1. Which of the following does not hold true about ECM?
a) Lower current density leads to poor surface finish
b) Small gap between tool and the workpiece can cause short circuit
c) Gap between the tool and the workpiece doesn’t affect the process parameters
d) MRR is dependent on feed rate and electrolyte composition
Explanation: Current density is not only controlled by the amount of current that the power supply is delivering, but also by the size of the gap. A small gap results in the highest current density. However, when it’s very small, there is a danger of sludge particles bridging the gap and causing a short circuit. When the gap is too large, current density is reduced, resulting in a poor surface finish and decreased MRR.
2. Pick the incorrect one from the following options.
a) Voltage across the cutting gap influences the current and the MRR
b) Higher voltage decreases the equilibrium machining gap
c) Increased current leads to electrolyte heating
d) ECM can be used for facing and turning complex 3D surfaces
Explanation: Low voltage decreases the equilibrium machining gap and results in a better surface finish and finer tolerance control. Voltage across the cutting gap influences the current and the MRR and is controlled in most ECM operations. Increased current leads to electrolyte heating – low temperature of the electrolyte is conducive for a better surface finish and tolerances
3. Which of the following largely affects the MRR?
a) ECM cell size
b) Tool shape
c) Feed rate
d) Complexity of the product
Explanation: Feed rate determines the current passed between the tool and the work. High feed rate results in higher MRR. High feed rates also decrease the equilibrium machining gap resulting in improvements of the surface finish and tolerance control. At slower feed rates, the MRR decreases as the gap increases resulting in the rise of resistance and drop in the current.
4. Conductivity of the solution can be increased by_____
a) increasing the gap between the tool and the workpiece
b) increasing the temperature of the solution
c) increasing the concentration of the solution
d) using conductive workpiece
Explanation: Electrolyte conductivity can be increased in following ways-
• Increasing the concentration will cause conductivity to rise
• Temperature increases of the electrolyte also increases conductivity
Low concentration and low temperature will results in lower MRR. Electrolyte conductivity affects the resistance across the gap, but the gap doesn’t affect the conductivity of the electrolyte
5. The velocity and the electrolyte flow through the gap is also an important parameter affecting the surface finish and MRR.
a) True
b) False
Explanation: If the velocity is too low, the heat and by-products of the reaction build in the gap causing non-uniform material removal. A velocity that is too high will cause cavitation, also uneven material removal
6. There is a huge change in the mechanical properties of the material after ECM.
a) True
b) False
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.
7. _____ strength of stainless steel decreases when machined by ECM.
a) Yield
b) Tensile
c) Fatigue
d) Notched-tensile
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.
8. 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.
9. Sharp change in tool geometry or sharply divergent flow path affects the surface finish of the product.
a) True
b) False
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.
10.Striation, ripples on work surface occur because of ____
a) differential machining of material phases
b) incorrect tool alignment
c) cavitation
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.