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
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.
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.
4
Q
Non-traditional Machining Processes:
Mechanical
A
- Ultrasonic Machining
- Abrasive Jet Machining
5
Q
Electro-chemical
A
- Electro-chemical Machining
- Electro-chemical Grinding
6
Q
Thermal
A
- Electro-Discharge Machining
- Electron Beam Machining
- Laser Beam Machining
7
Q
Chemical
A
- Engraving
- Photochemical Machining
- Photochemical Blanking
8
Q
Electrical-Discharge Machining - EDM [Principle]
A
-Based on the erosion of metal by spark discharge.
9
Q
Principle
A
2 main variants:
- Conventional (sinker EDM, RAM EDM)
- Wire EDM
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
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
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)
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.
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.
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
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
Applications - EDM
- Small hole drilling:
* Pre-drilling for wire EDM
-Super Drill:
* stand-alone for conventional EDM
* Depth to hole ratio 400:1
26
Variations - EDM
- 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
Variations - EDM
- 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
Advantages - EDM
- 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
Advantages - EDM (continued)
- 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
Limitations - EDM
- 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
Limitations - EDM (continued)
- 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)
