Electrical discharge machining

An electrical discharge machine

Electric discharge machining (EDM), sometimes colloquially also referred to as spark machining, spark eroding, burning, die sinking or wire erosion,^[1] is a manufacturing process whereby a wanted shape of an object, called workpiece, is obtained using electrical discharges (sparks). The material removal from the workpiece occurs by a series of rapidly recurring current discharges between two electrodes, separated by a dielectric liquid and subject to an electric voltage. One of the electrodes is called tool-electrode and is sometimes simply referred to as ‘tool’ or ‘electrode’, whereas the other is called workpiece-electrode, commonly abbreviated in ‘workpiece’. When the distance between the two electrodes is reduced, the intensity of the electric field in the volume between the electrodes is expected to become larger than the strength of the dielectric (at least in some point(s)) and therefore the dielectric breaks allowing some current to flow between the two electrodes. This phenomenon is the same as the breakdown of a capacitor (condenser) (see also breakdown voltage). A collateral effect of this passage of current is that material is removed from both the electrodes. Once the current flow stops (or it is stopped - depending on the type of generator), new liquid dielectric should be conveyed into the inter-electrode volume enabling the removed electrode material solid particles (debris) to be carried away and the insulating proprieties of the dielectric to be restored. This addition of new liquid dielectric in the inter-electrode volume is commonly referred to as flushing. Also, after a current flow, a difference of potential between the two electrodes is restored as it was before the breakdown, so that a new liquid dielectric breakdown can occur.

History

The EDM process was invented by two Russian scientists, Dr. B.R. Lazarenko and Dr. N.I. Lazarenko in 1943. Agie launches in 1969 the world's first numerically controlled wire-cut EDM machine.^[2][3] Seibu developed the world first CNC wire EDM machine 1972 and the first system manufactured in Japan.

Generalities

Electrical discharge machining is a machining method primarily used for hard metals or those that would be very difficult to machine with traditional techniques. EDM typically works with materials that are electrically conductive, although methods for machining insulating ceramics^[4][5] with EDM have also been proposed. EDM can cut intricate contours or cavities in pre-hardened steel without the need for heat treatment to soften and re-harden them. This method can be used with any other metal or metal alloy such as titanium, hastelloy, kovar, and inconel. Also, applications of this process to shape polycrystalline diamond tools have been reported.^[6]

