Advances In Simulation of Electro - Discharge Machining

 

Dry EDM Process

The famous Electrical discharge machining (EDM) process is widely used to machine electrically conductive materials through the process of controlled spark generation. The dielectric fluid helps in flushing away the debris from the spark gap and also in cooling of the work piece and the tool electrode immersed in it. Mineral oil is typically used as the dielectric medium. This poses a threat in the immediate surroundings of the machine and to the environment due to the toxic fumes produced during the process.

The Dry EDM is a “green” environment friendly EDM technique in which a gas is used at high pressure as the dielectric medium instead of mineral oil based dielectric. This method also leads to various other advantages on the overall process.

Introduction to Dry EDM: 

Dry Electrical discharge machining (Dry EDM) is performed in a gas atmosphere. It typically used gases like air, oxygen, argon, helium, nitrogen or a gas mixture as dielectric. This gas based dielectric is injected inside the tool electrode at high pressure which removes the debris from working gap and helps in cooling the tool electrode and workpiece. It eliminates all environmental hazard risks and at the same time promotes operator’s health.

Difference between Conventional and Dry EDM:

Conventional EDM

Dry EDM

 

Conventional EDM	Dry EDM
Kerosene, deionized water etc. used as dielectric fluid.	Helium, nitrogen, argon etc. used as dielectric fluid.
Spark generated in liquid medium.	Spark generated in gaseous medium.
Dielectric fluid can be reused. Usually cooled down and cleaned before next operation.	Dielectric gas is not recycled. Hence, cleaning and cooling apparatus is not required.
Liquid dielectric is a good coolant. High thermal capacity.	Gaseous dielectric is a poor coolant. Low thermal capacity.
Cost of dielectric fluid is comparatively high.	Gas based dielectrics are cheaper.
Liquid dielectric posses high risk of environment pollution and operator health hazards.	Gaseous dielectrics eliminate environment pollution risks.
Liquid dielectric prevents atmospheric oxygen to come in contact with hot sparking zone and avoids oxidation.	High chances of oxide layer formation on hot machined surface during machining of ferrous materials.

 

Machining setup of Dry EDM:

The setup includes a dry EDM unit attached to the machining head. Here, the gas is filled through a free-wheeling bush. In order to seal the gas inlet region, two o-rings were mounted the shaft. To prevent the eccentric rotation of the electrode a ceramic pipe guide is provided.

Processes parameters:
The input parameters are current, pulse on-time (t_on), pulse off-time (t_off), duty factor, electrode rotational speed, pressure and the type of gas used as dielectric.
The duty factor (DF) can be calculated by:
DF = t_on/(t_on+t_off)

The output parameters include process responses like:

Material removal rate (MRR): It is the amount of material removed per unit time(usually taken in minutes).

Tool wear rate (TWR): Tool wear is the weight losses in the electrode to capture the extent of wear.

Surface roughness (R_a): It is the measure of finely spaced irregularities of the surface texture.

Radial overcut (ROC).

Factors affecting processes responses:

Effect of gas type on MRR:

 Compared to air and oxygen, mixture of air and argon shows the highest material removal rate. This is because argon in the mixture has higher viscosity and density which leads to a narrower plasma channel that focuses its energy on smaller area and creates a greater crater depth.

 Graph 1: MRR v/s Current; Graph 2: MRR v/s Tool rotational speed.

Effect of input parameters on MRR: 

The MRR is directly proportional to the current. This is due to the increase in pulse energy by increasing current which leads to higher heat transfer rate in the spark zone.

The pulse on time (t_on) is directly proportional with the MRR. However this relation is not linear. Increasing pulse on time leads to higher pulse energy.

Similarly, the MRR is directly proportional to the duty factor.

The MRR increases with an increase in gas pressure as it creates a better flushing condition.

The MRR initially increase with the increase in electrode rotational speed. But on much higher speeds the discharge may be interrupted by the movement of tool during the pulse on time. This leads to a decrease in MRR. (Graph 2) 

Effect of gas type on R_a: 

Nitrogen has the minimum R_a compared to air and argon air mixture. This is due to the lower viscosity which leads to lower crater depth and hence R_a decrease.

