Manufacturing Processes Multiple Choice Questions on “Abrasive Jet Micromachining – 3”.
1. In AJMM, material removal starts when the fracture strength is reached.
a) True
b) False
Answer: a
Clarification: During AJMM, plastically deformed surface layer is formed near the eroded surface, and therefore, the yield strength of the material increases. Upon further deformation, the yield strength at the surface of the material will eventually become equal to its fracture strength. The surface becomes brittle and its fragments may be removed by subsequent impacts.
2. Brittle fracture can occur due to indentation rupture.
a) True
b) False
Answer: a
Clarification: Brittle fracture may take place due to:
1) Indentation ruptures
2) Elastic–plastic deformation
3) Critical plastic strain theory
4) Radial cracking and propagation or surface energy criterion.
3. The mass loss of workpiece is proportional to_____
a) power supplied
b) nozzle diameter
c) amount of abrasive
d) carrier gas
Answer: c
Clarification: The mass loss of workpiece is proportional to the amount of abrasive.
Mass loss = (Kρmv2)/(2H), where, K (> 2) is a dimensionless factor, m and v amount and velocity of
particles, and ρ and H are density and hardness of the eroded material, respectively. The above relation is true for brittle erosion but not for softer materials (elastomers and some metals) due to time variant erosion behaviour.
4. In AJMM, there is initial weight gain in the workpiece.
a) True
b) False
Answer: a
Clarification: Especially at normal impact angle, particles tend to embed in the material, resulting in an initial gain in weight of the specimen. After this incubation time steady-state erosion is established and mass loss from the eroded material is proportional to the amount of abrasive particles.
5. Erosion rate is directly proportional to velocity of the jet.
a) True
b) False
Answer: a
Clarification: The principal empirical relation between erosion rate Erate, expressed as the quotient of mass loss and amount of abrasive, and particle velocity is given as a power function by
Erate α vk , where k= velocity coefficient.
6. For metal, velocity coefficient ranges from_____
a) 0.5—1
b) 1.8—3
c) 2.3—3
d) 2—4
Answer: c
Clarification: velocity coefficient k commonly reported for,
1) for metals – between 2.3 and 3,
2) for glasses – between 2 and 4 and
3) for elastomers – between 1.8 and 3.2.
7. AJMM can also machine poly methyl methacrylate (PMMA).
a) True
b) False
Answer: a
Clarification: AJMM can machine much steeper side walls and flatter bottom section in PMMA than those in glass. For PMMA, there is only a small probability of a particle rebounding from the steep side wall and hitting the opposite side.
8. There is appearance of the dimples during AJM.
a) True
b) False
Answer: a
Clarification: Appearance of the dimples during AJM for 10s. for various machining sets of abrasives and ceramic materials. Properties of the dimples do not differ in terms of the removed volume, but also the roughness of the struck face for different combination of abrasive particles and workpiece material.
9. In AJMM, the substrate has to be shielded by a mask.
a) True
b) False
Answer: a
Clarification: In AJMM, the substrate has to be shielded by a wear resistant mask that is patterned with the desired contour. The mask determines the accuracy of the dimensions in the plane of the desired structure.
10. During blasting, the workpiece is exposed to an abrasive air jet pressure of _____ MPa.
a) 0—1
b) 0—0.2
c) 0.2—0.8
d) >1
Answer: c
Clarification: During blasting, the workpiece is exposed to an abrasive air jet pressure of 0.2-0.8 MPa and abrasive particles average diameter of 10-100 μm. The scan strategy and the particle beam profile of the nozzle are of great importance.