This set of Manufacturing Processes Multiple Choice Questions & Answers (MCQs) focuses on “Abrasive Jet Micromachining – 10”.
1. AJM nozzles are made from which of the following materials?
a) Low carbon steel
d) Stainless steel
Explanation: Generally, the nozzle is made of;
• Material – WC / sapphire
• Diameter – (Internal) 0.2 ~ 0.8 mm
• Life – 10 ~ 300 hours.
2. Material removal in AJM of glass is around_____
a) 0.1 mm3/min
b) 15 mm3/min
c) 15 mm3/s
d) 1500 mm3/min
Explanation: Following are the limitations of AJMM;
• MRR is rather low (around ~ 15 mm3/min for machining glass)
• Abrasive particles tend to get embedded particularly if the work material is ductile
• Tapering occurs due to flaring of the jet
• Environmental load is rather high.
3. Material removal takes place in AJM due to _____
a) electrochemical action
b) mechanical impact
c) fatigue failure of the material
d) sparking on impact
Explanation: In Abrasive Jet Machining (AJM), abrasive particles are made to impinge on the work material at a high velocity. The high velocity abrasive particles remove the material by micro-cutting action as well as brittle fracture of the work material.
4. As the stand-off distance increases, the depth of penetration in AJM _____
c) does not change
d) initially increases and then remains steady
Explanation: The depth of penetration varies inversely with the stand-off distance. In AJM, generally, the abrasive particles of around 50 μm grit size would impinge on the work material at a velocity of 200 m/s from a nozzle of I.D. of 0.5 mm with a stand-off distance of around 2 mm.
5. If air is used as a carrier gas, the pressure of the air is maintained at around_____
a) 1 bar
b) 3 bar
c) 5 bar
d) 3.5 bar
Explanation: 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. 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.
6. The mass flow rate of abrasive entering the chamber is independent of the amplitude of vibration of the sieve (in the mixing chamber).
Explanation: 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.
7. In AJMM, impingement angle is kept approximately in range of_____
a) 100 ~ 200
b) 300 ~ 450
c) 600 ~ 900
d) 500 ~ 600
Explanation: Generally, abrasive jet has,
• Velocity : 100 ~ 300 m/s
• Mixing ratio : mass flow ratio of abrasive to gas = mass of abrasive/ mass of carrier gas
• Stand-off distance : 0.5 ~ 5 mm
• Impingement Angle : 600 ~ 900.
8. Which of the following is used as abrasive in AJMM?
Explanation: Abrasive used in AJMM is:
• Material – Al2O3 / SiC / glass beads
• Shape – irregular / spherical
• Size – 10 ~ 50 μm
• Mass flow rate – 2 ~ 20 gm/min.
9. Which of the following assumption is true about AJMM?
a) Abrasives are irregular in shape.
b) The abrasive particles are characterised by the mean grit diameter
c) Brittle materials are considered to fail due to fatigue
d) The kinetic energy of the abrasives is partially lost in overcoming the friction in the nozzle
Explanation: Modelling in AJMM is done with the following assumptions:
(i) Abrasives are spherical in shape and rigid. The particles are characterised by the mean grit diameter
(ii) The kinetic energy of the abrasives are fully utilized in removing material
(iii) Brittle materials are considered to fail due to brittle fracture and the fracture volume is considered to be hemispherical with diameter equal to chordal length of the indentation
(iv) For ductile material, removal volume is assumed to be equal to the indentation volume due to particulate impact.
10. By AJMM, holes of intricate shapes can be drilled easily.
Explanation: Following are the advantages of AJMM process;
• For drilling holes of intricate shapes in hard and brittle materials
• For machining fragile, brittle and heat sensitive materials
• AJM can be used for drilling, cutting, de-burring, cleaning and etching.
• Micro-machining of brittle materials.
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