1. In ECM, one needs to understand fluid flow while designing the tool for machining complex shapes.
a) True
b) False
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.
a) True
b) False
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
d) Non-corrosiveness
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.
a) True
b) False
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.
a) True
b) False
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
b) roughness
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.
a) True
b) False
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.