A miniature thunderstorm...

A controlled spark, struck between two metal parts, causes a series of small craters, with consequent material removal.

 

 

Craters

The micro-craters determining te material removal are produced by local fusion of the material caused by the very high temperatures generated by the spark.

 

 

The principle

The principle of EDM (Electrical Discharge Machining) is simple.

• The workpiece and tool are placed in the work position so that they do not touch.
• A gap remains, filled by the "dielectric". The workpiece and tool are connected to a d.c. power  source via a cable.
• A switch is inserted in the connection. When closed, an electrical voltage is set up between the workpiece and tool.

 

 

Workpiece and electrode

• The part used as tool is the one where craters are minimum. It is called "ELECTRODE".
• The part where there is to be the most amount of material removal is the one where the craters are greater in size. It is called "WORKPIECE".
• The EDM process is performed in a fluid known as "DIELECTRIC".

 

 

The electrical discharge

• At first no current flows because the dielectric acts as an insulator.
• When the gap is decreased to a given distance (very small), an electrical discharge with  spark is struck.
• During the discharge, the voltage is decreased and the current causes considerable increase in temperature at the spark point.
• This causes small quantities of metal to melt and evaporate.

 

 

 

Formation of the surface

• When the switch is opened, the molten material is dispersed just like an explosion, the discharge channel is deionized and a small crater remains.
• If a sequence of discharges is applied, several craters are produced one close to the other, therefore continuous material removal is obtained on the workpiece surface.

 

 

Two different processes: die sinking EDM and wire-cut EDM

In both processes the electrical energy from the power supply is transformed according to the desired result by means of highly developed, controlled pulse generators.

The capacities of the present-day equipment are extreme: high cutting speed and removal, extremely effi-cient automatic operation, interlinking and storage of very long and recur-ring machining cycles, as well as a practically one hundred per cent monitoring of the final shape and sur-face of the metals being machined - from dead-smooth to extremely rough

 

Die-sinking EDM

In the case of die-sinking EDM, the equired shape is formed negatively in the metal with a three-dimensional electrode.

By superimposed movements in the main axes x, y, c, z, the most varied shapes, indentations and cavities are created, such as can not in part be achieved by any other machining system.

 

 

Wire-cut EDM

In the case of the wire-cut process, the required shape is "computer" stored and passed as comprehensible codes to the machine which then cuts the shape independently by guiding the wire along the given stretch. 

• In wire EDM, the electrode is a metallic wire
• The wire cuts into the material a programmed path
• The wire guides are attached to the X-Y and U-V axes
• The wire renewal is continuous

 

 

Wire-cut materials and wire used

Materials:
All the electrical conductive materials can be machined by WEDM regardless its hardness. (All the metals, graphite, hard metals,…)

The wire:
The most used wire are the coated wire or the brass wire

Structure of the coated wire SW25X

Diameter: 0.25mm

Basic material :copper

Coating: Zn, 15 µm

 

 

Examples of applications

Die sinking or wire EDM are used in the manufacturing process of various objects

 

 

The direct cut

The machining is made in a single and simple operation. There is no roughing and no finishing cut. The cutting speed may be reduced. The direct cut is very often used in general mechanics.

 

 

 

The finishing cut

Following the roughing cut, this operation improves the surface finish

 

 

Limits of WEDM

• Max. cutting speed: about 500 mm2/min.
• Machining of blind cavities impossible.
• Necessity to drill a pre-hole.
• Max achievable taper: about 45°

 

Main advantages of WEDM

• «Clean» process.
• Very convenient process to machine different pieces regardless their hardness
• Excellent profitability: unattended machining during several days thanks to the automation capabilities.
• Cheap wire. The used wire is recuperated.
• Easy connection to CAD-CAM systems.