Mumbai University > Mechanical Engineering > Sem 5 > production process 3

Marks: 10M

Year: Dec 2015

• Principle:

  • AJM uses a stream of fine grained abrasives (of size 10 to 40 microns) mixed with air or some other carriers gas at high pressure.
  • This stream is directed on the work surface by using a suitable nozzle.
  • The velocity of carries gas or air is up to 200 to 400 m/sec
  • Due to this high speed, impact on the work surface erosion takes places by abrasive particles and metal removes from the work piece.

• Construction:

  • Above figure shows typical setup for abrasive jet machining which consists of mixing chamber in which fine grained abrasive particles are filled through a holding device like a “Hopper”.
  • The mixing chamber vibrates (upto 50 cycles/sec) and amplitude of these vibrations controls the flow of abrasive particles. To control the amplitude of vibration, regulator is placed in the system.
  • Compressed air or high pressure gas is supplied to the mixing chamber through a pipe line which carries a pressure gauge to control its pressure.
  • These particles mix in the stream of gas, travel via hose and pass through a pipe line, which carries a pressure gauge to control its pressure.
  • These particles mix in the stream of gas, travel via hose and pass through a nozzle. This stream of mixture of gas and abrasive particles is called as Abrasive Jet.

• Working:

• Through hopper, fine grained abrasive powder is filled in a mixing chamber.

• The gas as air is supplies through the pipeline which carries a pressure gauge and regulator to control the flow.
• The gas or air is supplied under pressure into the chamber, the pressure of gas varies from 2 to 3 kg/cm2 .

• This mixture of high pressure gas and compressed air is passed through a nozzle on the surface of workpiece and due to high speed mixture erosion is caused and metal removal takes place.

• Parameters in AJM:

  The parameters which affect MRR and accuracy of the machining process are:

  • Carrier Gas:
    • A carrier gas used in the process must not flare excessively when discharged from the nozzle into the atmosphere.
    • A gas used should be non-toxic, cheap, easily available and capable of being dried easily.
    • Commonly used gases are air, nitrogen and carbon di-oxide.
  • Type of Abrasives:
    • The abrasive used in the process should have a sharp and irregular shape and should have excellent flow charactertics.
    • Commonly used abrasives are aluminum oxide and silicon carbide for general machining and grooving whereas sodium bi-carbonite for line finishing and dolomite for etching and light cleaning purpose.
  • Jet Velocity:
    • The kinetic velocity of the abrasive jet is utilized for metal removal by erosion.
    • The velocity is a function of nozzle pressure, nozzle design and abrasive grain size.
    • Higher nozzle pressure results in greater MRR. Also, higher grain size produces higher MRR. The inside diameter of the nozzle is about 0.04mm.
  • Stand-off Distance(SOD) or Nozzle Tip Distance (NTD):
    • It is the distance between the face of the nozzle and working surface of the workpiece to be machined.
    • Generally, it is kept about 0.7mm to 1.0mm.
    • The shape and size of cavity produced as well as the surface of the workpiece is affected by NTD.
    • If NTD increases, the velocity of abrasive particles striking on the workpiece also increases, hence MRR also increases.
    • Initially, MRR increases with NTD after which it remains unchanged for a certain NTD and then falls gradually. Fig. shows.
  • Advantages of AJM:
    • Brittle Materials of thin sections can be easily maintained.
    • No direct contact between the tool and workpiece, hence less damage to the workpiece surface.
    • Holes of any shape and intricate cavities can be machined.
    • Initial investment is low as compared to other methods.
    • Power consumption is low.
  • Disadvantages of AJM:
    • AJM is suitable only for brittle materials as MRR is high for brittle materials.
    • Machining accuracy obtained is poor. i.e. upto +50microns.
    • MRR is low i.e. up to 0.05cm2 /hr.
    • There is always a chance of abrasive particles getting inserted in the work materials hence cleaning needs to be done after machining.
    • The used abrasive powder cannot be resused.
  • Applications of AJM:
    • The process 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 substance deburring, polishing, etc.
    • It is used for producing high quality surface.
    • It is used for reproducing design on a glass surface with the help of masks made of rubber, copper, etc.