This set of Manufacturing Processes Interview Questions and Answers for freshers focuses on “Electro-Chemical Machining – 3”.
1. In ECM, one needs to understand fluid flow while designing the tool for machining complex shapes.
Explanation: Because of the interaction of working-tip shape and dimensions, location of insulation, current density and feed rate, the design of tools for machining complex shapes requires understanding of fluid flow, electrical and electrochemical principles as well as experience and ingenuity.
2. Which of the following does not hold true about ECM?
a) ECM cell must avoid flutter and arcing
b) Part and the cathode must have adequate current-carrying capacity
c) Tool must have shape exactly same as that of the mirror image of the part
d) It gives burr free surface
Explanation: Tool dimensions must be slightly different from the nominal mirror dimensions of the completed part to allow for ECM overcut. ECM cell must have strength and rigidity to avoid flutter and arcing.
3. For which of the following ECM cannot be used?
a) Tapering a predrilled hole in iron block
b) To make stepped hole is a nickel alloy
c) To generate ribs on metal surface
d) To machine Al2O3
Explanation: Open-flow cathode (tool) is used to generate the outside diameter wall and leave an embossment. Stepped tool is used for sinking a stepped through-hole. Cone shaped tool is to generate taper in a predrilled hole. Al2O3 cannot be machined by ECM as it is ceramic.
4. Electrolyte should carry away the heat generated and products of the reaction.
Explanation: Electrolyte used in ECM should-
• Complete the electrical circuit between the tool and the workpiece
• Allow the desirable machining reactions to take place
• Carry away the heat generated during the operation
• Carry away products of reaction from the zone of machining.
5. Which of the following is not a desirable electrolyte property?
a) Low viscosity
b) High specific heat
c) Lower resistance to film formation on the workpiece
Explanation: Desirable electrolyte properties are as follows-
• High electrical conductivity – ease in ionisation
• Low viscosity – for easy flow
• High specific heat – to carry more heat
• Chemical stability – to be chemically neutral or does not disintegrate during the reaction
• High resistance to formation of passivating film on the workpiece surface
• Non corrosiveness and non-toxicity
• Inexpensiveness and easy availability.
6. Electrolyte flow plays an important role in ECM.
Explanation: Need for sufficient electrolyte flow between the tool and the workpiece-
• To carry away the heat and the products of machining
• To assist the machining process at the required feed rate, producing a satisfactory surface finish, cavitation, stagnation and vortex formation should be avoided since these lead to bad surface finish
• There should be no sharp corners in the flow path. All corners in the flow path should have a radius.
7. Which of the following is a sludging type electrolyte?
a) NaOH solution
b) KOH solution
c) NaCl solution
d) CuSO4 solution
Explanation: There are two types of electrolytes-
(1) Sludging type: These are solutions of salts which forms precipitate (sludge) after reaction with metals. For example: solutions of typical salts like NaCl.
(2) Non-sludging type: These are solutions of salts which do not form heavy precipitate or form soluble compounds after reacting with metals. For example: strong alkali solutions.
CuSO4 does not form any solution.
8. Tool with an electrolyte supply slot leaves small ridge on the work.
Explanation: Tool with an electrolyte supply slot is simple to manufacture, but such a slot leaves small ridges on the work. However, the ridges can be made very small by making the slot sufficiently narrow. The flow from a slot takes place in a direction perpendicular to the slot and the flow at the end is poor – therefore the slot is terminated near the corners of the workpiece surface.
9. The insulation in ECM should have_____
a) adhesion to the tool
c) high water absorption tendency
d) chemically active
Explanation: Desirable qualities of insulation are as follows-
• Adhesion to the tool: preformed insulation can be held to the tool by shrink fitting, adhesives or fasteners
• Sealing without pores or leaks that could cause stray machining by current leakage
• Adequate thickness
• Smoothness to avoid disturbing the flow of electrolyte
• Resistance to heat for continuous service at 200°C without breakdown
• Durability to resist wear in guides and fixtures
• Chemical resistance to the electrolyte
• High electrical resistivity
• Uniform application to minimize disturbance of the flow of electrolyte and to prevent interference
• Low water absorption.
10. Spraying or dipping method is used for applying insulation.
Explanation: Generally the simplest method of applying insulation is by spraying or dipping. Teflon, urethane, phenolic, epoxy, powder coating and other materials are commonly used for insulation. Sprayed or dipped coatings of epoxy resins are among the most effective insulating materials. For optimum effectiveness, these coatings should be used with a protective lip on the tool to protect the edge of the insulation from the flow force of the electrolyte.
11. 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.
12. 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.
13. 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.
14. 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.
15. The velocity and the electrolyte flow through the gap is also an important parameter affecting the surface finish and MRR.
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.
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