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MECH339 Manufacturing Processes > Advanced machining processes- Electrical Discharge Machining > Flashcards

Advanced machining processes- Electrical Discharge Machining Flashcards

1
Q

Traditional Machining Processes

A
  • Turning, drilling, milling, grinding etc.
  • Material removal mechanism is usually through chip formation, micro-chipping or 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

-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 to complex
=Surface finish or dimensional accuracy requirements

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3
Q

Non traditional machining process- Method

A
  • Means of material removal are chemical dissolution, etching, melting, and evaporation etc
  • Energy source is chemical, electrical, thermal and mechanical
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4
Q

Pros of Non-traditional machining processes

A
  • Negligible tool wear; no contract between tooling and workpiece
  • hardness of workpiece is irrelevant
  • usually negligible residual stress through machining
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5
Q

Mechanical Machining Processes

A
  • Ultrasonic Machining

- Abrasive Jet Machining

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6
Q

Electro-chemical Machining Processes

A
  • Electro-chemical Machining

- Electro-chemical Grinding

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7
Q

Thermal Machining processes

A
  • Electro-discharge Machining
  • Electron Beam machining
  • Laser beam Machining
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8
Q

Chemical Machining processes

A
  • Engraving
  • Photochemical Machining
  • Photochemical Blanking
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9
Q

What is EDM

A

Electrical Discharge Machining

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10
Q

Electrical Discharge Machining- Method

A

-Based on the erosion of a metal by spark discharge
-2 varients
=Conventional EDM(sinker EDM)
=Wire EDM
-Shaped tool and an electrically conductive workpiece are connected to a DC supply and 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 to the workpiece
-spark caused the workpiece to erode as it melts or vaporises

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11
Q

Electrical discharge machining - process mechanism

A
  • a servomechanism maintains a gap of 0.01-0.02mm between the electrod and the workpiece
  • discharge of an electrical current, normally stored in a capacitor bank, when the PD between tool and workpiece is sufficient (50-280v, 0.1-500A, roughly 10,000°C)
  • to generate spark and flush molten material away the curernt must be switched on and off at high speeds (200-500 kHZ)
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12
Q

Electrical Discharge Machining- process characteristics (Material removal rate)

A
  • Ranges from 10^-6 to 10^-4 mm^3 per spark
  • 15mm^3/hr to 400cm^3/hr
  • MRR= 410^4I*Tw^-1.23

when Tw= melting point of workpiece(°C)

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13
Q

Electrical discharge machining- process characteristics(surface finish)

A

-Ranges from 0.05-micrometers Ra depending on electrode and workpiece material and MMR
-Low MRR improve surface roughness; it can also be improved by oscillating the electrode at amplitudes of 10-micrometers
-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

What is an over cut? (EDM)

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 Tool wear

A
  • Dependant on melting point of the material, the lower the melting point the higher the tool wear
  • dependant 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

=Volume of workpiece material removed/volume of tool wear

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16
Q

Electrode for conventional EDM- Materials

A
  • Graphite
  • Brass
  • copper
  • copper-tungsten alloys
17
Q

Electrode for conventional EDM- Tool wear

A
  • adverse effects on tool geometry and 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
18
Q

Electrodes- Copper

A
  • capable of producing “glass like” surface finish Ra roughly equal to 0.05 micrometers
  • copper electrodes are easy and quick to machine
  • good for small parts only
  • contamination of the dielectric due to oxidation
  • favoured for machining low melting temperature alloys (aluminium, brass, stainless steel)
  • copper-tungsten are strong and resistant to damage; good for high precision work and intricate details
19
Q

Electrodes- graphite

A
  • surface finish (Ra roughly equal to micrometers) due to large grain structure of graphite
  • good for high volume production when no smooth finish is required (Ra roughly equal to 6-micrometers)
  • graphite electrodes are easy and quick to machine
  • favoured for machining high temperature alloys and steel
  • infiltrate graphite electrodes with copper to combine the positive characteristics of both materials
20
Q

Dielectric fluids for EDM-

Properties

A

Insulator- to prevent short circuit between tool and 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 and flash point)

21
Q

Dielectric fluids for EDM- Medium

A
  • Mineral oil, kerosene, de-ionised water, paraffin, silicon fluids
  • Wire EDM - De-ionised water only
22
Q

What is conventional EDM?

A
  • shape of the cutting area of the electrode is a mirror image of the finished shape of the workpiece
  • electrode design is critical as the cut cavity is marginally larger than the electrode’s surface area -defined as overcut
23
Q

Wire EDM- Method

A
  • Strand of wire feeds from a supply reel and passes through the workpiece at a determined velocity
  • A stream of dielectric surrounds the wire and the electrical current in the wire is pulsed on and off crossing the gap between wire and work piece
24
Q

Wire EDM- Process characteristics

A
  • wire travels continuously under tension- wear of electrod does not affect accuracy
  • no special shaped electrode required (typically diameter of 0.1-0.4mm
  • cutting speed of up to 200cm^2/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
25
Q

Electrodes for wire EDM

A

-materials
=Brass
=Tungsten and molybdenum
=Stratified wires(zinc-copper,zinc-brass, silver-brass, copper-steel)

26
Q

Characteristics for wire EDM electrode material

A
  • Precision uniform diameter
  • High melting point
  • High tensile strength
27
Q

Applications for EDM

A 
-tool making and die industry
=punch and die sections
=forming and forging dies
=plastic and die casting moulds
=internal and external electrodes
  • jewellery and coinage stamping
  • prototype and production parts

-Small hole drilling
=pre-drilling for wire edm

-super drill
=standalone for conventional EDM
=depth to hole ratio 400:1

28
Q

What is electrical discharge grinding?(EDM)

A
  • 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 and dies; also used for fragile and very small parts
  • faster than conventional EDM but higher power consumption
29
Q

what is electrical discharge sawing? (EDM)

A

-Electrical discharge sawing
=set up similar to band or circular saw (without teeth)
=high material removal rates possible
=narrow cuts
=good for small parts as negligible cutting forces

30
Q

pros of EDM (part 1)

A
  • manufacture of difficult and intricate cavities and contours
  • multi-axis CNC machines allow complex profiles with simple electrodes
  • complex internal shapes can be machined
  • high accuracy for MRR and surface finish due to good spark control
  • good for brittle components that would not withstand conventional machining forces and temperatures
31
Q

pros of EDM (part 2)

A
  • machining of hard or tough materials, eliminating deformation caused by heat treatment
  • no burrs- often no secondary operation required
  • quick tooling setup times
  • minimum operator attention during cutting process required
  • high versatility of machining options (vertical, orbital, rotational, spin, indexing) compared to traditional processes
32
Q

cons of EDM(part 1)

A
  • Machine and tooling can be expensive
  • only conductive materials (tooling and workpiece) can be machined
  • spark action will create some unwanted erosion and taper on the tool; cavities on the workpiece may be tapered but this is controllable
  • overcut of workpiece requires tools to be manufactured undersize
33
Q

cons of EDM (part 2)

A
  • some thermally induced stress can lead to fine sub-surface cracking
  • work surface and heat affected zone can affect structural and surface integrity (can be a +)
  • slow material removal rates
  • swarf cannot be recycled, dielectric must be filtered
  • potential health hazard from contact with dielectric fluid or the inhalation of spark-induced flames

MECH339 Manufacturing Processes (11 decks)

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