff distance may damage the nozzle when they are used for cutting.

Applications:

  1. This process is used for the glass abrading and frosting very economically when compared to the other processes as etching and grinding.
  2. Ceramics, oxides on metals etc. are cleaned by this process.
  3. AJM process is used to manufacture the electronic devices, glass wafers drilling, burring of plastics, and permanent marking on nylon or Teflon parts, rubber stencils and titanium foils.
  4. The small castings, registration number engraving on toughened glass (car windows) etc. are done by AJM process.
  5. Germanium, silicon type fragile materials are cut with these processes.
  6. Micro module fabrication process for electrical contact process and streaming registration, processing of semi-conductors can be done easily and very effectively.
  7. Used in hard and brittle material drilling, cutting, de-burrying operations.
  8. Glass, quartz, sapphire, mica, ceramics, silicon and gallium are the most common types of heat sensitive materials that are machined with AJM process.
  9. Hypodermic needle holes can be de-burred easily and milled slots in hard metallic components can be done more easily.

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