EDM is often included in the ‘non-traditional’ or ‘non-conventional’ group of machining methods together with processes such as electrochemical machining (ECM), water jet cutting (WJ, AWJ), laser cutting and opposite to the ‘conventional’ group (turning, milling, grinding, drilling and any other process whose material removal mechanism is essentially based on mechanical forces).^[7] Ideally, EDM can be seen as a series of breakdown and restoration of the liquid dielectric in-between the electrodes. However, caution should be exerted in considering such a statement because it is an idealized model of the process, introduced to describe the fundamental ideas underlying the process. Yet, any practical application involves many aspects that may also need to be considered. For instance, the removal of the debris from the inter-electrode volume is likely to be always partial. Thus the electrical proprieties of the dielectric in the inter-electrodes volume can be different from their nominal values and can even vary with time. The inter-electrode distance, often also referred to as spark-gap, is the end result of the control algorithms of the specific machine used. The control of such a distance appears logically to be central to this process. Also, not all of the current flow between the dielectric is of the ideal type described above: the spark-gap can be short-circuited by the debris. The control system of the electrode may fail to react quickly enough to prevent the two electrodes (tool and workpiece) to get in contact, with a consequent short circuit. This is unwanted because a short circuit contributes to the removal differently from the ideal case. The flushing action can be inadequate to restore the insulating properties of the dielectric so that the flow of current always happens in the point of the inter-electrode volume (this is referred to as arcing), with a consequent unwanted change of shape (damage) of the tool-electrode and workpiece. Ultimately, a description of this process in a suitable way for the specific purpose at hand is what makes the EDM area such a rich field for further investigation and research.^[8] To obtain a specific geometry, the EDM tool is guided along the desired path very close to the work, ideally it should not touch the workpiece, although in reality this may happen due to the performance of the specific motion control in use. In this way a large number of current discharges (colloquially also called sparks) happen, each contributing to the removal of material from both tool and workpiece, where small craters are formed. The size of the craters is a function of the technological parameters set for the specific job at hand. They can be with typical dimensions ranging from the nanoscale (in micro-EDM operations) to some hundreds of micrometers in roughing conditions. The presence of these small craters on the tool results in the gradual erosion of the electrode. This erosion of the tool-electrode is also referred to as wear. Strategies are needed to counteract the detrimental effect of the wear on the geometry of the workpiece. One possibility is that of continuously replacing the tool-electrode during a machining operation. This is what happens if a continuously replaced wire is used as electrode. In this case, the correspondent EDM process is also called wire-EDM. The tool-electrode can also be used in such a way that only a small portion of it is actually engaged in the machining process and this portion is changed on a regular basis. This is, for instance, the case when using a rotating disk as a tool-electrode. The corresponding process is often also referred to as EDM-grinding.^[9] A further strategy consists in using a set of electrodes with different sizes and shapes during the same EDM operation. This is often referred to as multiple electrode strategy, and is most common when the tool electrode replicates in negative the wanted shape and is advanced towards the blank along a single direction, usually the vertical direction (i.e. z- axis). This resembles the sink of the tool into the dielectric liquid in which the workpiece is immersed, so, not surprisingly, it is often referred to as die-sinking EDM (also called Conventional EDM and Ram EDM). The corresponding machines are often called Sinker EDM. Usually, the electrodes of this type have quite complex forms. If the final geometry is obtained using a usually simple shaped electrode which is moved along several directions and is possibly also subject to rotations often the term EDM-milling is used.^[10] In any case, the severity of the wear is strictly dependent on the technological parameters used in the operation (for instance: polarity, maximum current, open circuit voltage). For example, in micro-EDM, also known as μ-EDM, these parameters are usually set at values which generates severe wear. Therefore, wear is a major problem in that area.

Definition of the technological parameters

Some difficulties have been encountered in the definition of the technological parameters that drive the process. The reasons are explained below.

On the one hand, two broad categories of generators, also known as power supplies, are in use on EDM machines commercially available: the group based on RC circuits and the group based on transistor controlled pulses. In the first category, the main parameters that a practitioner may be expected to choose from at set-up time are the resistance(s) of the resistor(s) and the capacitance(s) of the capacitor(s).

In an ideal condition these quantities would then affect the maximum current delivered in an ideal discharge, which is expected to be associated with the charge accumulated on the capacitors at a certain moment in time. Little control, however is expected to be possible on the time duration of the discharge, which is likely to depend on the actual spark-gap conditions (size and pollution) at the moment of the discharge. Yet, this kind of generators can allow the user to obtain short time durations of the discharges relatively easier than with the a pulse controlled generator. This advantage is however going to be diminished with the progress in the production of electronic components.^[11] Also, the open circuit voltage (i.e. the voltage between the electrodes when the dielectric is not yet broken) can be identified as steady state voltage of the RC circuit. In the second group of generators, based on transistor control usually enables the user to deliver a train of pulses of voltage to the electrodes. Each pulse can be controlled in shape, for instance quasi-rectangular. In particular, the time between two consecutive pulses and the duration of each pulse can be set. The amplitude of each pulse constitutes the open circuit voltage. In this framework, the maximum time duration of a current discharge is equal to the duration of a pulse of voltage in the train. Two pulses of current are then expected not to occur for a duration equal or larger than the time interval between two consecutive pulses of voltage. The maximum current during a discharge that the generator deliver can also be controlled. Design of generators different from that described above is likely to be commercially available. Thus, the parameters that a user may actually set on his own machine may be quite different and generator-manufacturer dependent. Moreover, the manufacturers are usually quite reluctant to unveil the details of their generators and control systems to their user base not to give a potential competitive advantage to their competitors. And, conversely, the average users are usually more interested in a ‘machine that can do the job’, rather than in through understanding of the EDM process. This circumstance constitutes another barrier to the attempt of describing unequivocally the technological parameters of the EDM process. Moreover, the parameters affecting the phenomena occurring between tool and electrode are related not only to the generator design but also to the controller of the motion of the electrodes. A framework to define and measure the electrical parameters during an EDM operation directly on inter-electrode volume with an oscilloscope external to the machine has been recently proposed by Ferri et al.^[12] These authors conducted their research in the filed of μ-EDM, but the same approach can be used in any EDM operation. This would enable the user to estimate directly the electrical parameter that affect their operations in an open way, without relying upon machine manufacturer's claims. Finally, it is worth mentioning that, quite unexpectedly, when machining different materials in identical nominal set-up conditions the actual electrical parameters of the process are significantly different.^[12]

