EMD MACHINE

Electric Discharge Machining

EDM• The EDM process we know today started

with the observations of Joseph Preistly in 1770. He noticed that electrical discharges had removed material from the electrodes in his experiments. This is also known as electro-discharge erosion.

In the 1940's Soviet researchers developed a machining process that formed the foundation for modern EDM.

Principle

• EDM is a machining process which consists in removing material from a workpiece, using electrical discharges as the means of machining. This technique is characterized by its aptitude for machining all materials that conduct electricity (metals, alloys, carbides, graphite, etc. whatever their hardness may be. To machine with this process, 4 items are required:

Die electric fluid

• The purpose of the dielectric (water or mineral oil) is to lower the temperature in the machining area, remove the residual metallic particles, and enable sparks to be created

Electrical discharge machining

• Produced by a spark generator, the sparks at regular intervals create a succession of craters in the work piece. Each spark produces a temperature between 8,000 and 12,000° C. The size of the crater depends on the energy turned out by the spark generator. The range of the sparks varies from a few microns to 1 mm.

The EDM process takes place in 6 stages

• 1 The electrode approaches the work piece. The two units are energized.

• 2 Concentration of the electrical field towards the point where the space between the electrode and work



• 3 Creation of an ionized channel between the electrode and workpiece.


• 4 Breakdown of the spark. The work piece material melts locally and disintegrates. The electrode only wears out slightly.


• 5 The current is cut off, causing implosion of the spark.


• 6 Evacuation of the metallic particles by flushing with dielectric.

Surface finish and machining speed

• The surface finish depends on the dimensions of the sparks. If they are energetic, the surface finish will be rough, but on the other hand the speed of machining will be high.


• If the sparks are of low energy, the surface finish will be fine, but machining speed will be low.


• The finest surface finishes will be of the order of Ra 0.10, and the visual effect is almost like a mirror finish. Standard surface finishes, that are easy to obtain, are equivalent to Ra 0.8/1 (N5 - N6).

• Machining speeds in EDM are moderate. Depending on the energy of the sparks, material removal rates range from 1 to several thousand cubic millimeters per minute.

• Although it uses electrical sparks, the process entails no risk for users or the environment

Machining Type

There are two distinct EDM processes, both using the same physical principle:

• Die sinking

• Wire cutting

Die sinking

• EDM die sinking reproduces, in a metallic work piece, the shape of a tool called electrode.

• Injection molds for plastic parts are very frequently machined by die sinking.

• The shape given to the electrode is that of the object that is going to be molded.

• In the machining area, each discharge creates a crater in the work piece (material removal) and an impact on the tool (wear of the tool/electrode).

• There is never any mechanical contact between the electrode and work piece.

• The electrode is usually made of copper or graphite.

Die sinking

Wire cutting

• EDM wire cutting uses a metallic wire (electrode) to cut a programmed contour in a work piece.

• Extrusion dies and blanking punches are very often machined by wire cutting.

• Cutting is always through the entire work piece.

• To start machining it is first necessary to drill a hole in the work piece or start from the edge.

• In the machining area, each discharge creates a crater in the work piece (material removal) and an impact on the tool (wear of the tool/electrode).

Wire cutting

• The wire can be inclined, thus making it possible to make parts with taper or with different profiles at the top and bottom.

• There is never any mechanical contact between the electrode and work piece.

• The wire is usually made of brass or stratified copper, and is between 0.02 and 0.03 mm diameter

Wire cutting

Applications-Die Sinking

Applications-Wire EDM

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Types of applications using EDM

Which industries use EDM?

Die sinking

• Die s inking is a method involving e lectrica l discharges be tween an e lectrode and a conductive work piece in a die lectric fluid. Mate ria l is removed from the work piece by a controlled e lectrica l spark genera ted in the machine’s power supply and discharged be tween an e lectrode and work piece . The e lectrode progress ive ly “erodes” the workpiece , producing a cavity the same contour or shape as the e lectrode . In fact, making die cavities was such a common applica tion for ram EDM in the early days tha t it was often ca lled die s inking. The machines a re s till often re ferred to as die s inkers . Regardless of the applica tion, the work piece may be of any materia l, no matte r how hard, as long as it is e lectrica lly conductive . However, machining speed will be de te rmined by the materia l’s melting tempera ture , a s well as its e lectrica l and thermal conductivity.

