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Electrical-Discharge Machining Flashcards

1
Q

Electrical-Discharge Machining - Traditional Machining Processes

A
  • Turning, drilling, milling, grinding etc.
  • Material removal mechanism is usually through chip, formation, micro-chipping abrasion.
  • Forces (often high) exercised by a tool which is in contact with the work-piece removing material or separating it
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2
Q

Limitations of Traditional Methods

A

-The results of material removal achieved by mechanical means may sometimes not be satisfactory, economical or feasible

  • Reasons:
  • Material properties (too hard, too brittle, too flexible)
  • Component shape too complex
  • Surface finish or dimensional accuracy requirements.
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3
Q

Non-Traditional Machining Processes

A
  • Principle:
  • Means of material removal are chemical dissolution, etching, melting & evaporation etc; not produced by chips from using sharp cutting tools.

*Energy source is chemical, electrical, thermal & mechanical.

Advantages:

  • Negligible tool wear; no contact between tooling & workpiece.
  • Hardness of the workpiece is irrelevant.
  • Usually negligible residual stress through machining.
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4
Q

Non-traditional Machining Processes:

Mechanical

A
  • Ultrasonic Machining

- Abrasive Jet Machining

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5
Q

Electro-chemical

A
  • Electro-chemical Machining

- Electro-chemical Grinding

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6
Q

Thermal

A
  • Electro-Discharge Machining
  • Electron Beam Machining
  • Laser Beam Machining
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7
Q

Chemical

A
  • Engraving
  • Photochemical Machining
  • Photochemical Blanking
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8
Q

Electrical-Discharge Machining - EDM [Principle]

A

-Based on the erosion of metal by spark discharge.

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9
Q

Principle

A

2 main variants:

  • Conventional (sinker EDM, RAM EDM)
  • Wire EDM
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10
Q

EDM - Method

A
  • Shaped tool (electrode) and an electrically conductive workpiece are connected to a DC supply & placed in a dielectric fluid.
  • When dielectric breaks down and becomes an electrical conductor it permits the current (spark, or discharge) to flow through the fluid workpiece.
  • Spark causes the workpiece to erode as it melts or vaporises
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11
Q

EDM - Process Mechanism

A
  • A servomechanism maintains a gap of 0.01-0.02mm between the electrode & the workpiece
  • Discharge of an electrical current, normally stored in a capacitor bank, when the potential difference between tool * workpiece is sufficient (voltage 50-380V, current 0.1
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12
Q

EDM - Process Characteristics

A
  • Material Removal Rate (MRR):
  • Ranges from 10-^6 to 10-^4mm3 per spark
  • 15mm3/hr to 400cm3/hr
MRR = Material Removal Rate
I = Current (A)
Tw = Melting point of the workpiece (degrees C)
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13
Q

EDM - Process Characteristics

A
  • Surface finish:
  • Ranges from 0.05-0.1 micrometres Ra depending on electrode & workpiece material and MRR
  • Low MRR improve surface roughness; it can be also be improved by oscillating the electrode at amplitudes of 10-100 micrometres.
  • Capable of cutting hardened materials; forms can be machined after heat treatment thereby eliminating the problem of distortion caused by heat treatment.
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14
Q

EDM - Process Characteristics

A

-Overcut is the distance by which the machined cavity in the workpiece exceeds the size of the tool on each side of the tool.

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15
Q

EDM - Process Characteristics

A
  • Tool wear:
  • Dependant on melting point of the material; the lower the melting point the higher the tool wear.
  • Depending on the power usage; the higher the current the higher the tool wear.
  • Wear ratio of 3:1 for metallic electrodes up to 100:1 for graphite ones
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16
Q

Electrodes for Conventional EDM

A
  • Materials:
  • Graphite, brass, copper, copper-tungsten alloys.
  • Tool wear:
  • Adverse effects on tool geometry & dimensional accuracy.
  • Higher tool wear on electrode material with low melting point and also when a high current is used (graphite better than metal).
  • Contributed by spark frequency, polarity, duty cycle.
17
Q

Electrodes - Copper

A
  • Capable of producing “glass like” surface finish Ra ~0.05 micrometres
  • Copper electrodes are easy and quickly to machine.
  • Good for small parts only due to its thermal expansion characteristics.
  • Contamination of the dielectric due to oxidation.
  • Favoured for machining low melting temperature alloys (aluminium, brass, stainless steel).
  • Copper-Tungsten are strong & resistant to damage; good for high precision work and intricate details.
18
Q