Material removal mechanism

The first serious attempt of providing a physical explanation of the material removal during electric discharge machining is perhaps that of Van Dijk.^[13] In his work, Van Dijk present a thermal model together with a computational simulation to explain the phenomena between the electrodes during electric discharge machining. However, as Van Dijk himself admitted in his study, the number of assumptions made to overcome the unavailability of experimental data at that time was quite significant.

Further enhanced models trying to explain the phenomena occurring during electric discharge machining in terms of heat transfer theories were developed in the late eighties and early nineties. Possibly the most advanced explanation of the EDM process as a thermal process was developed during an investigation carried out at Texas A&M University with the support of AGIE, now Agiecharmilles, a company with headquarters in Switzerland. This stream of research resulted in a series of three scholarly papers: the first presenting a thermal model addressing the material removal on the cathode,^[14] the second presenting a thermal model for the erosion occurring on the anode^[15] and the third introducing a model describing the plasma channel that is formed during the passage of the discharge current through the dielectric liquid.^[16] Validation of these models is carried out also using experimental data provided by AGIE. These models constitute the most authoritative support for the claim that EDM is a thermal process, describing how the material is removed from the two electrodes because of melting and/or vaporization processes in conjunction with pressure dynamics established in the spark-gap by the collapsing of the plasma channel. However, from a careful reading of these papers it emerges that for small discharge energies the presented models are quite inadequate to explain the experimental data. Also, all these models hinge on a number of assumptions, often taken from such disparate research areas as submarine explosions, discharges in gases, and failure of transformers. So it is not surprising that alternative models have been proposed more recently in the literature trying to explain the EDM process. Among these, the model from Sigh and Ghosh^[17] re-connects the removal of material from the electrode to the presence of an electrical force on the surface of the electrode that would be able to mechanically remove material and create the craters. This would be made possible by the fact that the material on the surface has altered mechanical proprieties due to an increased temperature caused by the passage of electrical current. The authors simulated their models and showed how they might explain EDM better than a thermal model (melting and/or evaporation), especially for small discharge energies, which are typically used in μ-EDM and in finishing operations. In the light of the many available models, it appears that the material removal mechanism in EDM is not yet well understood and that further investigation is necessary to clarify it.^[12] Especially considering the lack of experimental scientific evidence to build and validate the current EDM models.^[12] This explains an increased current research effort in related experimental techniques.^[8]

Types

Sinker EDM

Sinker EDM sometimes is also referred to as cavity type EDM or volume EDM. Sinker EDM consists of an electrode and workpiece that are submerged in an insulating liquid such as, more typically ^[18], oil or, less frequently, other dielectric fluids. The electrode and workpiece are connected to a suitable power supply. The power supply generates an electrical potential between the two parts. As the electrode approaches the workpiece, dielectric breakdown occurs in the fluid forming a plasma channel^{[14] [15] [16] [8]}, and a small spark jumps. These sparks usually strike one at a time ^[18] because it is very unlikely that different locations in the inter-electrode space have the very identical local electrical charachetistics which would enable a spark to occur simultaneously in all such locations. These sparks happens in huge numbers at seemingly random locations between the electrode and the workpiece. As the base metal is eroded, and the spark gap subsequently increased, the electrode is lowered automatically by the machine so that the process can continue uninterrupted. Several hundred thousand sparks occur per second in this process, with the actual duty cycle being carefully controlled by the setup parameters. These controlling cycles are sometimes known as "on time" and "off time", which are more formally defined in the litterature ^[19], ^[12], ^[8]. The on time setting determines the length or duration of the spark. Hence, a longer on time produces a deeper cavity for that spark and all subsequent sparks for that cycle creating a rougher finish on the workpiece. The reverse is true for a shorter on time. Off time is the period of time that one spark is replaced by another. A longer off time for example, allows the flushing of dielectric fluid through a nozzle to clean out the eroded debris, thereby avoiding a short circuit. These settings can be maintained in micro seconds. The typical part geometry is a complex 3D shape ^[18], often with small or odd shaped angles. Vertical, orbital, vectorial, directional, helical, conical, rotational, spin and indexing machining cycles are also used.

