Advanced machining processes- Electrical Discharge Machining Flashcards
Traditional Machining Processes
- 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
Limitations of traditional methods
-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
Non traditional machining process- Method
- Means of material removal are chemical dissolution, etching, melting, and evaporation etc
- Energy source is chemical, electrical, thermal and mechanical
Pros of Non-traditional machining processes
- Negligible tool wear; no contract between tooling and workpiece
- hardness of workpiece is irrelevant
- usually negligible residual stress through machining
Mechanical Machining Processes
- Ultrasonic Machining
- Abrasive Jet Machining
Electro-chemical Machining Processes
- Electro-chemical Machining
- Electro-chemical Grinding
Thermal Machining processes
- Electro-discharge Machining
- Electron Beam machining
- Laser beam Machining
Chemical Machining processes
- Engraving
- Photochemical Machining
- Photochemical Blanking
What is EDM
Electrical Discharge Machining
Electrical Discharge Machining- Method
-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
Electrical discharge machining - process mechanism
- 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)
Electrical Discharge Machining- process characteristics (Material removal rate)
- 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)
Electrical discharge machining- process characteristics(surface finish)
-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
What is an over cut? (EDM)
Overcut is the distance by which the machined cavity in the workpiece exceeds the size of the tool on each side of the tool
EDM Tool wear
- 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
Electrode for conventional EDM- Materials
- Graphite
- Brass
- copper
- copper-tungsten alloys
Electrode for conventional EDM- Tool wear
- 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
Electrodes- Copper
- 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
Electrodes- graphite
- 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
Dielectric fluids for EDM-
Properties
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)
Dielectric fluids for EDM- Medium
- Mineral oil, kerosene, de-ionised water, paraffin, silicon fluids
- Wire EDM - De-ionised water only
What is conventional EDM?
- 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
Wire EDM- Method
- 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
Wire EDM- Process characteristics
- 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
Electrodes for wire EDM
-materials
=Brass
=Tungsten and molybdenum
=Stratified wires(zinc-copper,zinc-brass, silver-brass, copper-steel)
Characteristics for wire EDM electrode material
- Precision uniform diameter
- High melting point
- High tensile strength
Applications for EDM
-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
What is electrical discharge grinding?(EDM)
- 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
what is electrical discharge sawing? (EDM)
-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
pros of EDM (part 1)
- 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
pros of EDM (part 2)
- 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
cons of EDM(part 1)
- 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
cons of EDM (part 2)
- 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
