Electrical Discharge Machining Flashcards
What are the two variants of EDM?
- Conventional EDM (sinker EDM, RAM EDM)
- Wire EDM
EDM – Principle/Method
- Shaped tool (electrode) 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 causes the workpiece to erode as it melts or vaporises
EDM Process Mechanism
- Development of the spark erosion process, positive and negative charged particles
- As temperature and pressure increase, the vapour bubble expands
- The molten metal particles are cooled and removed by the dielectric fluid
What does the term overcut meant?
Overcut is the distance by which the machined cavity in the workpiece exceeds the size of the tool on each side of the tool
What causes tool wear in regards to EDM?
- Dependent on melting point of the material; the lower the melting point the higher the tool wear
- Dependent on the power usage; the higher the current the higher the tool wear
The function of Dielectric fluids for EDM
Insulator – to prevent short circuit between the tool and 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 and flash point)
Medium of dielectric?
Mineral oil, Kerosene, de-ionised water, paraffin, silicon powder
Wire EDM Principal/Method
- Strand of wire feeds from a supply reel and passes through the workpiece 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
Applications – EDM
Tool making and die industry Punch and die sections Forming and forging dies Plastic and die casting moulds Internal and external electrodes
What are the two variations of EDM?
Electrical Discharge Sawing
Electrical Discharge Grinding
Advantages of EDM?
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
Machining of hard or tough materials, eliminating deformation
caused by heat treatment
No burrs – often no secondary 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
EDM Limitations?
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 workpiece may be tapered but is
controllable
Overcut of the workpiece requires tools to be manufactured
undersize
Some thermally induced stress can lead to fine sub-surface
cracking
Work surface (recast) and Heat Affected Zone (HAZ) can
affect structural and surface integrity (can be advantageous!)
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 fumes
Name the effects of incorrect flushing pressure?
- Too much dielectric fluid pressure will remove the chip before it can assist in the actual cutting
- Too little dielectric fluid pressure will not remove the quickly enough and may cause short-circuits(melting of surfaces possible)
- In wire EDM excessive dielectric flow candeflect the wire causing erratic cutting and loss of machining accuracy