Wire 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, which is typically deionised water ^[18]. This process is not typically used to produce complex 3D geometries ^[18]. It is instead typically used to cut plates as thick as 300mm and to make punches, tools,and dies from hard metals that are too difficult to machine with other methods. 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 occurs from the sides of the wire to the work piece, causing erosion ^[18]. This "overcut" is necessary, for many applications it is adequately predictable and therefore can be compensated for (for instance in micro-EDM this is not often the case). 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.

Along with tighter tolerances multiaxis EDM wire-cutting machining center have many added features such as: Multiheads for cutting two parts at the same time, controls for preventing wire breakage, automatic self-threading features in case of wire breakage, and programmable machining strategies to optimize the operation.

Wire-cutting EDM is commonly used when low residual stresses are desired. Wire EDM may leave residual stress on the workpiece that are less significant than those that may be left if the same workpiece were obtained by machining. In fact in wire EDM there are not large cutting forces involved in the removal of material. Yet, the workpiece may undergo to a significant thermal cycle, whose severity depends on the technological parameters used. Possible effects of such thermal cycles are the formation of a recast layer on the part and the presence of tensile residual stresses on the workpiece. If the process is set up so that the energy/power per pulse is relatively little (typically in finishing operations), little change in the mechanical properties of a material is expected in wire-cutting EDM due to these low residual stresses, although material that hasn't been stressed relieved can distort in the machining process.

Applications

Prototype production

The EDM process is most widely used by the mould-making tool and die industries, but is becoming a common method of making prototype and production parts ^[20], especially in the aerospace, automobile and electronics industries in which production quantities are relatively low. In Sinker EDM, a graphite, copper tungsten or pure copper electrode is machined into the desired (negative) shape and fed into the workpiece on the end of a vertical ram.

Coinage die making

For the creation of dies for producing jewelry and badges by the coinage (stamping) process, the positive master may be made from sterling silver, since (with appropriate machine settings) the master is not significantly eroded and is used only once. The resultant negative die is then hardened and used in a drop hammer to produce stamped flats from cutout sheet blanks of bronze, silver, or low proof gold alloy. For badges these flats may be further shaped to a curved surface by another die. This type of EDM is usually performed submerged in an oil-based dielectric. The finished object may be further refined by hard (glass) or soft (paint) enameling and/or electroplated with pure gold or nickel. Softer materials such as silver may be hand engraved as a refinement.

EDM control panel (Hansvedt machine). Machine may be adjusted for a refined surface (electropolish) at end of process.

Master at top, badge die workpiece at bottom, oil jets at left (oil has been drained). Initial flat stamping will be "dapped" to give a curved surface.

Small hole drilling

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.

Advantages and disadvantages

Some of the advantages of EDM include machining of:

• complex shapes that would otherwise be difficult to produce with conventional cutting tools
• extremely hard material to very close tolerances
• very small work pieces where conventional cutting tools may damage the part from excess cutting tool pressure.
• There is no direct contact between tool and work piece.Therefore delicate sections and weak materials can be machined with out any distorsion.

Some of the disadvantages of EDM include:

• The slow rate of material removal.
• The additional time and cost used for creating electrodes for ram / Sinker EDM.
• Reproducing sharp corners on the workpiece is difficult due to electrode wear.
• Specific power consumption is very high.

See also

• Electro chemical machining

References

External links

• Research carried on EDM process
• New Arc Detection Technology for Highly Efficient Electro-Discharge Machining

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