Electro discharge machining ( EDM ) | Interview question and Answers
Electro Discharge Machining ( EDM ) | Interview question and Answers
1. What is the principle of EDM?
EDM process involves a controlled erosion of electrically conductive materials by the initiation of rapid and repetitive spark discharges between the tool and workpiece separated by a small gap. The controlled pulsing of the direct current between the tool and the workpiece produces the spark discharge. Heat transfer from the spark to both the tool and workpiece makes the latter to melt and partially vaporise and partially ionise the metal in a thin surface layer.
2. What causes the material to be removed from the workpiece surface? (Or) How does the dielectric assist in removing the material from the workpiece?
Due to the inertia of the surrounding fluid, the pressure within the spark becomes quite large and may possibly assist in “blasting” the molten material from the surface leaving a fairly flat and shallow crater.
3. What are the principle components of EDM process?
Power supply, Dielectric system, electrodes: workpiece and tool and servo system (tool feed)
4. What is the use of dielectric in EDM process?
- spark conductor that must ionize under an applied voltage
- deionize the spark gap after the discharge has occurred
- effective cooling medium (coolant)
- carry the swarf particles away from the working gap (flushing medium)
5. What the dielectric fluids commonly used in EDM?
- Petrolium based hydrocarbon fluids.
- Parafin, white sprite, transformer oil.
- Kerosine, mineral oil.
- Ethylene glycol and water miscible compounds.
6. List the desirable characteristics of a dielectric.
The following are the desirable characteristics of a dielectric:
- It should have sufficient and stable dielectric strength to serve as an insulation between the tool and work till the breakdown voltage is reached.
- It should deionise rapidly after the spark discharge has taken place.
- It should have low viscosity and good wetting capacity to provide an effective cooling mechanism and remove the swarf particles from the machining gap.
- It should be chemically neutral so as not to attack the electrode, workpiece, table or the tank.
- It should not emit any toxic vapours or have any unpleasant odour.
- It should have high flash point so as to avoid any fire hazards.
- It should maintain its properties over a wide range of temperature, contamination by debris and products of decomposition.
- It should be economical and easily available.
7. What is tool wear in the EDM process?
Partial removal of the tool material form the tool surface while machining the workpiece due to the discharge spark produced between the tool and the workpiece.
8. How does tool wear occur in the EDM?
Due to the sparking action, the intense heat generated near the zone melts and evaporates the materials in the sparking zone. Since the tool is also within this zone, it also gets eroded.
9. How to minimise tool wear in EDM?
Tool wear can be minimized by using a tool material that has a high melting point and high thermal conductivity. Also by properly configuring the tool design, tool wear can be minimized.
10. Please identify the characteristics of an electrode material in order to serve as a good tool.
- It should be a good conductor of electricity and heat
- It should be easily machinable to any shape at a reasonable cost
- It should produce efficient material removal rates from the workpieces
- It should resist the deformation during the erosion process
- It should exhibit low electrode (tool) wear rates
- It should be easily available in a variety of shapes
11. Name some of the tool material used in EDM?
- Copper, brass, alloys of Zinc &tin.
- Hardend plain carbon steel
- Copper tungsten, silver tungsten, tungsten
- Copper graphite and graphite.
12. Why is graphite the most preferred electrode material in EDM?
Graphite has very high melting point (3727C). It does not melt rather vaporizes. At the same time, the tool wear is minimum.
13. How does the melting temperature of work material affect the MRR in EDM?
Work materials having higher melting temperature have lower MRR’s.
14. What is the effect of capacitance in EDM?
Increasing the capacitance causes the discharge to increase and increase both the peak current and discharge time.
15. Define wear ratio
It is the ratio of volume of work removed to the volume of tool removed.
16. Define over cut
It is the distance by which the machined hole in the work piece exceeds the electrode size and is determined by both the initiating voltage and the discharge energy.
17. What is recast layer?
If molten material from the workpiece is not flushed out quickly, it will resolidify and harden (as a martensite) due to cooling effect of the dieletric, and gets adhered to the machined surface. This thin layer (about 2.5 to 50m) is known as re-cast layer.
18. Name the best electrode material for finish operations on a small die made of WC by the EDM process.
Best electrode material for finish machining a small die made of WC – steel. Steel is used because it can match the parting planes of the moulds in which half the mould is used as the electrode and the other half is used as the workpiece.
19. Explain why tool shape in the EDM process should be complementary to the final form?
Since the arc is always struck at a point between the workpiece that is closest from the tool (electrode), the complementary tool surface will be reproduced in the workpiece.
20. What are the factors upon which the material removal rate and surface roughness depends?
- Increase in current or spark voltage increases MRR and produces higher surface roughness
- Increase in spark frequency results in improved surface finish but the MRR can be maintained
- Low inter electrode gap (IEG) results in lower MRR, higher surface finish and better accuracy
21. What are the design factors to be considered while selecting the machine tool?
- Number of parts to be produced.
- Size of work piece.
- Size of electrode.
- Depth of cavity.
22. Why is a servo-controlled system required in EDM?
EDM requires that a constant arc gap be maintained between the electrode and the work piece to obtain maximum machining efficiency. Therefore EDM tool incorporates some form of servo control.
23. What are the various applications of EDM?
- Mould –making industry –manufacture moulds for plastics
- Tool and die industry- manufacture extruding, heading, drawing, forging and die lasting dies.
- Aerospace and electronic industry – to make prototypes and production parts- made of difficult to machine material
- Small or odd – shaped holes
- Miniature parts and parts made from thin or fragile materials (delicate)
24. What is meant by Wire cut EDM?
The principle and working of an WEDM is similar to the EDM but instead of using a solid electrode, a wire (about 0.05-0.30mm diameter) is used as an electrode in EDM.
25. What are salient features of WEDM?
WEDM uses a small diameter wire as tool electrode, which produces a very narrow kerf in the work. Because of this reason, very sharp corners can be easily produced.
26. Please identify various applications of Wire cut EDM.
- Wire cut EDM is used for the manufacture of:
- Press tools
27. What are the different types of the electrical circuits that are used for electro spark machining?
- R.C. Circuits
- Rotary impulse generator
- Pulse controlled circuits
- Hybrid circuits
28. What is an arc gap?
Optimum distance between the tool and the workpiece so that spark can run and cause useful erosion.
29. How is the arc gap controlled in EDM?
Servo mechanism is used to control the arc gap –average voltage is measured and as long as it is within a certain limit, no correction signal is fed to the feed drive.
30. What are the limitations of EDM process?
- Low material removal rate
- Only electrically conductive materials can be machined
- Tool electrode wear is very high
- More time in the fabrication of the electrodes.
- High cost involved.