Electrical discharge machining

[Amin 6457]

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.

[Amin 6457]

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 series-produced, numerically controlled wire-cut EDM machine.

 

The first numerically controlled (NC, or computer controlled) EDM was invented by Makino in Japan in 1980.

[Amin 6457]

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 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.

 

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). 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. 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. 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. 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.

[Amin 6457]

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.[2] 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.[3] 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.[3]

[Amin 6457]

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 [4]. 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 [5] , the second, presenting a thermal model for the erosion occurring on the anode [6] and the third introducing a model describing the plasma channel that is formed during the passage of the discharge current through the dielectric liquid [7]. 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 [8] 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 reduced 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.[3] Especially considering the lack of experimental scientific evidence to build and validate the current EDM models.[3]

[S.F 24932]

ماشين كاري به روش تخليه الكتريكي (edm)

ماشين كاري به روش تخليه الكتريكي

 

ماشين كاري با روش تخليه الكتريكي

روش هاي توليد مخصوص است كه كاربرد وسيعي يافته است. در اين روش براي براده برداري هيچگونه تماس مستقيمي بين قطعه كار و الكترود بر قرار نمي‌شود و در نتيجه نيروي فيزيكي نخواهيم داشت. آهنگ جداشدن فلز يا براده برداري به رسانايي الكتريكي قطعه كار بستگي دارد نه سختي آن.

 

اساس اين روش:

 

اين روش براي

ماشين كاري كليه مواد هادي جريان به كار مي رود با هر مقدار سختي كه داشته باشند و از چهار بخش تشكيل مي شود

:

 

1- الكترود

 

2- قطعه كار

 

3- سيال دي الكتريك

 

4- منبع

تامين جريان

 

 

هدف از استفاده از دي الكتريك (آب يا نفت سفيد) كاهش دما در

منطقه ماشينكاري و انتقال ذرات ماشين كاري شده از منطقه ماشين كاري مي‌باشد تا جرقه

ها مناسب زده شوند و اصطلاحا پديده آرك (

Arc

) اتفاق نيافتد

.

 

چنانچه بين دو الكترود

(قطعه كار و الكترود) اختلاف پتانسيلي اعمال شود در اثر برخورد شديد الكترون ها به

دي الكتريك بين دو الكترود مولكولهاي دي الكتريك يونيزه مي شوند و كانالي از يون

بين دو الكترود به وجود مي آيد كه به آن كانال پلاسما گويند.(پلاسما حالت چهارم

ماده است). و در اثر بر خورد شديد يونها به قطعه كار باربرداري صورت مي گيرد.

 

 

با زدن جرقه از يك سو و پيشروي ابزار به سمت قطعه كار از سوي ديگر (به صورت ارتعاش رفت و برگشتي با فركانس بالا) به مرور زمان شكل

ابزار در قطعه كار براده برداري مي شود. هر جرقه درجه حراتي بين 8000 تا 12000 درجه

سانتيگراد توليد مي كند . اندازه چاله اي كه هر جرقه از قطعه بار برمي دارد به

ميزان انرژي جرقه بستگي دارد كه مهمترين عامل موثر منبع تامين جريان است عمق چاله

به وجود آمده از چندين ميكرون تا 1 ميليمتر متفاوت است

.

 

 

فرآيند

EDM

شش مرحله دارد:

 

1-الکترود به قطعه کار نزديک

شده. هر دو بار دار ميشوند (معمولا قطعه کار مثبت و الکترود منفي)

 

2-چون سطح الکترود

و قطعه کار هر دو در اشل ميکروني داراي پستي و بلندي مي باشند بنابراين بين دو

نقطه که نزديکترين فاصله را نسبت به جاهاي ديگر با هم دارند جرقه الکتروني شکل مي

گيرد.

 

3- کانال پلاسما شکل مي گيرد.

 

4- در اثر تمرکز بالاي کانال پلاسما چاله اي از

قطعه کار ذوب مي شود.

 

5- فشار کانال پلاسما بسيار بالا است .با قطع شدن جرقه و در

پي آن قطع شدن کانال پلاسما چون مذاب در آن دما و فشار نمي تواند دوام داشته باشد

به يکباره با حالت انفجاري به اطراف پراکنده مي شود.

 

6-دي الکتريک با شستشوي خود

ذرات پراکنده شده را جمع آوري مي‌کند.

 

صافي سطح و سرعت ماشيکاري:

صافي سطح به ابعاد جرقه توليدي بستگي دارد. هر چه جرقه قوي تر باشد سطح خشن تر ولي سرعت ماشين کاري خيلي بيشتر خواهد بود. با اين روش به صافي Ra 0.10 مي توان رسيد، سطحي که مثل آينه عمل مي کند. صافي سطح هاي استاندارد معادل Ra 0.8/1 (N5 - N6) مي باشد. بسته به انرژي جرقه سرعت بار برداري از 1 تا چند صد ميليمتر مکعب بر دقيقه مي‌باشد.

اضافه مي شود که جرقه حداقل بايد 5 سانتيمتر زير دي الکتريک زده شود تا خطر اشتعال را در پي نداشته باشد چون انرژي جرقه بسيار بالا است.

از دستگاه هاي متداول مي توان به اسپارک و وايرکات اشاره کرد .

 

كارايي اين سيستم با آهنگ براده‌برداري بر حسب ميليمتر مكعب يا اينچ مكعب بر دقيقه سنجيده مي‌شود و توسط سيستمهاي كنترل عددي كنترل مي‌شود.

الكترود اين فرايند معمولا از جنس مس(در اسپارك) و مسس يا تنگستن (در واير كات) مي‌باشد.

 

يك نمونه از ماشين اسپارك:

[serj 34]

باعرض سلام وخسته نباشید

لطفا ویدئویی از عمل ماشين كاري به روش تخليه الكتريكي (edm)برامون بگذارید.

باتشکر فراوان.

[S.F 24932]

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