Electrode - graphite

A
  • Surface finish (Ra ~ 0.5 micrometres) due to large grain structure of graphite.
  • Good for high volume production when no smooth finish is required (Ra ~6-12micrometres)
  • Graphite electrodes are easy & quickly to machine.
  • Favoured for machining high temperature alloys & steel.
  • Infiltrate graphite electrodes with copper to combine the positive characteristics of both materials.
19
Q

Dielectric fluids for EDM

A
  • Function & Properties:
  • Insulator - to prevent short circuit between the tool & the workpiece; act as conductor only when conditions are right.
  • Coolant - eroded material is cooled and solidified; any heat generated should be removed by the fluid.
  • Flushing agent - to remove eroded particles from the spark gap (viscosity & flash point).
  • Medium:
  • Mineral oil, Kerosene, de-ionised water, paraffin, silicon fluids.

*Wire EDM - de-ionised water only

20
Q

Conventional EDM

A
  • Shape of the cutting area of the electrode is a mirror image of the finished shape (cavity) of the workpiece.
  • Electrode design is critical as the cut cavity is marginally larger than the electrode’s surface area - called overcut.
21
Q

Wire EDM

A
  • Strand of wire feeds from a supply reel & passes through the work-piece at determined velocity.
  • A stream of dielectric surrounds the wire and the electrical current in the wire is pulsed on and off, creating sparks that cross the gap between wire and workpiece.
22
Q

Wire EDM - Process Characteristics

A
  • Wire travels continuously under tension, wear of the electrode does not affect accuracy.
  • No special shaped electrode required (typically Ø0.1-0.4mm)
  • Cutting speed of up to 200cm2/hr
  • Accuracy up to 0.004mm
  • Cutting path or kerf as small as 0.12mm using Ø0.1mm wire.
  • Capable of cutting plates up to 300mm thickness.
23
Q

Electrodes for Wire EDM

A
  • Materials:
  • Brass
  • Tungsten & molybdenum
  • Stratified (composite) wires; zinc-copper, zinc-brass, silver-brass, copper-steel.
  • Required characteristics for wire EDM electrode material:
  • precision uniform diameter.
  • high melting point
  • high tensile strength
24
Q

Applications - EDM

A
  • Tool making & die industry:
  • Punch & die sections
  • Forming and forging dies
  • Plastic and die casting moulds
  • Internal & external electrodes.
  • Jewellery and coinage stamping.
  • Prototype and production parts.
25
Q

Applications - EDM

A
  • Small hole drilling:
  • Pre-drilling for wire EDM

-Super Drill:

  • stand-alone for conventional EDM
  • Depth to hole ratio 400:1
26
Q

Variations - EDM

A
  • Electrical Discharge Grinding:
  • Grinding wheel of graphite or brass without abrasives
  • Material removal on the workpiece surface by spark discharge between wheel and workpiece.
  • Primarily used for grinding carbide tools & dies; also used for fragile and very small parts.
  • Faster than conventional EDM but higher power consumption.
27
Q

Variations - EDM

A
  • Electrical Discharge Sawing:
  • Set up similar to band or ciircular saw (without teeth)
  • High material removal rates possible
  • Narrow cuts
  • Good for small parts as negligible cutting forces.
28
Q

Advantages - EDM

A
  • Manufacture of difficult & intricate cavities and contours.
  • Multi-axis CNC machines allow complex profiles with simple electrodes.
  • Complex internal shapes can be machined.
  • High accuracy for MRR & surface finish due to good spark control.
  • Good for brittle components that would not withstand conventional machining forces & temperatures.
29
Q

Advantages - EDM (continued)

A
  • Machining of hard or tough materials, eliminating deformation caused by heat treatment.
  • No burrs - often no econdary operation required.
  • Quick tooling set up times.
  • Minimum operator attention during cutting process required.
  • High versatility of machining options (vertical, orbital, rotational, spin, indexing) compared to traditional processes.
30
Q

Limitations - EDM

A
  • Machine & tooling can be expensive
  • Only conductive materials (tooling & workpiece) can be machined.
  • Spark action will create some unwanted erosion and taper on the tool; cavities on work-piece may be tapered but is controllable.
  • Overcut of the work-piece requires tools to be manufactured undersize.
31
Q

Limitations - EDM (continued)

A
  • Some thermally induced stress can lead to fine sub-surface cracking.
  • Work surface (recast) and HEAT Affected Zone (HAZ) can affect structural & surface integrity (can be advantageous)

Manufacturing Processes (12 decks)

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