This set of Manufacturing Processes Questions and Answers for Experienced people focuses on “Electro-Chemical Machining – 5”.
1. There is a huge change in the mechanical properties of the material after ECM.
Explanation: There is no appreciable change in the mechanical properties, such as tensile strength, yield strength, hardness, ductility, etc. of the material due to ECM. Values of notch-tensile strength, notched-sensitivity, and the sustained-load characteristics of the ECM parts are comparable with those of conventionally machined parts.
2. _____ strength of stainless steel decreases when machined by ECM.
Explanation: Fatigue strength of stainless steel is found to decrease by ECM. This can, however, be overcome by cold working the surface of the product after ECM.
3. In ECM, electrode or tooling cost is fixed because_____
a) there is little wear of the tool
b) tool can be used only once
c) tool can be regenerated
d) one tool can be used for any product
Explanation: Electrode or tooling cost is fixed because there is little wear of the ECM tool. There occurs, however, a negligible abrasion wear of electrode due to electrolyte flow across the gap.
4. Sharp change in tool geometry or sharply divergent flow path affects the surface finish of the product.
Explanation: Sharp change in tool geometry or divergent flow path causes cavitation which further leads to small irregular raised area, often with bright work area. This can be avoided by reducing electrolyte supply pressure, blending out sharp radii on tool, or by increasing tool feed.
5. Striation, ripples on work surface occur because of ____
a) differential machining of material phases
b) incorrect tool alignment
d) electrical field concentration of machining current
Explanation: Differential machining of material phases causes striation and ripples on the work surface which leads to poor surface finish. It can be minimised by increasing electrolyte supply pressure, reducing tool feed rate, or by reducing the voltage to maintain the same gap.
6. Foreign particle in electrolyte or insulation failure within tool can cause_____
a) random inaccuracy in work
b) ripples on work
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.
7. 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
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.
8. Discrepancies are sometimes observed between theoretical and actual metal removal rates and electrode feed rates.
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.
9. There is a limit to the minimum cross-section of the current carrying parts.
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.
10. 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.
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