Electrode Discharge Machining

EDM works by eroding material in the path of electrical discharges (sparks) that form an arc between an electrode tool and the work piece. EDM manufacturing is quite affordable and a very desirable manufacturing process when low counts or high accuracy is required. Turn around time can be fast and depends on manufacturer back log.

Ram EDM machines range in size from tabletop models to large CNC units. Ram EDMs have four sub-systems:

• a DC power supply to provide the electrical discharges, with controls for voltage, current, duration, duty cycle, frequency, and polarity

• a dielectric system to introduce fluid into the voltage area/discharge zone and flush away work and electrode debris, this fluid is usually a hydrocarbon or silicone based oil

• a consumable electrode, usually of copper or graphite
  
• a servo system to control infeed of the electrode and provide gap maintenance
  

EDM is the thermal erosion process in which metal is removed by a series of

recurring electrical discharges between a cutting tool acting as an electrode

and a conductive work piece, in the presence of a dielectric fluid. This

discharge occurs in a voltage gap between the electrode and work piece. Heat from

the discharge vaporizes minute particles of work piece material, which are then

washed from the gap by the continuously flushing dielectric fluid.

• EDM is a non-contact process that generates no cutting forces, permitting the production of small and fragile pieces

• burr-free edges are produced

• intricate details and superior finishes are possible

• EDM machines with built-in process knowledge allow the production of intricate parts with minimum operator intervention

• low metal removal rates compared to chip machining

• lead time is needed to produce specific, consumable electrode shapes
  

In ram EDM, the electrode/tool is attached to the ram which is connected to one

pole, usually the positive pole, of a pulsed power supply. The work piece is

connected to the negative pole. The work is then positioned so that there is a

gap between it and the electrode. The gap is then flooded with the dielectric

fluid. Once the power supply is turned on, thousands of direct current, or DC,

impulses per second cross the gap, beginning the erosion process. The spark

temperatures generated can range from 14,000° to 21,000° Fahrenheit. As the

erosion continues, the electrode advances into the work while maintaining a

constant gap dimension.

The finished EDM'd work piece can exhibit several distinct layers. The surface

layer will have small globules of removed work piece metal and electrode

particles adhering to it, which are easily removed. The second layer is called

the “white” or “recast” layer where EDM has altered the metallurgical structure

of the work piece. The third layer is the heat-affected zone or “annealed” layer.

This layer has been heated but not melted. See picture above.

We have lightly covered the Ram type above. The other type is the wire-cut.

Each are used to produce very small and accurate parts as well as large items like automotive stamping dies and aircraft body components.

The largest single use of EDM is in die making.

Materials worked with EDM include hardened and heat-treated steels, carbide, poly-crystalline diamond, titanium, hot and cold rolled steels, copper, brass, and high temperature alloys.

However, any material to be machined with the EDM process must be conductive.

Electrical Discharge Wire Cutting (EDWC) involves a continuously spooling conductive wire (the most widely used is brass). A power supply generates rapid electric pulses that create a discharge between the work piece and electrode (the wire). The discharge causes the melting, and probably the vaporisation, of a minute piece of material, slowly eating into the work piece. Any electrical conductive material can be machined irrespective of hardness. The position of the wire with respect to the work piece is controlled in the x and y planes usually by CNC. On some machines the wire can be tilted to create tapered parts. An advantage of this process is that no mechanical stresses are created in the work piece because the wire does not make contact with it.

• Cutting and shaping of metals and conducting ceramics that are difficult to shape in any other way: dies for moulding, stamping, extrusion and forging; making tool fixtures; aircraft and medical parts.
  
• Prototype parts.
  
• Burr-free parts.
  

The first picture is a cavity plate from a plastic injection mould tool.

The second picture is of some miscellaneous parts created by EDWC

There are no tooling costs as the wire creates the required profile. However, the slow rate of cutting (measured in a few mm/min) limits its use to small batch sizes. The process can be fully automated with low labour costs, giving an overall cost of about £20/hour. Typical machining times are 1 to 6 hours.
CNC is common.