About 70% of the heat is liberated near the anode in DC arc. That is the main reason. Positive polarity improves metal removal rate and negative polarity improves surface finish.

Weng, F. T., & Her, M. G. (2002). Study of the batch production of micro parts using the EDM process. The international journal of advanced manufacturing technology, 19(4), 266-270.

http://www.ijest.info/docs/IJEST11-03-06-091.pdf

http://www.edm.kd-solution.com/en_edm15.html

Please refer my paper entitled as "EFFECT OF TOOL ELECTRODE RE-SOLIDIFICATION ON SURFACE HARDNESS IN ELECTRICAL DISCHARGE MACHINING" which has been accepted and in press in the journal of "Materials and Manufacturing processes". I have given one section regarding the effect of tool polarity on machined workpiece. Please refer it.

Normally positive workpiece polarity gives higher removal rate whereas negative workpiece polarity gives higher surface finish. Genearlly micro EDM is used for higher surface finish. Thats why negative workpiece polarity is utilized for MIcro EDM. Best wishes for your research

My first problem is the term positive polarity, because it does nit indicate which electrode is positive or anodic. Examinating the removals of he anode and the cathode, then we find that the anodic removal for time ranges up to about 1 µs is substantially larger than the cathodic removal. For most metals this turning point (anodic and cathodic removal is equal) between 0.8 and 2 µs. For WC-Co-alloys at this point is 20 µs and higher. WC-Co alloys are therefore machined with anodic workpiece polarity reversal.

The differences between EDM and Micro-EDM are the micromachining is performed exclusively with needle pulses under 2 µs. The same statement applies to the Wire-EDM to.

Therefore, it is incomprehensible to me when working in Papers in Wire-EDM with pulse duration greather than 100 µs. (????)

In the final analysis thus depends on the chosen polarity of processing of the used process energy source (generator).

Thank you Dr Rao, Dr Muthuramalingam and Dr Schulze for your valuable replies.

Sirs, I still have a very silly query

If we give positive or anodic polarity to the tool electrode, the electrons will migrate towards the tool electrode. My doubt is, will these electrons result in high tool wear?

I read in a literature that due to large energy accumulation at the anode, the dielectric dissociation happens intensely and result into generation of a carbon layer over the tool electrode, which protects it from wear. But I think this for a higher pulse ON time condition.

Although I am a physicist, I go out from a technical current direction, i.e. the electrons emerge from the cathode and move towards the anode. The faster electrons, the anode is heated and worn away earlier than the cathode, that is, the anodic removal is higher for short pulses. The eypansion of the dichsrge channel (Plasma channel plus suttounding gas bubble) the anodic foot spot area is growing and the relative cathodic erosion always lower.

Best regards

Hans-Peter

PS: From 15. - 17. April 2015, there is the ESAFORM conference in Graz (Austria) and in the Minisymposium (MS) "Non-conventional processes can these issues are discussed. As one of the organizers of the MS, I would be happy even if interesting papers submitted by side of the EDM and ECM communoty.

to 15. September 2014 (abstracts, see website) and until 1. December 2014 the final paper.

           IN EDM process, the machining is happened owing to the electrical discharge happened between two conducting layer separated an insulating medium.  In EDM, the carbon layer is deposited  both an toll and workpiece surface ( Mostly due to the higher pulse energy ratings and usage of hydrocarbon insulating medium like kerosene). Since the carbon layer is also a conducting layer, it will not reduce the tool wear as per my opinion.

Best regards,

Muthuramalingam

To the latter answer:

Kerosene may no longer be used as a dielectric in most countries of toxic reasons, so I think it's not good when scientific investigations are carried out with kerosene. As a very good dielectric is n-dodecane offers, especially since here the composition is very defined and constant.

When there is a carbon layer during processing, then it suggests that very poor process conditions are present, which may have occurred through

incorrect process parameters, inadequate process control or ene insufficient gap flushing.

Best regards

Hans-Peter Schulze

Please, I would like to ask about the meaning on both positive and negative polarity and how we can do it practically during EDM.

in positive polarity, practically 70% of the total heat is generated at workpiece while 30% at electrode surface. Incase of Macro EDM ,where objective may be to economise the prodction , positive polarity is used. and negative polarity , 30% heat at workpiece gives comparatively better control on material erosion hence . better surface finish and dimensional accuracies can be obtained..

however, which polarity is to be used , largerly decided by the final application i.e. for material removal, positive and material deposition, negative polarities are to be used in case of macro as well as micro EDM..

I think it is going to get confusing.

To the latter answers: Why do they want to use potable water, I think there are better uses. If you want to use water, then the electrical conductivity has about 1 to 10us / cm, and that they reach with de-ionized water. Drinking water also includes too many salts which can be disturbing. Then I do not understand why they still mix graphite in the already "highly conductive" water. This "bad" electrolyte they might carry out an EC-ED-combination, but I think that I was not their goal.

To penultimate Answer: The power distribution on the cathode, anode and plasma channel is wrong. I think it is assumed that the pulse energy and since various studies have achieved differente results, but the absolute maximum amount of energy is used for the creation and preservation of the discharge required (70%). The anodic and cathodic shares now depend on the parameters pulse duration and also on the steepness of the current pulse. For very short pulses (

Heat generated in EDM is due to particles bombarding the surface as a result of electrical field. Electrons with negative charge are lighter in mass and are therefore accelerated faster. As a result, with a short pulse duration, electrons bombarding the anode electrode generates more heat than positively charged particles bombarding the cathode electrode. So applications with short pulse duration (micro EDM) choose the tool to be the cathode to reduce tool wear. 

On the other hand, if pulse duration is longer (macro EDM), then the positively charged particles start to generate more heat with their larger mass. In that case, people choose the tool to be anode to reduce tool wear.

Well, that's the simplified view. Hope it helps.

Your statement on the bombardment of anodic and cathodic electrode of the surface is only true at the time, where no plasma channel was formed. In addition, bombarding of the anodic surface after a very short time by the formation of a space charge cloud reduced (increased surface also).

For short and long pulses the bombing described effects of them are almost the same when the Arbeeitsstrom is approximately equal. It is an important explanation for the wear component at EDM.

This is why the polarities are different in WEDM with anode as work-piece(short on-time < 2 microsec). I think the paper by Dibitonto D.D will be useful to understand:

http://dx.doi.org/10.1063/1.343994 

Three output from electro discharge machining are influenced when using positive electrode [ metal removal rate (MRR) and surface roughness (Ra) and heat effected zone (HAZ) ] and each of them has a good impact in case of positive electrode.

Due to the fact that, increasing the discharge time in plunge EDM, the erosion of Cathode is drastically increase whereas the Anode's erosion asymptotically die out.

Thus, with sufficient discharge time. It is proven to use Positively charged Electrodes as tool, resulting lower wear.

However, increasing discharge time result in poor surface finishing.