 Effect of input parameters on R_a: 

The R_a value decreases with an increase in current.

Higher the pulse on time, deeper the crater will be. This could result in higher R_a values.

Effect of MRR on Radial overcut (ROC): 

Higher MRR will lead to an increase in ROC as it places more debris between the machined hole and the tool electrode hence there is occurrence of spark between these particles and the workpiece.

Effect of flushing condition on ROC: 

Better flushing conditions could reduce ROC to a great extent as fewer particles will be placed between the crater and electrode wall.

Effect of gas type on ROC: 

Amongst nitrogen, air and air argon mixture, nitrogen gas showed the lowest ROC values. This is due to its low viscosity which leads to low concentration of plasma channel. This in turn leads to a decrease in effective pulses. 

Effect of input parameters on ROC:

 Higher current leads to higher ROC as current is directly proportional to MRR.

As pulse on time increases, ROC increases. This is due to the increase in MRR and plasma channel expansion.

Increasing gas pressure will help in decreasing ROC due to better flushing conditions.

Advantages of Dry EDM machining:

·       Eliminates risk of environmental pollution: Dry EDM is said to be a ‘green’ environment friendly EDM process as there are no toxic fumes generated.

·       No fire hazard: Ensures safety in the machine surroundings.

·       Decreased dielectric medium costs.

·       Lower tool wear rate: In Dry EDM, as the removed material gets deposited on the electrode, a protective carbon layer is shaped. This leads to near zero or in some cases negative tool wear rate.

·       Lower residual stresses.

·       Smaller Heat affected zone (HAZ).

·       Electrolytic corrosion of workpiece is avoided (in comparison with liquid dielectrics).

·       Arbitrary machining directions are possible, regardless of gravity due to the absence of dielectric tank.

 Major limitations of Dry EDM:

·       Lower material removal rate: Although Dry EDM proves to be a promising EDM technology with a number of advantages over the traditional EDM process like lower tool wear rate, thin recast layer and general overall environment friendliness; as this processes experiences unconstrained plasma expansion, it acts as a barrier in achieving desirable material removal rate.

·       Poor surface quality: As liquid dielectric fluids are better coolants than gaseous dielectrics, the workpiece being machined in dry EDM experiences higher surface temperatures which lead to poor surface quality.

·       Decreased spark intensity: This is caused due to a comparatively lower breakdown voltage for the gaseous dielectric.

 Research based solutions to improve Dry EDM machining:

The following solutions were proposed to improve the material removal rates in a Dry EDM:

1)  Use of electrodes with peripheral slots.

Ineffective debris disposal is a major factor hindering the performance of Dry EDMs. Providing peripheral slots to the electrode will ensure more space for the flow of dielectric for effective debris disposal and thus improving the material removal rate (MRR).

 

2)              Introducing oxygen into the dielectric gas mixture.

Electron negativity of oxygen was a major factor in improving the material removal rate. Experimental results stated that oxygen mixed Dry EDM showed a 200% increase in the material removal rate compared to the conventional Dry EDM process.

 Conclusion:

The conventional EDM is a commercially exploited process which provides various excellent machining characteristics like good surface finish, machining of complex geometries etc. On the contrary, the conventional EDM poses a threat to the environment due to the highly toxic gases produced and could cause severe health hazards to the operator.

The Dry EDM is a potential alternative process which eliminates all kinds of environmental threats and carries out ‘green’ environment friendly machining. However, there are certain performance drawbacks in Dry EDM regarding the material removal rates and surface finish. Currently researches are being carried out and certain solutions have been proposed to improve these parameters. With these improvements the Dry EDM is a candidate to be used as an alternate environment friendly process.

References:

[1] L. Liqinga, S. Yingjieb, Study of dry EDM with oxygen-mixed and cryogenic cooling approaches,  Harbin Institute of Technology, No.92 West Dazhi St., Harbin, 150001, China

[2] Saman Fattahi and Hamid Baseri, Analysis of dry electrical discharge machining in different dielectric mediums.

 [3] Govindan Puthumana1 and Suhas S. Joshi1,Investigations into Performance of Dry EDM Using Slotted Electrodes,International journal of precision engineering and manufacturing vol. 12, no. 6, pp. 957-963