1. Foreign particle in electrolyte or insulation failure within tool can cause_____
a) random inaccuracy in work
b) ripples on work
c) cavitation
d) spark damage to work
Explanation: Foreign particle in electrolyte or insulation failure within tool causes spark damage to the tool or work at the point near to electrolyte entry into machining gap. To avoid this, check internal filters, electrolyte ducts, seals and joints. Also check insulation of the tool frequently.
2. Composition of a Nickel superalloy is as follows: Ni = 70.0%, Cr = 20.0%, Fe = 5.0% and rest Titanium. What will be the rate of dissolution if the area of the tool is 1500 mm2 and a current of 1000 A is being passed through the cell? Assume dissolution to take place at lowest valency of the elements.
ANi = 58.71 ρNi = 8.9 νNi = 2
ACr = 51.99 ρCr = 7.19 νCr = 2
AFe = 55.85 ρFe = 7.86 νFe = 2
ATi = 47.9 ρTi = 4.51 νTi = 3
a) 2.14 cc/min
b) 3.14 cc/min
c) 4.25 cc/min
d) 1.66 cc/min
Explanation: Now,
ρalloy = 1/ (Σαi/ ρi), where αi = % of the respective element in the alloy
= 8.07 gm/cc ρi = density of respective element
Now, MRR = I/ [Fρ*(Σαiνi)] where F= 96500 coulomb
= 0.0356 cc/sec
= 2.14 cc/min.
3. Discrepancies are sometimes observed between theoretical and actual metal removal rates and electrode feed rates.
a) True
b) False
Explanation: In practise, metal removal rates are often higher than the estimated ones because;
(1) The exact valency at which a metal behaves in the reaction is generally unknown.
(2) ECM continuously exposes a new and clean surface to the electrolyte which is easily attacked chemically. It also varies with electrolyte used and metal being machined.
4. There is a limit to the minimum cross-section of the current carrying parts.
a) True
b) False
Explanation: To prevent over-heating, there is a limit to the minimum cross-section of the current carrying parts. For 1000 A, it is about 6 cm2 for copper, 25 cm2 bronze and brass, and 250 cm2 for stainless steel.
5. Which of the following is suitable if work surface finish is important?
a) High machining voltage
b) High concentration
c) Larger gap
d) High current
Explanation: If work surface finish is important, high machining voltage, dilute electrolyte and a small gap between the workpiece and the tool should be employed instead of low voltage and concentrated electrolyte.
6. Etching refers to the removal of material from ___________
a) the soft surface
b) the hard surface
c) the sticky surface
d) the wafer surface
Explanation: Etching refers to the removal of material from the wafer surface. The process is usually combined with lithography in order to select specific areas on the wafer from which material is to be removed
7. Deposition is a complimentary process to etching.
a) True
b) False
Explanation: The complementary process to etching is deposition (or growth), where new material is added. Unlike oxidation (or nitridation), where the underlying Si is consumed to form the oxide (nitride) layer, in deposition, new material is added without consuming the underlying wafer
8. In wet etching material is removed by ___________
a) absorption
b) sublimation
c) chemical reaction
d) the force exerted due to flow of solvent
Explanation: A controlled portion of the wafer surface is exposed to the etchant (mix of chemicals) which then removes material by chemical reaction.
9. ________________ is used to protect the remaining area of the wafer while machining.
a) Tin foil
b) Wood
c) Photoresist layer
d) Sodium bicarbonate
Explanation: There is a chemical reaction between the wafer surface and the etchants that helps in material removal. Either a photoresist layer or a hard mask like oxide or nitride layer is used to protect the rest of the wafer.
10. The time for etching is independent of material to be removed.
a) True
b) False
Explanation: The time for etching depends on the amount and type of material that needs to be removed. KOH (potassium hydroxide) is a common etchant used to remove Si.