Electrical Discharge Machining

Electrical Discharge Machining

Electrical discharge machining is a process in which electrical discharges are used to shape and form metal parts. It is done through the use of sparks that generate heat from eight to twenty thousand degrees. A shaped tool, electrode, or wire can be used to generate the sparks, depending on which process is used. A conductive path is established between the electrode and the material being machined so that there is no actual contact between the two. The electrical discharge machining takes place within a bath of dielectric fluid in order to prevent premature sparking and to flush away debris. It also conducts electricity between the electrode and the work piece, flushing out the melted material when it is done.

Common uses for electrical discharge machining include plastic molds, die casting dies and forging dies. It is also used for manufacturing engine parts such as titanium alloy or nickel based super alloy compressor blades. Prototype and production parts are more often made through electrical discharge machining methods, especially in the aerospace and electronics industries because of the low production quantities. Other industries that use electrical discharge machining include food and beverage, automobile, stamping, extruding, defense and medical.

Because it can create intricate and complex parts, electrical discharge machining is often used over different machining techniques. It is known for the ability to machine hard materials and extremely small parts. Another reason that electrical discharge machining is so popular is because the work piece does not get deformed through the process, the finished product will not have burrs or heat damage. Many electrical discharge machining electrodes can rotate about two-three axis, which is another advantage because it allows for the cutting of internal cavities.

Electrical discharge machining From Wikipedia, the free encyclopedia

Electrical Discharge Machining (or EDM) is a machining method primarily used for hard metals or those that would be impossible to machine with traditional techniques. One critical limitation, however, is that EDM only works with materials that are electrically conductive. EDM can cut small or odd-shaped angles, intricate contours or cavities in pre-hardened steel without the need for heat treatment to soften and re-harden them as well as exotic metals such as titanium, hastelloy, kovar, inconel and carbide.

Sometimes referred to as spark machining or spark eroding, EDM is a non-traditional method of removing material by a series of rapidly recurring electric arcing discharges between an electrode (the cutting tool) and the workpiece, in the presence of an energetic electric field. The EDM cutting tool is guided along the desired path very close to the work but it does not touch the piece. Consecutive sparks produce a series of micro-craters on the work piece and remove material along the cutting path by melting and vaporization. The particles are washed away by the continuously flushing dielectric fluid. It is also important to note that a similar micro-crater is formed on the surface of the electrode, the debris from which must also be flushed away. These micro-craters result in the gradual erosion of the electrode, many times necessitating several different electrodes of varying tolerances to be used, or, in the case of wire EDM machining, constant replacement of the wire by feeding from a spool.

There are two main types of EDM machines: Sinker EDM (also called Conventional EDM and Ram EDM) and Wire EDM.

History

The EDM process was improved by two Russian scientists, Dr. B.R. Lazarenko and Dr. N.I. Lazarenko in 1943

Advantages

Some of the advantages of EDM include machining of complex shapes that would otherwise be difficult to produce with conventional cutting tools, machining of extremely hard material to very close tolerances, and machining of very small work pieces where conventional cutting tools may damage the part from excess cutting tool pressure.

Disadvantages

Some of the disadvantages of EDM include

     • The inability to machine non conductive materials.
     • The slow rate of material removal.
     • The additional time and cost used for creating electrodes for ram EDM.
     • Reproducing sharp corners on the workpiece is difficult due to electrode wear.

Conventional EDM

Prototype production

The EDM process is most widely used by the mold-making tool and die industries, but is becoming a common method of making prototype and production parts, especially in the aerospace, automobile and electronics industries in which production quantities are relatively low. In Sinker EDM, a graphite or pure copper electrode is machined into the desired (negative) shape and fed into the workpiece on the end of a vertical ram.

Small hole drilling EDM

Is used to make a through hole in a workpiece in through which to thread the wire in Wire-cut EDM machining. The small hole drilling head is mounted on wire-cut machine and allows large hardened plates to have finished parts eroded from them as needed and without pre-drilling. There are also stand-alone small hole drilling EDM machines with an x–y axis also known as a super drill or hole popper that can machine blind or through holes. EDM Drills bore holes with a long brass or copper tube electrode that rotates in a chuck with a constant flow of distilled or deionized water flowing through the electrode as a flushing agent and dielectric. The electrode tubes operate like the wire in wire-cut EDM machines, having a spark gap and wear rate. Some small-hole drilling EDMs are able to drill through 100 mm of soft or through hardened steel in less than 10 seconds, averaging 50% to 80% wear rate. Holes of 0.3 mm to 6.1 mm can be achieved in this drilling operation. Brass electrodes are easier to machine but are not recommended for wire-cut operations due to eroded brass particles causing "brass on brass" wire breakage, therefore copper is recommended.

Wire Cut EDM

In wire electrical discharge machining (WEDM), or wire-cut EDM, a thin single-strand metal wire, usually brass, is fed through the workpiece, typically occurring submerged in a tank of dielectric fluid. The wire, which is constantly fed from a spool, is held between upper and lower diamond guides. The guides move in the x–y plane, usually being CNC controlled and on almost all modern machines the upper guide can also move independently in the z–u–v axis, giving rise to the ability to cut tapered and transitioning shapes (circle on the bottom square at the top for example) and can control axis movements in x–y–u–v–i–j–k–l–. This gives the wire-cut EDM the ability to be programmed to cut very intricate and delicate shapes. The wire is controlled by upper and lower diamond guides that are usually accurate to 0.004 mm, and can have a cutting path or kerf as small as 0.12 mm using Ø 0.1 mm wire, though the average cutting kerf that achieves the best economic cost and machining time is 0.335 mm using Ø 0.25 brass wire. The reason that the cutting width is greater than the width of the wire is because sparking also occurs from the sides of the wire to the work piece, causing erosion. This "overcut" is necessary, predictable, and easily compensated for. Spools of wire are typically very long. For example, an 8 kg spool of 0.25 mm wire is just over 19 kilometers long. Today, the smallest wire diameter is 20 micrometres and the geometry precision is not far from +/- 1 micrometre. The wire-cut process uses water as its dielectric with the water's resistivity and other electrical properties carefully controlled by filters and de-ionizer units. The water also serves the very critical purpose of flushing the cut debris away from the cutting zone. Flushing is an important determining factor in the maximum feed rate available in a given material thickness, and poor flushing situations necessitate the reduction of the feed rate.