Die sinking

• For EDM, there must always be a small space, known as the spark gap, between the electrode and the workpiece. The gap is filled with a circulating dielectric fluid. On a ram EDM machine, this fluid is usually oil. The dielectric fluid becomes ionized during the course of a discharge. As ionization occurs, positively charged ions strike the work piece, increasing the temperature at the surface of the workpiece and electrode. The temperature is so high at the surface, often reaching more than 10,000° C, that it causes the work piece to melt or vaporize. A bubble of hydrogen gas begins to form. When current flow is turned off, the hydrogen bubble implodes. The force of this implosion forces molten metal to be evacuated from the surface of the work piece and flushed away by the dielectric fluid. Although most of the molten metal created by the discharge is carried away from the work piece, a small amount fuses onto the machined surface, creating what is called the “recast layer.”

•

Die sinking

• Fine surface finishes can be achieved with current, conventional EDM technology. The technique involves copper rather than the more commonly used graphite electrode material. Copper lends itself to fine finishes with conventional EDM because it does not have the granular structure of graphite. Copper electrodes can achieve a “mirror” or “glazed” finish (that is, 2µinch Ra) on cavities with a surface area of 2 square inches or smaller.

• As the work area becomes larger, however, the finish will deteriorate because a copper electrode cannot easily be contained in the cavity. This is due, in part, to the thermal expansion characteristics of copper. As a result, a fine finish becomes increasingly difficult to achieve as cavity size increases.

Features of the Wire EDM Process

• Low work holding forces

• Low cutting forces

• Very accurate process tolerances held +/- 0.0001"

• Complex profile capability

• No tool wear (the wire is continually replenished)

• Environmentally friendly (by products are easily recycled)

• Hardened materials are easily machined

Wire EDM

• Depending on the accuracy and surface finish needed a part will either be one cut or it will be roughed and skimmed. On a one cut the wire ideally passes through a solid part and drops a slug or scrap piece when it is done. This will give adequate accuracy for some jobs but most of the time skimming is necessary. A skim cut is where the wire is passed back over the roughed surface again with a lower power setting and low pressure flush. There can be from one to nine skim passes depending on the accuracy and surface finish required.

Wire EDM

• Usually there are just two skim passes. A skim pass can remove as much as 0.002" of material or a as little as 0.0001". During roughing ( i.e. the first cut) the water is forced into the cut at high pressure in order to provide plenty of cooling and eliminate eroded particles as fast as possible. During skimming (accuracy / finish cuts) the water is gently flowed over the burn so as not to deflect the wire.

What Is Small Hole EDM?• Small hole EDM is a specialized component of electrical

discharge machining. A small hollow electrode spins about a spindle much like a drill and drill bit (EDM drill). The electrode is electrically charged by a servo-controlled generator producing the spark. Water based dielectric flushes through and around the electrode providing a

controlled environment for the spark to jump to the work piece. The electric spark erodes the surface of the work

piece creating very small pockets. Eventually millions and millions of these microscopic pockets create the small

hole. The size of the hole is controlled by the diameter of the electrode. The location and depth of the holes are driven by CNC ISO codes, reducing and eliminating

operator error. Machines shown have automatic electrode changers for unattended machining.

Applications

• Small hole EDM is in many ways similar to a drilling operation but it offers the advantages of EDM. Very low machining and work piece hardness are not barriers to performance. A short list of common applications follows.

• Addition of wire EDM start holes after heat treating

• Removal of broken drill bits

• Placement of holes too small/difficult for conventional drilling

• Coolant holes in hardened machine tool bits

• Taps

• End Mills

• Drill

Small Hole EDM

Advanced Wire E.D.M.

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Advanced Wire E.D.M.

EMD MACHINE

Engineering

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